CA1339322C - Interactive television and data transmission system - Google Patents

Abstract

A spread spectrum system provides bidirectional digital communication on a vacant television (TV) channel for simultaneous use by more than 75,000 subscribers using time and frequency division multiplex signals locked to horizontal and vertical sync pulses of an adjacent channel Host TV station. The system, whose operation is analogous to a radar system, comprises: (1) the Host TV station to send down-link sync and data pulses to subscribers during the horizontal blanking interval (HBI), (2) subscriber "transponders" which detect those signals and transmits up-link "echo" data pulses only during the HBI to eliminate interference to TV viewers, and (3) a central receiver which also uses the host TV sync pulses to trigger range gates to detect the up-link data pulses. In a preferred embodiment the central receiver employs directional antennas to determine direction to transponders and to define angular sectors partitioning the service area into pie-like "cells" which permit frequency re-use in non-contiguous sectors (like cellular radio).
The system thus operates like a radar to measure elapsed time between receipt of TV sync pulses and receipt of transponder response pulses and measures bearing to transponders to thereby determine the location of fixed or mobile subscribers as well as provide data links to them. Transponders may share user's existing TV antenna or may operate on cable TV and could be packaged as "RF modems" for personal computers, as transceivers for mobile or portable use, or they may be integrated with a TV
receiver to provide "interactive television".

Description

~339322 1 R~ck~rol~n~ Qf ~h~ ~nve~t~o~

This invention relates to a new system referred to a~
~~-NET~ which provides bidirectional communication of digital lnformation to a plurallty of fi~ed or mobile subscrlber~ on a vac~nt TV s~nnel ~djacent to, and cooperating with an existlng ~~ost~ telev~sion (TV) station. The horizontal and vert~cal sync pulses of the host TV ,signal are used as a wide-~rea clock to coordinate time and fr~quency d~vision multiple~ing o~ subscriber transponder~ and to t~igger up-link responses from them only 1, ) .
during the hori~ontal ~l~n~irg interval ~EBI) to prev~nt inter-ference to television viewers. Down-link signals to ~ub~cribers are also ~ent within the ~BIo In a preferred embodimont the typical subscriber-to-central receiver data rate i8 3t)0 or 1200 baud and that signal's ,spectrum is ''spread- into subchAnnels 187.5 R~z wide by virtue of modulating it on a stream of 5 microsecond pulE;es ~hirty-two of these subchannel~ ~it in a standard 6 M~z TV channel. More than 300 transponderE; can operate simultaneously on eac~ ~iubchannel. The Fiame ~;ubchannels may be used for up-link and down-link commun~cation~ even simultaneously on the iame 6~hchannel Means to multiplex information to TV receivers on an existing TV s$gnal during itia ~orizontal or vertical bl~nking lnterval are ln use or have been contemplated (eOg. pre~ent day I ~Teletext~ owever,,l the advantage of using TV horizontal and/or vertical sync pul~es to synchronize both down-link and up-link radio signals, on the same or an adjacent TV ch~n~el, 80 --they effectively exifit only within TV horizontal or vertical '~
blanking intervals, thus are invisible to television viewers, and 133~322 1 for the furtber purpose of enabling time and freguency division multiplexing of many signals, has not heretofore been discovered.
The present $nvention teaches that technology.
A ma~or portion of the U.S. radio spectrum has been sllocated to broadca8t 6ervlces and more specif$cally to television. A substantial number of television chAn~el~ are unused in most c~ties because of physical limitations caused by inadequate television receiver 6electivity. As a conseguence of thi~ at least one vacant channel exists between ass$glled lo teleYision ~tations and those channels have heretofore been unusable. As a practical matter, ~ntermodulation interference and other con6iderations further limit the number of us~ble television channels ~o that ~ubstantially less than h~lf th-allocated TV channels are in use in a given area. Unusable 1~ channels are sometimes referred to as ~taboo~ freguencies. A
principal object of the instant invention is to make practical u~e of this presently unusable spectrum.
~ Described another way, typical television receiv~srs, particularly when operating at ~F frequencies, have relatively poor frequency 6elect~vity con6equently radio tranL.~ission in c~nnels adjacent to a TV signal i8 prohibited because ~t would cause unacceptable interference. For example, even a low power conventional radio device whicb transmits one watt could easily cause unacceptable inter$erence to adjacent chAnnel television viewers who live within a radius of several city blocks ~urroundinq it because it~ power would oyerwhelm the TV ~ignal.
Clearly, thousands of such conventional transmitters deployed throughout a city for the uses contemplated here would generate unacceptable interference.

--1 S~nce televi~ion broadca~t c~an~els are by gover~ment regulation allocated to ~mass media~ use~ lt ~8 ~mpllcit that ~uch channel~ are not lntended fo~ low capacity pri~ate radio commun~cations such as associated wlth point-to-po~nt or land 5 mobile rad$o applications- Consequently appllcations for the aforement~oned vacant ~V ch~nels, ~ they could be used at all, would be e~pected to benefit the public en masse as contemplated in the $nstant inventlon for such uses ~5 future home ~nformation ~ystems, interactive television, remote ~hopping, b~in~, electronic ma~l, reservations means, security a~arm communications, and the like~
Ea6e of installation and simplicity of operatlon ~re lmportant considerations for mass applications. Thus sSaring of the userl~ existinq television antenna as taught hero i- an lS important feature. Integratinq this invention with a television receiver to provide interactive TV controllable from remote Sand-held devices comparable to those used today for remote TV channel ~witching are other features taught in this pecificationO
An object of the present invention is to provide ~eans to accurately partition subscribers into geogr~phic ~radio cells~
within which ~pecific ~ubscriber tran~ponder subchannels may be assigned and isolated from transponder~ in other cell-0 This permits re-use of subchannel frequencie~ in non-contiguou~ cells to significantly expand the number of users that can operate on 25 one previously vacant TV channel in a given city. These desirable frequency re-use features are commonly identified today with ~Cellular Radio~.

:~ i 1 The instant invention is al60 applicable to two-way cable TV
systems ICATV) to provide improved isolation of up-link and down-link signals compared to existing methods.
-A further ob~ect of the present invention is to provide S lmproved means for locating and tracking the position of mobile or portable subscriber tran~ponders to provide economical ~ervices somet$mes referred to a~ automatic veh$cle location or automatic vehicle monitoring (AVM)~
Aut~:atic ~hand-off~ of present day cellular radio telephone ~ubscribers a~ they move ~rom cell-~o-cell ~n a city i~; a problem because it i8 based on signal amplitude measurement~ ~nd these vary widely at dlfferent places and at differen~ times. An ~ndependent means such as T-NET to locate subscribers ~:an form the basis for an alternative hand-off method which could minimize or solve the existing problem and thi0 constitutes ano~ther T-NET
application.
Yet another application of the invention is for so-called ~video conferencing~ which usually comprises a dedicated TV
network connecting a centrsl office with many remote offices for such applicatlons as over-the-air teaching, presentations by management, or even TV monitoring of banks or other businesses for ~ecurity alarm purposes. Such T-NET applications would employ the down-link to send pictures ~video) and the ~p-link could either be digital or digitized ~slow-voice~, all 2s multiplexed ~imultaneously with the existing TV programO
It is clear that simultaneous synchronization o~ the T-NET
sy~tem with several IY stations in a city as contemplated by the inventor could be a problem. Thus a further ob~ect of the invention is to teach an operatin~ means wherein the horizontal ~339322 1 8ync pulses of several co-located television transmitter~ are locked together ln time 80 that subscr$ber transponder~ work$ng ln cooperation w~th one or several such stations wlll always transmlt with$n the hor$zontal blank$ng interval ~sI) of all th-television ~ignal~ simultaneously, thus el$m$nating intcferenceto viewers of all of them. TV tran~m$tter co-loc~tion ~8 ~
practice in many large citiea te.g, Los Angeles and New York) to establish ~ common antenna direction for ~11 TY viewers.
Alternatively, it is taught that if T~N~ subscrlbers are located in a boundary ~ervice area ~etween televis$on stations not co-located ~e.g, ~etween TV tranEmitters ln adjacent cities), then those subscr$beE transponders can be progra~med l:o transmit only during the vertical blanking interval (VBI, wh$cb $8 much longer in time duration than the ~BI) and thus w$11 not $nterfere w$th TY viewer~ of either city, provided those televil;$on 6tations are synchronized to cause thei~ vertical blanking ~ntervals to overlap as taught in this invent$on~
Two nsw and improved ~ethods ~re also taught for se~ding digital informatisn to subscribers ~down-link) by eitl~ers ~1) co-chAn~el modulation of the ~o~t TV signal in a non-interferr$ng manner or (2) modulating new ~out-of-channel~ subcarr;ler ~ideb~nd~ ~n adjacent upper or lower tor both) TV channels.

Brief Summary of the Invention -The invention relates to a bidirectional radio communlcation system for use on pre~ently vacant TV channels in cooperat$on with a host television transmitter, In one embod$ment the host provides down-link digital s$gnals to a plurality of 6ubscriber ~, ~339322 1 transponder~ using the improved methods herein set forth.
Subscriber transponder devices detect these signals and transmit carefully synchronized up-link digital signals to central rece$vlng sites which are preferably located along the path between subscribers and the host television transmitter. The inventor caIls the system ~T-NET~.
A network control center ~NCC) interconnects the host televislon transmitter ~nd central receivers with Information Providers using convention~' trunk-line paths 80 as to furnish the Information Providers with means to cc u~cate with their subscribers, or to provide virtual circuits for ~ubscribers to cc un1cate w$th each other. T~e Informat~on Providers may be organizations ~uch as banks, retail stores, vehicle di~patcher~, Data Banks, entertainment sources such as pay IV, and the like.
1~ The horizontal and vertical synchronizing pulses normally transmitted by the host television transmitter are employed in the invention as a clocking mechanism to coordinate time and frequency division multiplexing of subscriber receiver/transmitter devices (herein called transponders).
Subscriber transponders are triggered by the host TV signal ~orizontal sync ~hereinafter called ~-sync~ pulses 80 that they transmit on1y during the horizontal blanking interval (EBI? or vertical blanking interval ~YBI). i;.e B I is typically eleven microseconds in duration and viewers living within a radius of 25 about one mile ~urrounding a transponder are simultaneously blanked out during this time period. Consequently they would not see the transponder'6 signal, thus will not be interfered by it, providing itC transmission duration is on the order of a few microseconds.

.
, ~ b 133~322 1 The T-NET system 1~8 mo6t ea6ily described by compar~ng its operation to ~ radar sy~tem. ~he ~ost TV ~-sync pulses are analogous to the outgo$ng radar pul~e8 and these trigger transponder reply pulses ~echos~). The reply-echos, each -- -comprising one bit of lnformat~on, a~e recelved at a central recelver after a ~ranslt delay and that delay is a measure of the distance to tbe subscrlber. In the ~nlted St~tes the TV ~-sync pulse recur~ence frequency (called PRF in radar) is 15,73~ ~z and provides an unambiguous ~adar ~range~ of about 81~ miles because lo rad~o waves travel at about 10.7 microseconds (two-way) per m~le and the time between ~-sync pulses is 63.555 microseconds.

In one aspect, the present invention relates to a bidirectional wireless digital communication system comprising a television broadcast station for transmitting ordinary ~5 televlsion programming including vertical sync and horizontal sync signals and associated blanking intervals ~n a preassigned ; -~~
television channel and as~ociAted video carrier;
broadcast mean6 for controll~bly transmittlng downlink digital data slgnal~;
a plural$ty of eubscriber receiver-transmltter~ dietributed about an area within the broadcast range of said televieion broadcast etatlon and 6ald broadcast means, each eubscriber receiver-tran6mitter having a 6ubscriber receiving means for receiving 6aid video 6ignal and detectlng 6ync eignal6, and for receiving and detecting 6aid downlink digital data 6ignals, each subscriber receiver transmitter having modulating means for modulat~ng uplink digital data eignal6 on an uplink carrier of a frequency within the ~requency band of a televie~on channel ad~acent eald preas61gned televielon channel, each eubscriber transmltting mean~ coupled to said eubscriber recslving mean6 for l transmitting the modulated uplln~ digital data sign~ls durlng at least 60me of the blan~ing lnterval5 o~ the received video signal; and a plurality of central receiVers, each central receiver being located and having a directlonal antenna to predominately receive the modulated uplin~ digital dat~ signal6 from a respective 6ubarea within sald area, each 6aid central receiver being a means for receiving and detecting 6aid uplink digital data 6ignals transmitted by the respective subscriber transmitting means within the respective subarea.

In 8 further aspect, the present invention relates to a wireless digital communication sy~tem comprising a broadca~t station for transmitting a video signal a wireless digital communication system comprising:
a broadcast station for transmitting a vide~ signal at least including blanking intervals on a video carrier;
broadcast means for controllably transmitting downlink signals on a second carrier; and a plurality of ~ubscriber receiver-transmitters, each subscriber receiver-transmitter having a subscriber receiving means for receiving said video signal and detecting said blanking intervals, and for receiving and detecting said downlink signals, each subscriber receiver-transmitter also having a subscriber transmitting means coupled to said subscriber receiving means for transmitting uplink signals only during at least some of the blanking intervals of the received video signal; and at least one central receiver, each said central receiver being a means for receiving and detecting said uplink signals transmitted by each subscriber transmitting means.

.
., ~' ~ 3 3 ~ 3 ? 2 1 In a still further aspect, the present invention relates to a cable television system comprising:
a broadcast station for transmitting a video signal over a cable television channel; and broadcast means coupled to said broadcast station for transmitting a downlink digital data signal so that Qaid broadcast station broadcasts through said cable regular television programmins and said downlink digital data, said downlink digital data being transmitted as video information by adding said downlink digital data to at lea~t part of the video signal during one video frame and by subtracting said downlink digital data from the corresponding part of the video signal during the next video frame.

In a further aspect the present invention provides in an interactive television system, a method of communicating information from a plurality of remote receiver locations, each connected to a cable television system, to a central location, the cable television system receiving ordinary television programming over the air and providing the same over a cable and having amplifiers at various points along the cable, each amplifier serving a plurality of remote locations, comprising the steps of:
(i) at each remote location:
a) modulating the information to be communicated from each remote location onto a carrier having a frequency in the frequency band of an unused cable television channel;
b) transmitting the modulated information on the cable;

7b ~: ~

1 3 3 ~ 3 r l 2 1 (ii) adjacent each cable amplifier:
c) transmitting the modulated information received on the cable over the air only during at least some of the blanking intervals of a first television channel broadcasting ordinary television programming over the air and in the frequency band of a television channel adjacent said first television channel; and (iii) at at least one central location:
d) receiving the transmissions of step (c) and detecting the information therein.

Further aspects of the invention reside in providing a method of communicating information within an area served by a broadcast station transmitting on a broadcast station carrier and within a broadcast station frequency band comprising the steps of:
a) modulating the information to be communicated by a carrier other t~an the broadcast station carrier;
b) transmitting at least one side band of the modulation of step (a);
c) receiving at a remote location a signal containing the carrier of the broadcast station, and the at least one sideband transmitted in step (b);
d) demodulating the signal received in step (c) using the carrier of the broadcast station as a reference to recover a signal corresponding to the cignal transmitted in step (b); and e) demodulating the signal recovered in step (d) to recover the information to be communicated.

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,. ,_.....

~3~322 1 Further aspects of the invention reside in providing an apparatus for communicating information within an area served by a commercial broadcast station transmitting over the air on a broadcast station carrier and within a broadcast station frequency band comprising:
modulation means for modulating the information to be communicated by a carrier other than the broadcast station carrier and for providing an output responsive thereto;
transmltter means coupled to said modulation means for transmitting, from a transmitter other than the broadcast station transmitter, the output of said modulation means;
a receiver at a remote location with respect to said transmitter means for receiving as a receiver slgnal a signal containing the carrier transmitted by the broaccast station and the signal transmitted by said transmitter means, s3id receiver means having;
a first demodulator for demodulating said receiver signal using the carrier of the broadcast station as a reference to recover a signal corresponding to the signal '~
transmitted by said transmitter means, and;
a second demodulator coupled to said first demodulator for demodulating the signal recovered by the first demodulator to recover the information to be communicated.

7d 1 3~3~2 ~P-r.INl~ S~tBSYSTE~
A 6tandard V.S. color TV frame consi6t6 of 525 horizontal ~lines~ and 29.97 frames are transmitted per second to yleld 15,734 horizont~l lines with ~-sync pulses per second. Thu~
subscriber tran6ponder up-link reply-echos could be triggered by the TV horizontal sync pulses at a rate up to 15,734 pulses per second. Bowever, this data rate 18 much faster than typical transponders requlre because they are usually designed for ~
performance comparable to telephone modems (300 to 1200 bits per second). In one embodiment of the invention the transponders tr~nsmit a RF reply pulse to 6end a logic ~1~ or no pulse ~no emis~ion) for logic ~0~ ~hen interrogated by a ~V ~-sync pul~e.
Consequently, if tran6ponder~ are designed to transmit ~t 300 baud they ~ill respond ~.e. provide an echo) on every 52nt ~V
15 B-6ync pul~e (15,73~/300 ~ 52).

13~322 1 Several transponders could therefore be scheduled to $nitlate transmission on different TV ~lines~, that i6~ different ~-syn~ pulses, and on every 52nd horizontal line thereafter ~modulo 52~. For example, one transponder could use the ~-sync S pulse of TV horizontal lines 1, 53, 105, . . . 469. Another tr~nsponder at the same location could be programmed to transmit on horizontal lLnes 2, 54, 106, 158, . . . 470, and 80 on. Thus up to 52 different sub~cribers living at the same range could effectively tran~mit on the same subchannel ~.:om one range ~cell~
(i.e. one range gate) location, but operating on 52 different TV
~-~ync line~ of the S25 horizontal line~ ~vailable on each TV
frame. Thi8 permits each of them to send 10 bits on each TV
~rame, which re6ults in about 300 baud transmission r~.te each.
This multiplex method is defined herein as a ~coarse~ t~me 15 division multiplex process to distinguish lt from the ~fine~ time division ~actually space division) multiplex process t.hat occurs because different subscribers live at different distances from the TV transmitter, thus at different transit time (r~.nge gate) interval~0 For esample, a system using six range gates, each ~ive microseconds wide, would provide an unambiguous T~NET service area radius of about six miles. Thirty-two different radio frequency subchar.nels could be created in one vacant 6 M~z wide 5 TV channel. consequently 9984 different subscribers (52 ~-sync 25 lines ~ 6 range cells x 32 subchannels) could simulataneously operate in one angular sector without signal ~clashes~ or range ambiquity. A similar number could operate in a 12, 18, or 24 mile service radius using a software routine to eliminate the ~3~322 .

1 ~radar range ambiquity~ which ari~es wben more tban one ~-sync pUlfie i8 ~n transit at one time between the TV tran~mitter and t~e ~ubscribers.
At the central receivlng site each basic timing pro~e6s s commences w$t~ each IV *~ame. ~hls occurs upon ~ece~pt o~ ~
vertical 6ync~ronizing (V-6ync) pulse from th- televlJlon ~tat~on and the 525 B-sync pulses that follow ~t ~in the ~.S. standard NTSC format~ In a pre~erred embodiment the ~-sync pul~es each tr~gger the start of a ~range ~ddress generator~ at e~ch cen~ral lo receiver which generates 8 series of delayed receiver range gates, e~ch having a width of 5 micro~econds. Thi8 width i8 adjusted to match the width of the pulse ~ignal tr~ns~itted from each subscrlber. Five microseconds is ~lso the approx~mate width of tbe standard TV ~-sync pulses. The central receiver, which has previously stored in its memory the range to each subscriber transponder, opens up a range gate at the expected ti~e of arrival of each transponder digital bit pulse to thereby determine if the transponde~ has fient a logic ~1~ (i.e. a transmitted pulse) or a logic ~0~ (no transmitted pulfie).
T-NET system ~erving a 24-mile radius would expect a 257 microsecond -~Y~mum duration between receipt of a TV ~-~ync pulse and receipt of a delayed transponder reply pul~e ~f the TV
transmit'-r and the central receiver are co-located.
A computer at the central receiver collects all of the time interleaved ~0~ and ~1~ responsefi from Its many ~ub~cribers, sorts and groups them into 6eparate packets and appends the appropriate ~ubscriber address. One preferred packet ~tructure which can be employed in the invention is the Eo-called X.25 public packet switching protocol which is expected to be 1 3 ~ 2 l universally accepted. These packet~ are then forwarded through conventional co~ ication trunk lines to a centrally located network control center ~NCC) where the packets are further routed ' by conventional means to various Information Providers ~uch as 5 data bank~, electronlc mail ~erv$ces, financial institution~, ~nd the llke. Their replles are slmilarly routed back to each subscriber a8 described below.
DOWN--T.TNl~ SUBSYST~
Present aL~. In the instant invention, dig~ta~
c~ --r~cations to transponders could be ~uperimposed Gn a televi-~ion ~ignal using either conventlonal techn~ques known today ~e.g. ~eletext) or the enhanced new methods disclosed in this specification. In the present art, teletext digital ~ignals are transmitted on up to 8 of the 21 TV horizontal lines that lie lS within the vertical blanking $nterval (VBI) sf conventional television 6ignal formats. One such method is called the North American Basic Teletext Specification (~ABTS) and thiE standard permits approx~mately 288 bits o$ information to be packed in each of eight of the twenty-one horizontal lines that lie within the TV vertical blanking interval. Since the YBI repeats at the rate of 6C times a ~econd, this results in an average down-link traffic capacity of approximately l38,000 bits pe~ ~econd ~288 bits ~ 8 lines x 60 ~z). We refer to these methods a~ co-channel techniques because they lie within, and share the ~ost TV'~
2s channel.
New and improved means are taught fn this ~pecification for ~ending information down-link to ~ubscribers at greater ~peeds and more reliably. These are ~ubdivided into two classes: (l) .. . ......
. ~ . ~, ': ~

~ 339322 1 those that operate on the adjacent upper or lower c~annel ~or both), and (2) co-channel techn$gues that share the same c~Pn~e Z18 the TV ~ost station~ The $nventor's improvements are ~ummarized in the following paragraphs.
s ~ace~t ~hannel n~wn-T~n~. A preferred embodiment of the pre~ent invention packs 4 bits of information on each of up to 32 t$me-gzlted ~ubcarriers. These subcarriers are tuned to a chAnnel ad~acent to the ~ost TV stat$on chAnnel and are gated to exist - -~ only within its horizontal bl~n~ng interval (HBI). S.ince the EBI repeat~ at the rate of 15,734 times per second, th:is provides a potentlal capacity of approximately 2 million bits ~!r ~econd ~15,734 ~z x 4 bits ~ 32 ~bc~Anne~s)~ a substantizll il~provement over exicting Te~etext~ While this methot reguires us~ of a vacant adjacent channel above or below ~or both) the host 5V
station, some of the ~ame s~hch~nnels used for the T-NIiT up-link can be used for the down-link as well, even on the ~ame 6uhchAnnPl at the ~ame timeO
Ço-c~an~el ~own-T.~k. The ~econd improved ~ethod to increase the digital tr2ffic capacity of down-link datzl stream~
8uperimposed on the TV transmission is taught here~ It; uses the ~ame channel as the TV 6ignal (co-channel) and involve~ the '_ ~eguential adding and subtracting of identical digital data Fitreams to the existing video picture information at corresponding TV picture elements of ~equential TV frames. The process is a~ follows: the existing TV videojinformation of each horizontal line of a first frame is stored and compared to the video on the corresponding lines of the following frame to locate non-moving (~frozen~ port~ons of each ficene. The desired digital data is first added then ~ubtracted on corresponding 11 ~

1~

~ ~3Q3~2 1 line~ of the first and second frame, preferably at only the frozen scene portions. For example, each digital bit added to each line could be a pulse about 185 nanoseconds ~n duration as ln e~isting Teletext to yeild 288 bits per line. On the following frame tbe same digital data i6 inverted and, in effect subtracted from the frozen vldeo of the previous corresponding horlzontal lines and again transmitted. ~he result l~ that at ~ny 5V picture spot (piYtel) of the telev~6ion viewerls screon the digital information whic~ is first added then subueguently subtracted, cancels and becomes invis~ble. Each of tbe 525 lin-s per frame could carry data $n thls manner. Note that each frcme consists of 525 lines in two interleaved ~fields~ of 261.5 lines each in the U.S. Standard.
The invisibility of data is primarily due to the known psychological canceling process of human vision, but also because the phosphors of the television screen have a slight ~veraging -effect that cmoothes out TV ~cenesO This cancelling effect can be optimally adjusted for data added/~ubtracted from Eixed televised scenes as well as to i~ ~ze ~beat~ effect~ caused by it8 presence with the color TV chroma subcarrier. ~owever, for televised scenes that include motion there is a slight difference ln the video level from one frame to the next in the motion part of each scene and therefore superimposed digital data, followed ~y inverted data, may not completely cancel. Fortunately, if the data is transmitted at the high rate propo~ed here, one can capitalize on the fact that the frequency response of the human eye to this ~high fidelity noise~ is masked by the motion in those portions of the moving scene. In other words, the human ~ 33~322 1 eyes' resolutlon deteri~rates ~nd does not see high freguency ~xtraneous component~ of a scene wbich is in motion and one could gend data with motion ~cene~ with some gacrifice of picture qusllty. Alternatively, data transmission could be inhibited in wene segment~ which contain motion as guggested above.
The technique of adding and gubtracting dig$tal data ~ust descr$bed can be implemented using known digital TV gcene store and forward technigues. This is rather simple on televiged black and white programs. The description of how the new proce6s works 1~ on color televi~ion transmissions ~8 ~omewhat more complex, though es~entially s~milar a8 will now be described.
Color television basically transmits three different signal~
related to the primary colors (red, green, blue) ~nd these are generally refered to a6 the in-phase (I), quadr~ture ~Q) and lS luminance (M) components. For re~so~ related to c~ar wteri6tics of human vision it turns out that the freguency bandwidth requirements of the luminance component ~M~ is substantially greater than the other two. The ~I~ and ~Q~ components sre in ~act superimposed on a chroma subcarrier channel havi~g a useful information bandwidth of only one-third to one-half that employed for the luminance component. Consequently, the in6tant invention provideg for the modulation of digital dats on the lu~inance component only and in such a manner that the digital dat~
manifest frequency spectra well above the gpectra of ~I~ and ~0~
components, thus invisible to them, and in ~uch a manner that superimpo~ed digital data followed by inverted data superimposed on correspond~ng locations of a following frame visually cancel .

13~93~2 1 substantially as described before. Thus regular lV ~ideo and p~ggyback data may ~e transmitted simultaneously on the same TV
channel.
Th~s process could be accomplished at ~ data rate r~presenting the proper harmon~c ratio oi the horizontal ~ync r~te to optimize visual cancellat~on in much the same manner used - to ~elect the proper TV chroma oscillator freguency in present day color T~ receiYers~
This improved method of p~ggyback down-link co-ch~n~el lo transmission is particularly attractive for such appl:cations a8 video conferencing where a speaker in a centr~l locat~on may w~sh to addre~s ~ large num~er of remotely located off~ces and in which he use~ a series of charts and graphs~ thus a l~lrge part of the ~V scene comprises low data content fixed video consistent w~th the capability of this invent~on which is slower than regular IV. Most of the motion is primarily in the s]?eaker's llp8. Thi~ point-to-multipoint video conferencing mode uperimposed on regular TV i8 yet another a~tractive i~pplication of the T-NE~ 6ystem and has the additional benefit of having a return pa~h o that the listeners can Jtalk back~.
Angular Sector~. One preferred embodiment o~ the instant ~nvention would employ directional antennas at the central receivers, for example, each having a gain of approximately 20 dB
and beamwidth of about 18 degrees at ~EF. Twenty such antennas would provide a full 360 degree omni-directional coverage if all were located ~n one central location. Central receivers could be located near the host television transmitter or they could be disper~ed throughout a city area depending upon the local topography and coverage desired. In one preferred embodiment ' ~

1 previously de6cribed, each ~ubscrlber tr~nsponder transmits for about f$ve microseconds when it is lnterrogated by cvery 52nd ~-sync pulse. The radio frequency ~R~) bandwidth required to carry 6uch a signal is on the order of 187 R8z. Thus 32 tifferent transponder ~subchannels~ could be asslgned within one typical 6 M~z telev$sion cbAnnel. For example, sixteen even numbered transponder ~ubchAnne~s could be assigned to one directional recelving antenna sector while the ad~acent antenna sectors could use the 16 odd numbered ~ubchannels. 8uch a plan would permit the re-use of the even and odd numbered transponder subchannels many times within a city to significantly increase the overall system digital traffic capacity.
Veh~cle r~c~tio~. Because this invention operate~ ln a manner analogous to a radar ~ystem, wherein the IV horizontal 1~ sync pulses a~e equivalent to the radar's outgoing transmission pulse, and where the transponder pulses triggered by it comprise reply echo, it is clear tbat distance to each ~ubscriber can be accurately de~ermined~ This i~ used to advantage in two ways:
for f~xed subscribers the central receiver can ~ecur~tely predict the time at wh~ch each subscrlber transmission pulse will be received, consequently it can optimumly schedule subscriber responses in a space, time and frequency divi~ion ~anner to optimize the system traffic capacity. Alternatively, if the range to each subscriber i8 unknown, a~ for ex~mple in portable 2s or vebicle mounted devices, then specific frequency subchannels can be dedicated to those mobile transponder applications 80 that one can measure the position of each vehicle and automatically '~ ' 133~22 1 keep track of lt~ location using target acqui~ition and trscking techniques well known ln radar. Thi~ 18 done simultaneously with data transmi6~ion wlth the vehicle.
In the ~llustrative system previously described, the ~ngular bearing mea~urement to the unknown vehicle posit~on would be rather cruae because of the relatively wide beamwidth (i.e. 18 degrees). On the other hand, the range to transponders can be precisely mea~ured to the order of ~ hundred feet or 80.
Consequentl~, a T-NET sy~tem can be optimlzed to provide much better vehicle location accuracy by using two separated central receiver6 properly programmed so each mea~ures range to eacb mobile transponder and thereby more accurately dete~ ~nes veh~cle location ~about 300 feet accuracy is anticipated).
CA~Y ~ppl$cation. Yet another application of the invention lies in the area of cable television (CATY). The isolstion of siqnals to and from subscribers in present day CATV sy6tems has been found to be a problem~ partly because of the fact that when many subscribers transmitters are connected to the television cable they each contribute undesirable noise. ~ince this noi~e i8 additive in present day continuous wave (CW~ techni~ues, the umulative noise of all two-way CATV subscribers pose a serious problem; The instant invention ~olves this problem in essentially the same manne~ as described above for o~er-the-air applications. The horizontal blank~ng interval of a cable TV
program is on the order of one mile as in the examples discussed before. In the instant invention the transponders' up and down-link emissions exist only during the ~BI, hence are invisible to subscribers living within a mile of each other. One embodiment of a cable TY application of this invention would install T-NET

. ~ .
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~33~2 1 ~master~ (multiplesed) repesters within CATV ~mplifier bo~e~
which typ~cally are at intervals on the order of one mile slong the TV cable, and regular transponders at each subscriber'~ home.
~-NET ~ignals a~e collected at the master repeat~r and relayed ~over-the-air~ to a T'NET central receiver ~nd proces6ed in essentially the same manner previously discu~sedO
The var~ous technigue~ ~ust described compri~e the es6ential building blocks from which various system architectures ~ay be devi6ed to practice this lnvention. For example, it i8 obvious each transponder could reply to all B-sync pulse~ to provide a 15,73q baud rate and thereby ~bur6t~ its up-link message much faster~ comb$ninq this with a different time-sharing arrangement between transponders provides yet another mode of operation.
Thus the various applications described herein and others will lS become evident to the skilled co ~ication5 y6tem designer upon careful study of the operating details of these building blocks hereinafter described.

13393~2 ~f r~scrlption Qf. ~h~3. nrawingE;

Further object~ And advantages of the inventi~n will become apparent from the following specifications taken in connection with the accompanying drawing~, wherein like reference charw ter~
S ldentify parts of llke funct$ons throughout the different views thereof.
Figure 1 is a block diagram of the overall 5-NET ~ystem configuration.
Figure 2 i~ a plctorial of the invention employing directional antennas to segment the service area into pie shaped sector6 covering a city area and illustrating possible locations for a receiver ~ubstat1on.
Figure 3 i~ block diagram and pictor$al illustrating the manner in which TY horizontal sync pul~es trigger subscriber transponder repl~es and a pictorial illustrating the appearance of the TV ~ync pulses followed by range delayed subscriber replies, as ln a radar ~A~ scope.
Pigure 4 is a plan view of one central receiver directional antenna coverage ~ector segmented into range cellsO
Figure 5 illustrates the manner in which the television horizontal blanking $nterval (BBI) is superimposed on~ and thus masks the subscriber transponder rul6e trAns~issions.
Figure 6 is a graph ~llustrating the rapid drop-off (attenuation) in the strength of the subscriber transponder ~ignal pulses with propagation distance.
Figure 7 is a top view of one typical communication path between TV ~tation and subscriber transponder, illustrating the area blacked out during the BBI.

., 13~9322 1 ~igure 8 is anoth~r graph illustrating the typ~cal ~ignal level~ found in t~e HBI o~ ~ standard televislon ~aveform.
Pigure 9 illustrates the gated subcarrier ~own-link adjacent ch~ l embod$ment.
F~gure 10 illustrates two methods of modulation which can bc employed in gated subcarrier down-l~nksO
Figure 11 is an illustratlon of the invention as applied to simu~taneous up-l~n~ and down-link operation on the s~me ~ubchannel~
Pigure 12 is a block diagram of one embodiment of a typical - 8ub~cr$ber transponder.
F~gure 13 1~ a block diagram illustrating one way in whic~
an antenna duplexer may be constructed to permit sh~ring of an esi6ting TY antenna between the transponder and the existing television receiver.
Fifure 14 is a block diagram of a radio central office.
Figure lS ~ a block diagram of one embodiment cf the central rece~ver~
Figure 16 i a block diagram of one embodiment Gf the digital interface circuit~ section of a central receiver.
Figure 17 is a block diagram of the invention a~ applied to provide two-way cable televisionO
Figure 18 i8 a bloch diagra~ of t~e invention as applied to automatic vehicle location, including ~ digitized ~slow voice~
up-link~
Figure 19 illustrates the co-channel down-link digital video transmission techniqueO
Figure 20 illustrates the application to ce~lular radio.

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1 3 ~ ~ 3 r ~ 2 1 Detailed Description Q~ $h~ ~nvention Reference now should be made to the drawing~ in wbich the same reference numbers are used throughout the ~arious ~igures to designate the 6ame or ~milar components~ -~
Pigure 1 illustrates the majo~ components of an entire ~ystem of the ~vent~on for three appl~catlons serving a large number of: mobile transponder~, ~adio modems ~~ed ~n conjunction with per60nal computers (~C), and two-way interactive television ~iewers having remote hand-held control means. ~he system of figure 1 i8 $ntended to provide communications facilities for a plural~ty of host computers 4 who provide information to subscribers, or ~o that one or more ~osts, acting as switch centers, may establish what are sometimes called virtual circuits that enable subscr~bers to commun~cate with each other. The princlpal device used by subcribers to communicate o~l the system ~ 9f this invention comprise receiver-transmitter devices usually referred to herein as ~tr~n~ponders~ but ~omet~mes c~llled ~radio modem~ or ~RP modems~ whe~ used with personal computers.
Referrlng to figure 1, the network control center 2 employs conventional computer hardware, ~oftware and trunk llnes 26 to rece~va, tempo~arily store, roùte, ~nd ~orward digital messages between the host computer~ 4, the broadcast station ~nterface unit 8, and radio central offices 6. For example, a subcriber at a fixed location 15 sitting at personal computer 20 may c ~nicate digital information packets through radio modem 14 via radio signals transmitted throug~ antenna 12 to a centrally located antenna 28 and radio central office 6 which detects and reformats these messages into ~tandard packets and forwards them ~ --13393~2 1 to network control center 2. The network control center reads the destination address portion of these packets and forwards them to the appropriate host computer 4, which is one of a plurality of ho~ts. If a reply is required, the host computer 4 ~enerates the reply message and sends it to the network control center ~ where it i8 reformatted and placed into a transmission queue where, at the appropriate time, broadcast statlon ~nterface unit ~ transmits it over TV transmitter station 10 where the message is radiated o~er the air and detected by antenna 12, demodulated by radio modem lq and sent t~ personal computer 20 to complete the message loop.
The transponder device is functionally tbe same whether it is incorporated within 8 plurality of mobile subscriter packages 25, radio modems 14, or integrated in interactive televisions 16.
Device 2~ could be a portable computer terminal or simply a "Two-Way Pager~ which has the added benefi~ of being able to acknowledge "beeps~ t or even send and receive alpha-rumeric messages, If the subscr~ber transponder is integrated within television 16 then i may be conveniently operated tbrough a remote hand-held device 18, which could communicate with televi~on 16 using conventional wireless techniques, such as infrared signalling, thus providing ~Interactive TV~ The antennas 28 employed with radio central offices 6 may be directional in design so that each receives only from a specified direction and thereby partitions the service area into pie shaped sectors~ The communication paths 26 connecting t~e various components of the common central equipment comprise conventional communication trunk lines such as microwave links, dedicated phone lines, or other suitable means.

'~ ' 1 3 3 9 3 r~ 2 1 Figure 2 ~ 8 a pictorial illustration of the lnvent~on using a plurality of directional antennas to cover a city ~rea from a radio central office 6 ~nd a receiver substatioA 7, which ~ubstation is es~entially the ~ame as 6 but displaced from it BO
5 n8 to estend coverage $nto areas that may not be ~cce~sable to radio central office 6 because of mountains or other obstructions. Figure 2 al80 illustrates the definition of a radio cell which, for purposes of this ~pecification, i8 considered to compri6e a geographic area defined by t~e beamwidth of each antenna 28 and a range gate interval such as ~he di~tanc~
between ~2 and R3. It will be pointed out in subseguent discussion that the distance between R2 ~nd R3 is pro~ortional to the propagation distance covered during the subscribec transponders pulse-width which is on the order of flv~e microsecond~ in a preferred embodimentO Conseguently the distance between R2 and ~3 is on the order of ~ mile.
Figure 2 al~o illustrates how network control center 2 may be provided with intercity communication means through u~e of satellite commnication link 30. Messages may be communicated ~o between cities by these and other well known methodsO
Figure 3 is intended to facilitate the explanation of essential features of the invention. For purposes of illustration, operation of the invention is considered analogous to the operation of a radar system. A radar ~ystem typically trans~its brief radio pulses which lmpinge on ~targets~ in its transmission path that reflect back pulse energy ~called the echo in radar) that is detected at a receiving point after an elapsed time t. The elapsed time t is proportional to the pulse propagation distance to and from the reflecting targets and .~

~339322 1 consequently di6tance to targets tsubscribers) may be determined by measuring t. When directional antennas ~r~ employed to either send or receive ~ignals ~or both) then the direction to the target may ~180 be determined.
Referring to figure 3, TV transmitter 10 radiates ~
conventional telev$sion signal including horizontal sync pulses 31 and digital data which are detected by the transponder antenna ~2 and sent through antenna dupleser 32 to the receiv~r 34.
Rece$ver ~4 locks on to the TV signal and extracts from it the borizontal and ~ertlcal synchronizing pulses whlch ar~
~ubseguently employed to detect the T-NE~ digital down-link fiignal~ that are synchronized to and ~ccompany the TV signal nd al80 to coordinate the radio modem's reply pul~e tranEtmission time slots 80 reply pulses exist only in the ~BI. The digitsl information and synchronizing pulses are connected to microprocessor 36 where the address portion of the meEsage packet i8 examined to determine i~ it is a signal intended for that specific subscriber. If it i8~ it i8 forwarded to the! c: _~n; on personal computer ~0. The link between microproces~or 36 and computer 20 could employ the well known RS-232 standardO
Computer 20 digests that information and if a seply is necessary it Will generate it ~nd transmit it back to microprocessor'36.
where it i8 temporarily buffer-stored and prepared for transmission at appropriate time slots using transmitter 38.
Transmitter 38 generates an RE pulse in synchronization with the horizontal ~ync signal time slot received from microprocessor 36 and transmits that pulse through duplexer 32 and antenna 12 back to the central receiver antenna 28. Antenna 28 may be one of a ~r 133~322 1 multiplicity of direct~onal ~nt~nn~ to provide the desired city coverage. Antenna 28 is connected to the radio central office 6 where the up-l$n~ information ~8 detected, reformatted, and sent to the-network control center 2 ill~strated in ~igure 1- Another s antenna 27 at the radio central office detects sync signals from telev~sion 6tation 10 and connects them to the radio central off$ce where they are employed to in$tiate the desired timing processes based on the TV signal's horizontal and vert~cal ~ynchronizing pulses.
Referrin~ again to figure 3, the ~folded A scopeU receiver monltor shown in the center of the ~llustration is intended to ~acil~tate the description o~ the T~NET system operatlon ln analogy to ~ radar ~ystem. ~A~ scopes are c~ .~ly u~ed ln radar to display range to various targets. In theEe examples the trace ~8 ~folded~ into many lines. The ~A~ ~cope monitor 6hows a Eeries of horizontal line sweeps ~like a TV raster scan) each of ~ich starts when it is triggered by the horizontal sync pulse transmitted by ~V ~tation 10. This $s called the start pulse in that illustration and a short time later an echo pul~e! from a 29 6ubscriber appears~ ~he time duration t between the start pulse and the echo is indicative of the range to t~a~ subscriber.
The first line of the folded ~A~ 6cope is called 81, the second 82, and so on through 852 for a modulo 52 system. Since each line is triggered by 8-sync pulses from TV station 10, each ~ine has a duration of 63.555 microseconds in the U.S. TV
standard. The length of each line thus corresponds to a distance of ~bout 6 miles. This folded ~A~ scope monitor may also be v~ewed as eguivalent to a TV screen rastor 6can which sweeps out 52 lines then repeats lt~elfL

- 133~322 l In r~dar j~rgon each horizontal llne is called a ~A~ scan but ~n th$s illustr~tion, having many borizontal sweeps, we refer to it as a folded ~A- wope. The folded A scope monitor shown ln figure 3 could be employed within a radio central office s 6 for the purpose of monitoring the rad~o s1gnal activlty, or p~rhaps for techn~cal evaluation or trouble shooting and to sbow at a glance the strength of, ana r~nge to, various ~ubscribers.
In actual practice the detection, storing, ~nd routing of si~nals 18 all done ~utomatically by computers and such a d~sp1ay would not be regu$red for those funct$ons.
Figure 4 i5 a top view of one angular sector of a radlo central office ~ervice area. Antenna 28 provide~ reception of 6ignals in an angular sector ~pproximately 18 degrees ~n width and that sector is further partitioned into range cells numbered from l through 6 in the first range interval9 and similarly in the Eecond and third range interval. Each of these range cells is one mile 1n length and this corresponds to a time duration of S microsecohds~ which duration i5 al~o the width of each transponder reply pulfie.
It was pointed out earlier in this specification that the unambiguous range of the ~-NET sy~tem i8 proportional to the time duration between TV horizontal ~ync pulses and this turns out to be 63.555 microseconds, about 6 milesO This i8 called the first range interval. The ~econd range interval, also numbered l through 6 extends from 6 to 12 miles and the third range interval extends from 12 to 18 miles. Obviously, the number of range lntervals required depends on the ~ize of the city.

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t,--~ r 1339322 ' - --1 Since each ~V signal horizontal ~ync pulse i~ n~mhered from 1 to 525 starting ~rom the first vertical sync pulse which defines a TV frame, lt is clear t~at the central receiver 6 as well a8 each subscriber transponder, i~ each capable of s unambiguously counting and keeping track of all 525 horizontal sync pulse number~. Consequently it i~ clear that a software algorithm can be devised to remove any ambiguity that might e~lst as to w~ether a subscriber lives in range interval 1~ 2, 3, etc., and which ~pecific ~-sync pulse they have been a~igned to reply~
on.
- It was also pointed out earlier that about 10,000 subscribers could operate ~imultaneou~ly at 300 baud each within one ~ix-mile range intervalO That maximum number of subscribers would remain the ~ame even though more range intervals might be employed to yleld a 12, 18, or 24 miles service area. ~he number of subscriber~ could be increased, however, by using more angular sectors or more subchannels. For example, if 18-degree beamwidth -antennas are u~ed to cover a 360-degree are~, then ~ total of 20 antennas would result and this would service close to 200,000 subscribers simultaneously. ~owever, in a practical world the ~ubscriber~ are not uniformly distributed throughout a service area because of terrain and service boundaries, therefore less than the optimum number of subscribers could be serviced simultaneously in a practical system. Obviously a much greater number could be erved on a time-shared basis because each subscriber typically use the system only momentarily for a few minutes per day.

~339322 1 Figure 5 illustrates in a series of micro~econd time-steps t~e manner in which subscriber transponder pul~es are masked ~i.e. rendered invi~ible to TV viewers) by the horizontal bl~nking interval of the Bost telev$sion signal. The television s ~tation 18 as~umed to be at the left sLde of figure 5 and its signal is assumed to propagate from lef~ to right. When the ~ost television signal ~-~ync pulse lmpLnge~ on the subscriber's television antenn~, it triggers the generation of a tsansponder reply pul6e techo)~ the leading edge of t~is reply pu.l~e is illustrated in flgure 5a as a 6traight vertical lLne beginning at the leadLng edge of the ll mLcrosecond square pulse i,~beled ~B8I~. All this occuro at initlal ti~- t.
P$gure ~b Lllu~tr~tes the almo6t fully developed reply pulse at a time t + 4.5 m~croseconds. The subscriber' 8 repLy pulse 18 shown in diagonally shaded lines ~80 long as it is under the ~BI
pulse) and it is seen propagating both to.the left anl1 to the right, upstream and downstream, respectively. Since :r~dio waves travel at approYimately a thousand feet per microseco~ld~ the subscriber~s reply pulse would hqve propagated approx:Lmately 0.9 miles within 4~5 micro~econds and, if this illustratLon was 6een from the top view, one would see that the 6ubscriber~ reply pulse would represent a circle 1.8 miles in diameter centered on the 6ubscriber's antenna. The sguare pulse labeled ~BI above the ~haded ~ubscriber's pulse is the Bost TV horizontal bl~nkin~
~nterval and it is seen to propagate to the right at the same speed as the subscriber's pulse and all of the energy of the subscriber's pulse traveling that direction is seen to exist within the horizontal blanking interval. ~t will always do this in the downstream direction~

, ~ 33~322 1 Figure 5c illustrates the time waveforms as they exist at t ~ 8 microseconds. Since the subscriber's pulse i6 only 5 microseconds wlde it i~ seen that the subscriber' 8 transponder has cea6ed transm$tting and the reply pulse trail~ng edge has left the sub6criber's antenna and $8 now propagating ln all directions. If this illustration were viewed fro- the top, the reply pulse would appear as a doughnut with an outside diameter of about 3.2 m$1es and an inner diameter (hole) sllghtly over 1 mile in diameter, all centered on the subscrlber's antenna. Note the important point that waves propagating to the right ~downstream) st~ll exist underneath the horizontal bl~k~n~
interval of the ~ost TV signal but the waves traveling to the left ~upstream toward the TV ~tation) are no longer D~asked by the EBI5 in other words, TV viewers who live upstream mo~e than about 1.2 miles from the subscriber are not simultaneously blanked-out and they could, if the subscriber's signal was stroncl enough, see the subscriber's response pulses. We will point out shortly that the 6ubscriber'6 pulse ic quite weak by the time it reaches that distance~
' Figure 5d shows the waveforms which exist at t ~ 18 micro6econds at which point the subscriber's pulse waveform which propagates down6tream, ~hown in cross-hatched shadiny, ~till remains under the ~BI and is thus masked, but the pulse propagating upstream (to the left) shown without cross hatching, Z5 is about 3.4 miles upstream and, since it i6 out of the ~BI, could be seen by TY viewers, if it were strong enough.
Figure 6 shows graphically a plot of the signal strength of the subscriber transmission pulses as a function of distance from 1339~22 1 the subscriber. It is well known in radio wave propagation theory that ~n ~ree space the electric field $ntensity of a propagating rsdio wave falls linearly $n proportion to the propagation di6tance. The power ~n that wave falls as the sguare of the distance and i~ plotted in f$gure 6~ the radio wave power drops very rapidly in the first few hundred feet after leaving the radiating antenna and more slowly thereafter. By the time the radio waves reach a d$stance of approxlmately 500 feet they have fallen in magnitude by about ?~ dB. The point of figure 6 is to lo ~how that the strength of subscriber7s transmitting pulses drop 80 rapldly in the f$rst few hundred feet ~o a levcl ~hich ~ec~ ~8 insignificant in comp2rison to the strength o~ TV ra~io waves ~nd thus would not ~nterfere with television ~ignal~.
Consequently television viewers watching progra~s sent by the ~ost television station must be protected only if they live within a few hundred feet of the subscriber' 8 transpcnder antenna because this i8 where the transponder signal is strong and potentially capable of inter~ering witb the telev~sion program.
Fortunately, as shown in flgure 5 illustrations1 televi ion viewers living witbin a few hundred feet of the transponder are ~imultaneously blanked out by the horizontal blanking interval of the ~ost television signal and conseguently even though the subscriber's transponder signal is relativ~ly strong and potentially capable of interfering with adjacent chAnnel television viewer8, all those viewers' television receivers are blanked out and cannot see any video program at that instant.
By the ti~e the subscriber' 8 signal propagates to a distance outside the horizontal blanking interval it is about 90 ds weaker (i.e. about 1 billion ti~es ~eaker~ and will not interfere with ~,~

~3~22 -1 the ~ost televl6ion' 6 signal. Furthermore, since the ~ubscriber's transponder operate6 on a c~nnel adjacent to the ~ost television signal, lt i8 6uppre~sed further by radio freguency filters wh~ch are tuned to the ~ost television signal r~ther than the transponders signal and this suppression typically ~mounts to 35 dB or more~ In other words, a television receiver suppresses ad~acent channel ~gnals by about 35 d8 or more. The combined effect of the signal propagation ~ttenuation shown in f~gure 6 and the attenuation due to the televl~ion lo receiver8 tuned c$rcuits total over 115 dB. Tberefore, for all practical purposes the subscriber's transponder ~ignal cannot lnterfere with television viewers tuned to the Bost telev~sion ~Ignal.
In a practical environment (not ~ree space~ a~ plotted in figure 6~ the subscriber's transponder pulse attenuates even more rapidly and so the signal attenuation is even greater than 115 dB. TV viewers tuned to channel~ further away than the first adiacent channel ~uppress transponder pulses 59 dB ~r more because of their tuned circuits and they too are unef~ected as has been demonstrated in many field trials~
Figure 7 is a top view illustrating the ~BI masking geometry and It corre6ponds to tbe side view shown in figure 5. The horizontal blanking pulse 31 is shown propagating outward as a circular wave centered on TV station 10. The cross-hatched area shown around 6ub6criber 15 is the area which Is masked by the Host TV horizontal blanking interval (~BI) and all television viewers living within that cross-hatched area would not be able to 6ee the 6ubscriber' 8 transponder pulses because the screen of ' 1339372 1 tbeir television ~eceivfr ~ ~lanked out by the EBI at that - --moment. On the other hand, television viewers l$v$nq to the left of the ~ubscriber tn the area whicb is not cross-hatched would not be protected by the EBI masklng but would~ on the other hand, 5 be protected by the very low subscr$ber s$gnal ~trength wh$ch has already been discus6ed ln relation to figure 6.
We have thus shown that transm$ss$ons from subscriber transponders would not cause interference to television v$ewers looking at the ~08t televlsion signal ~ecau6e they are e$tber blanked out by its horizontal blank$ng interva~ if they l$vè
clo6e to the subscr$ber, or they are too weak to ~nterfere w$th the TV 6ignal if they l$ve outs~de the hor$zontal bl~n~ing ~nterval since they would then be at least a m$1e away.
There 1s another potent$~1 concern however and that i8 the '~
question of whether the subscriber's transponder signal may ~omehow interfere with certain telev$s$on receiver functions wh$ch mu6t be accompl~hed during the horizontal bl~nki~
lnterval. We shall now address that pointO
Figure 8 ~how~ a standard television waveform defined by the NTSC (National Televi~ion Standard Committee) during the hor$zontal blanking interval. T$me $s assumed to start at the left and increases to the right. ~hus the fir6t feature one sees after the start of t~e ~ i8 the front porch just preceding the horizontal sync pul~e. That front porch is used to define a reference for the so-called black level; signal~ weaker than that level lie within the visible range of the TV screen and ~ignals ~tronger than that level are black and cannot be seen. Thus the hor$zontal 6ync pulse, which is ~tronger than that reference level, cannot be seen. Since the sub6criber's tran6ponder ~3393~2 1 commences tran6mission with the begi~ning of the borizontal ~ync pulse tt is clear that it cannot interfere with tbe u~e of t~at reference black level becau6e it exists prior to the beginning of the horizontal 6ync pulse. The TV horizontal sync pul~e it~elf triggers the subscriber's transponder as well as all of the clrcuits needed by television receivers at the subscriber's home sr in,its neighborhood. Thus a subscriber transponder Qignal sccur$ng at this point would only appear like a regular horizontal 6ync pulse and it does not ~ntefere with the proper horizontal ~ync of any television receivers in its vlcinity.
Following the horizontal ~ync pulse (figurc ~ 8 ~chroma-burst~ waveform is transmitted on the back porch in the case of color,TV signals. That chrsma-burst represents apprcximately 8 cycles of a chroma subcarrier oscillator operating at a freguency of about 3.57 M~z and its purpose is to synchronize a crystal-controlled oscillator within each television receiver which i8 used to demodulate the color signalsO Interference with that process could cause degradation in the color balance of color TV
progra~s. Television receivers univer~ally employ a cry~tal-controlled chroma oscillator tightly locked to that chroma-burst and thi6 acts as a very ~harply tuned filter. The filter is in fact 80 narrow in bandwidth that the broadband energy density of the sub6criber'~ transponder pulse used in this invention has minimal effect upon it. In other words, the spectral power 2s density (watts per hertz) represented by the subscriber's transponder pulses as used in this invention is of such low value that the very small amount of energy which does exist within the very narrow TV chroma bandpass filter of television receivers is ~ 3~322 1 1nsufficient to interfere with it. Numerous experlment~
conducted by the inventor have ~hown that the transmissions contemplated in this specification have no ef~ect on the color guality of televi6ion programs, even if tran6ponders and IV
s receivers ~hare the ~ame antenna.
We have thus shown why transmi~sions by ~ubscrlber transponders designed in the manner cet forth herein will have no deleterious effect on television viewers in the neighborhood of the ~ubscriber or elsewhere, even lf they share the same antenna a~ the 8ubscriber's own television receiver.
One may ~uestion~ however, how weak the ~gnal m~1~t be in order to not inter~ere with television signals. Those ~fi~ues and related specificatlon~ are determined by the Federal Communications Commission in the Unlted States and by similar agencies in other countries. At the present time, the FCC has stipulated that an adjacent channel ~ignal must be equal to, or weaker than, a televi~ion signal 80 as not to interfere with it.
8tated another way1 the only protection afforded to tl~at television 6$gnal is the protection provided by the ~r receiver tuned filter~ which, a~ stated before, represents about a 35 dB
or more adjacent ch~nnel suppre~sionO On the other hand, if a potentially interfering ~ignal lies within the same ch~nnel as the t~.evision signal, then it must be weaker than the television signal by at least 50 dB under existing FCC rules and only 40 dB
under proposed new rules. Using existing FCC rule~ as a criteria, the inventor has found that a subscriber transponder may use a pulse power of approximately 2 watts peak and average power of a few milliwatts to meet tbe FCC criteria and this i8 also sufficient to provide a useable ~ignal to a T-~ET radio .~

3 ~ 2 1 central office at di~tance esceeding 20 miles. Battery operated transponders appear practical because of the low ~verage power of these subscriber transponders.
Thus far we have described the operation of the up-link from subscriber to radio central office. We have pointed out why sub6cr$ber tran~missions do not ln~erfere with televislon viewers. We have also pointed out how the horizontal and vertical ~ynchron$zing pulses of the ~ost televi~ion signal coordinate the subscriber transponder transmissions and permit many subscribers to be multiplexed on different hor~zontal lines of the TV frame. We shall now de~cribe the t~eory and spec~fic advantages of the new and improved down-link from the ~ost TY
station to subscribers.
Figure 9a shows the spectrum o~ the 80st televi~;ion signal and the 32 new down-link subcarriers of thi~ invention. In this illustration, they are shown to exist in the ~ost TV station's lower adjacent channel. It is well known that television signals employ what is referred to as upper sinqle sideband ISSB) modulation with a small vestigial lower ~ideband. The ~ost television signal carrier freguency Fc is shown at the left side of the TV 6 M~z signal channel and most of the video energy i~
shown in the upper sideband. That signal energy comprises the video picture information, a frequency modulated audio carrier at 4.5 M~z above Fc and a color subcarrier at 3.57 M~z. There are also additional subsidiary carriers (SCA's) which could exist within the TV channel for the purpose of providing stereophonic sound transmission and second audio programs but they are not shown in figure 9A. The lower edge of the TV channel is 1.25 MHz ~33~322 l below the v$deo carrier Fc. Frequencies lower than 1.25 below Fc are consldered to be ln tbe ne~t TV channel, which is referred to a8 the lower adjacent channel. As noted before, this has always been vacan~. It iB in this lower ad~acent e~Anne1 ~here tbe 32 S ~ubcarriers of the in~tant invention are positioned (they could al60 use upper adjacent channel~. The bandwidth of each of these 32 subcarriers l~ approximately 187.5 R~z and each of them i8 wlde enough to carry independent up-llnk pul~e 6ignalfi from ~ubscriber transponders a8 well a8 separate down link ~ignals to 6ubscribers a8 will now be described~
Figure 9b illustrates how ~ digital bits can be ~odulated within the time interval of a horizontal blanking intcrval which is approximately ll microsecond~ long (about 2.8 micro~econds per blt). Each of the subcarriers ~hown in figure 9b are gated 80 that they exist only during the ~BI and consequently 1:hey will not interfere with the video portion of the ~ost television program. Within this ~BI $nterval 4 bits of information are modulated on each 6ubcarrierO There are v~riou~ modulation methods which may be employed to ~ccomplish t~is. One preferred method is a phase modulation technique wherein the phase of the gated ~ubcarrier i6 advanced 90 degrees and brought b~ck to its 6tarting phase within l bit interval when ever-a logic ~l~ is to be tran~mitted. If more logic ~1~8~ are to be 6ent in succe6sion, the phase direction is rever~ed after each ~l~ bit~ that i6 the wave is advanced 90 degrees and brought back~to a starting phsse within l bit interval ~nd then retarded 90 degree~ and brought back to its starting phase within the 2nd bit interval. Thi6 i8 done in a sine wave fashion so as to restrain the 6ignal spectrum as much as possible to keep mo6t of its energy within its ,, 133~22 1 a6signed subchAnnel bandwidth. I~ a logic "on ils to be ~ent no phase advance or retardatlon will occur- ~his process iD 8hOwn ,,~
in figure 9b which illustr~Ates a 4-bit 6eguence as 1011. Since 4 ~~
bits are transmitted during each HBI for each subcarrier, and since the HBIls occur at lS,734 Hz, thls result~ in a down-lin~
data r~Ate of 62,936 bps per 8~A,hA,Anel. Each s~hA~h~n~el could carry lnformation indep~n~sntly.
Quadrature Amplitud~ Modulatlon (QAM). An altern-tlve modulatio~ method i8 disclosed in flA;ure 9c which ~pl~t~ ~ach 6ubcarrier into guadrature c~onents and ~ach Or thes~
~o, porAnt~ i6 indepen~ently modulated to provide a more narrow transmitted spectrum and therOby 1nl iz-~ ~nterferen¢e to ad~acent 6ubchAnnel signals~ It will be clear to those skilled in the art that 6aid quadrature method could employ e~ther binary (on-off~ modulation o~ each guadrature term, or each term could .
take on multiple value~ (e.g. quadrature ~mplitude modulation:
~AM) to define multiple symbol~ for greater data rates per assigned subchAnnel.
Two different equipment arrangements for transmitting the down-link subcarriers will now be described. Figure lOa show6 one method in wh~ch a subcarrier oscillator 40 followed by a frequency multiplier 42 generates the desired subcarrier radio frequéncy which is modulated by data in 44, amplified in 46 and radiated by antenna 48. Thi~ i6 one of thirty-two subcarriers tuned to exist within the lower TV ad~acent chAnn~l a5 shown in figure 9a. Those 6ubcarriers are al60 gated to exist for only 11 m~crosecon~ in the HBI. Figure lOa shows an assembly of thirty-two such subcarrier~ generators. The ou~ of all these 6ubcarriers can be summed together in S0 and amplified by 13~322 1 amplifier 46 and radiated through antenna 48. Antenna 4B and indeed the entire assembly of figure lOa, could be independent and distinct from the Rost television transmitter/an~enna~ This particular method has the advantage that antenna 48 could be a dlrect~onal antenina. Several subcarrier assemblies, eacb identical to figure lOa, ~nd their associated artellnas 48 could be provided to generate down-link tran~missions into other angular sectors to thereby cover an entire city.
Figure lOb and lOc show two methods for t~ansmitting down-link digital data. The RF subcarrier assembly o~ figure lOb,being already at the proper radio frequency, i8 ~Added~ to the regular TV video carrier in 54 and radiates through the TV
transmitter antenna. This is a method to piggyback T-NET data signals on an existing Host TV transmitter without interference because the T-NET subcarriers exist on an adjacent channel as t previously explained. The Host TV transmitter may hzve to be retuned somewhat to permit this~ however~ In figure lOc the subcarriers are generated at baseband freguencies ancl they in turn modulate the existing TV carrier ~n SSB modulators 11: the data carriers on the low sideband and video on the upper sideband (with slight vestigial low sideband)a The alternative quadrature modulation method for impressing four bits of data on each subcarrier during each horizontal blanking interval for down-link data transmlssion will now be described. The gated subcarrier oscillator 40 (figure 9c) is split into two quadrature components and each of these components is modulated with two bits of data during each ~BI. This is in contrast to the method described above wherein a single subcarrier component is modulated with four bits of data during 1 the ~BI. The method of using two quadrature ~ubcarr~er terms ~8 attractive from the standpoint of ~ni ~zing the required radio ~pectrum bandwidth. It is also very attractive because low-cost large integrated circuits ~called IC'~ or ~chips-) now esist for s ~color~ television receivers that incorporate within them all of the circuits necessary to demodulate the chroma sr~carrier and these can be adapted to demodulate the data subcarrier instead, a8 well ~8 to detect the horizontal and vertical sync pulses and necessary control s~gnals (AFT & AGC)o The manner ~n which the down-link 6ubcarriers are quadrature ~odulated at the down-link transmitter end is relat~vely straight forward. ~eferr~ng to figure 9c, the output of subcarrier oscillator ~0 is split into two guadrature components by shifting one signal path 90 degrees in phase shifter 41. ~he ln-pha~- and guadrature signal is then amplitude modulated independently by ~3 with 2 bits of data during each horizontal blanking interval. Of course subcarrier oscillator 40 itself only exist during the horizontal blanking interval (eleven micrcseconds) as explained before. Thirty-two oscillators identical to figure 9c could be provided for each antenna beam sector as previously e~plained.
It has been pointed out earlier in this specification that it is possible to use the same suhch~nnel to tr~nsmit up-link as well as down-link, even at the same time. The use of separate subchannels for either up-link or down-link transmission is fairly obvious. ~owever, to understand the use of a single subchannel for both up-link and down-link transmission at the same time requires some explanation. It has already been pointed ~ ~3~3~2 1 out that the gated subcarriers used ln the down-link e~ist only ~or ~pproximately 11 ~icroseconds coinciding with each ~ost TV
horizontal EBI. Tbe time between sync pulses is 63.555 microseconds and conseguently the down-link subcarriers e~ist for only 17.3~ of the total time (11/63.55 - .173). Bence the down-link sobchannel is actually ~off~ and unused 82.7~ of the time.
As noted before, the EBI time interval containing down-link subcarrier~ propagate~ away from the television station with the ~peed of light and 6weeps across the countryside to the maximum 0 ~xtent of the sy~tem's serv$ce area and beyond as shown in f~gure Since these gated subcarrlers are o~ly on for 17.3~ of the time this leaves U8 with approx~mately 82.7~ of the time fr-e to listen for reply ~echos~. These echos are in fact di~ital up-link data a8 pointed out in the prior discuss~on. In ordor to~hare subcha~els for simultaneous up-link and down-link transmissions one must be careful to permit only cert~in fixed ubscriber locations to operate in this manner 80 as ~IOt to cause the receipt o$ a reply pulse from prohibited location~; (figure 11) at the 6ame instant a down-link subcarrier transmission is occuring. It would be difficult, if not impossible, to detect the weak ~echos~ from prohibited locations which arrive at the same til!le as one is generating a strong down-link transmission.
These prohibited areas are ~hown in figure 11 as cros~-hatched 2s annular rings. The width of these rings, ~bout one mile, represents approximately 17.3~ of the total service range and occur every fi~e miles. Subscribers located within these ~rohibited rings would not be able to use the same subchannel for simultaneous transmission and reception, however, those ' 133~3~2 l 6ubscribers ln pro~ibited areas could use a different ~ubchannel for tr~ 6ion ~nd reception ~8 i8 customary ~n radio transmission. Alternatively, a receiver substation 7 (figure 2) could be positioned ~down-stream~ ~o as to effect~vely move its 5 proh~bited areas away from those of radio central office 6 and thereby provide continuous coverage.
Sy~ergetic Modul~tion. A point of novelty ln the instant invention should now be explained. It was po~nted out in the discussion relating to figure 9 that thirty-two 6ubcarriers are posltioned in the lower channel adjacent to the ~ost ~V stat$on ~ignal lor alternat~vely on the upper adjacent channe~l). These ~ubcarrier~ will in fact appear to the T-NET transpon~er receiver (figure 12b) as if they were lower ~idebands of TV ca~:rier FC, even though they may have been independently generat-d, and -ven though they ma~ be transmitted from a different location than the TV transmitter. Another way of explaining this is to point out that the ~beat~ frequencies which result when both th~-~iubcarriers and the ~Y main carrier Fc exis~ within the band~ss of the transponder receiver 34 (figure 12b) and are pr.ocessed by detector ~8; the result COmpri~ieS envelop-modulation, comparable to SSB modulation of carrier Fc by the ~ubcarriers. Thi~
envelope is demod~lated by detector 88 as explained shortly.
Since this process of effectively appending sidebands to an r'' existing signal ~i.e. the TV carrier) ~o exploit its carrier energy and/or 60me of the modulation which it already carries ~e.g ~ ~ V 6ync signals~ appears to be a unique concept, it bas consequently been labeled ~synergetic modulation~. Synergetic modulation ~s herein defined as follows: The creation of psuedo radio sidebands on an e~isting radio signal by means independent ~339322 1 of the generator of that signal whereln said means are located at the ~ame or a remote location to thereby enhznce the reliability of the psuedo ~ideband transmissions ~nd mini i2e mutual interference ~9~lLl9D~ ~esign Q~t~on~. ~pecific T~NET e~uipment and s sy~tem con~igurat$ons will now be deficribed in detall. It will become evident to 6killed communication workers that many variations of the basic T-NET ~ystem design concept ~an be implemented for var~ous applications and conseguently the following circuits and related illutratlons represent only one preferred embodiment.
Tr~n~ponder/Tr~nsm~er. Figure 12 shows t~e RF sub~ections of a typical radio modem (or transponder) wh~ch may be employed in the instant ~nvention. Figure 12A, the tran~mittel ection, is a relatively conventional radio transmitter design employing a fixed reference crystal 05cillator 60 and a suhc~Anne~ frequency ~ynthesizer comprising phase detector 62, low-pass filter 64, var$able oscillator 66 and programmable divider 74s all of these being combined ln A clrcuit commonly called a phase-lock loop (PLL). The programmable divider 74 is controllable by t~e microprocessor 36 previously shown ln figure 3. Thus the suhc~n~el frequency of the transponder is controllable by that microprocessor and it in turn may be controlled by the remote network control center 2 lfigure 1) 80 as to assign transponders to different subchannel frequencies dynamically at different 2s times to optimize overall system traffic management~
The output of variable oscillator 66 is amplified and frequency multiplied in 68 and pulse modulated in 70 by cosine ~quared modulator 76. Modulator 76 is in fact a waveform ,~ , ~3~322 1 generator that provides a pulse waveform having a smooth attack an~ decay shape (e.g. cosine squared) and this is done to opti~ize the spectral content of the transmitted pulses 80 that most of their radio energy falls within the des~red subchannel bandwidth. Alternatively~ the output of 68 can be split into quadrature terms, each term being modulated with one bit per HBI
(equivalent to the dow~-link QAM method previously described).
The pulsed output o~ modulator 70 is further amplified in 72 to a level o~ approximately 2 watts peak and connected through duplexer 32 to antenna 12 where it is radiated. Transmitted pulses are approximately 5 microseconds wide and the duty cycl-is very low~ the resultin~ average power of the transmitt-r ls about 1.5 milliwatts at 300 baud. This is a very lo~ average power and is therefore attractive for battery-powerecl operation.
Transponder/Receiver. Tbe ~ransponder's receiver subsection is shown in figure 12b. It is intended to employ corlventional lntegrated circuits designed for mass produced "blac~ & white~
television receivers and consequently uses relatively inexpensive and reliable piece parts. An alternative~ using ~color~ TV
circuits, is discussed later. Down-l~nk signals are intercepted by antenna 12 and are connected to TV tuner 80 through duplexer 32. Theze signals are amplified in 82 and sent through intermediate frequency (IF) bandpass filter/amplifier (BPP) assembly 84 and connected to TV receiver integrated circuit chip 86. Receiver 86 feeds back a control signal 87 to TV tuner 80 to provide automatic frequency tuning (AFT). These are all conventional TV components; for example, 84 could include ceramic 1 IF filters used in TV ~eceivers. Detector 88 demodulate~ the down-l~n~ s~gnal6 and removes ~he RF carrier to provide the TV
~ync and subcarrier b~eb~nd signal~ to both low pas~ f$1ter 90 and h$gh pa~s filter 92 connected in parallel.
S The intermediate frequency ~IF) tuned circuits of the receiver in f~gur!e 12~ are tuned ~o a8 to encompass all th$rty-two T-NE~ subcarri-r- a- well as th- televl-~on carrl-r FC.
8ince the television slgnal lncludes lower vestigla~ sidebands below fc, they are included w$thin the bandwidth of the receiver lo and are demodulated. Consequently, the output signals ~rom detector 88 include all of the th~rty-two subcarriers as well as most of the Host TV horizontal and vertical synchronizing pulse energy because that synchronizing pulse energy exists in the lower freguency components of the TV Jignal and pa~ through lS low pass filter 90. It consequently appears at the output of filter 90 as the ~ and V sync shown in figure 12Bo On the other hand, the data subcarriers exist between 1.25 and 7.~5 M~z below the TV carrier Fc and they are filtered out by high pass filter 92 and sent to mixer 94 where a phase-lock loop arrangement provides for the selection and demodulation of only one of the thirty-two subcarriers. ~hat pha6e-lock loop operates as follows. Freguency synthesizer 98, dynamically controlled by microprocessor 36 (figure 3), selects which of the thirty-two subcarriers will be demodulatedO Frequency synthesizer 98 may be controlled by either companion device~ such a5 a personal computer in the case where the receiver is inside an RF modem, or by the ~ystem network control center (NCC) $n the ~ame manner as it can control the progammable divider 74 of the transmitter 133~322 1 6ect~on. In any event, frequency synthesizer 98 controls voltage controlled oscillator 96 to set it at a ~pec~fic ~reguency precisely equal to the subcarrier frequency which i~ to be demodulated; this process occurs in mixer 9~, low pass filter s 108, amplifier 110, freguency control varactor 102, and crystal 06cillator 100. Their operation is identical to tbe operation of a common pha6e-lock loop (PLL) which ~8 well known. The result i8 that VCO 96 is kept precisely in tune with, and precisely in phase-lock with the average phase of the ~ubcar~ier which is to be demodulated. Phase fluctuations in the selected subcarrier will be smoothed out by low pa~s filter 108. However, fast phase fluctuations, which w~ll repre~ent the desir-d pha-e modulat-d dig$tal data, are passed through low pa88 filter 104 and amplifier 106 and are sent to the microproces~or 36 ~shown in figure 3). It will be recalled that the subcarriers are each gated to exist for only 11 microseconds and coincide with the ~BI
of the television signal~ Within this ~BI interval four bits of -~~
information is phase-modulated in the manner previously described in connection with figure 9. Thus the output of the receiver of figure 12B provides the ~ and V sync pulses of the ~ost TV signal a~ well as the down-link digital data in any one of the thirty-two subcarriers of the down-link subsystemO
The strength of the Host TV ~-sync pulses coming out of f~lter 90 is indicative of the radio path attenuation between the ~ost TV and the subscriber. It is therefore a measure of the power required in the return (up-link) path. Based on the principal of reciprocity, power level control 77 (figure 12) ., , .. . ,~ , ..

1,, .
~33~322 1 provides a control ~gnal to modulator 76 which cs~ablishes the des~rable output power level of the up-link transmitter 80 as not - -to radiate e~cessive power yet guarantee adequate levels.
~r~n~onder/O~dr~ture Rece~ver. Application of a TV colo~
~chip~ integrated circuit 81 of figure 12c to T-~ET transponders will be explained in the following discussion to illustrate practical economical design for detecting the alternative down-lin~ guadrature modulation (figure 9c) but it should be empha~ized that the essential feature of interest ~A this discussion has to do with the fact that this low-cost chip can be employed to demodulate both qu~drature terms of the ~-NET down-link subcarrier because it ~ppears like the TY quadra~ure modulated chroma signal.
A typical TV integrated circuit 81 includes an Il~
preamplifier 83 and IF amplifier 85 and detector 89 that are relatively conventional in design and include pro~is~on for ~utomatic ~requency tuning (AFT) circuit 87 and automatic gain control ~AGC) circuit ~1. It also include~ horizontal and vertical sync Eeparatisn and detection circuit~ 93 antl 95. It was pointed out earlier in this specification that ln ~.S. color televi~ion ~ystems the television signal is coded and transmitted as three components: a monochrome luminence component ~M~ and two color components ~I~ and ~Q~ that are superimposed on a ch~cma subcarrier having a flxed preci6e freguency of 3.579545 MHz.
That chroma subcarrier is quadrature modulated with the I and Q
color signals essentially in the same manner which can be employed for the alternative modulation of the T-NET gated subcarrier~ (figure 9c3.

~i ~3393~2 1 In tbe ca~e of color television transmission~, a brief ~chroma burst~ aynchronizing ~ignal i8 transmitted by the tele-vision transmitter tsee figure 8) on the ~back porch~ of e~ch hor~zontal ~ync pulse and its purpose i8 to pha~e-lock voltase s controlled oscillator 96 ~figure 12c~ with all televi~ion rece~vers. Therefore phase-locked oscillator 96 can be used a8 a continuous phase reference to demodulate the I and Q components of the transmitted color TV signal. In color TV receivers the chroma burst detector 103 accomplishes that 6ynchronizing proces~
lo by using a tlme gate derlved fro~ ~-sync d-tector 93 to gat- out.
the approximately 8 cycles of chroma burst~ tho-e 8 oycl-s ar-~pplied to pha6e-lock loop circuit 101 wh~ch controls~VC0 96 and thereby keeps lt precisely in phase with the 8-cycle chroma burst. A precisely tuned quartz crystal at 3.S79545 M~z i8 connected to terminal 105 and this causes phase-lock loop 101 and VC~ 96 to remain precisely in phase with the chroma bur~t 06cillation even a~ter the burst ceasesO VC0 96 in ef$ect ~coasts~ during the time interval between chroma bursts with neg~igible drift.
The manner in whieh the color TV integrated circuit 81 can be adapted 80 that it can instead detect down-link quadrature modulated subcarrier digital ~ignals of the instant invention will now be described. The objective is to use burst detector io3 i the I and Q detector 99, phase-lock loop 101 and VC0 96 for this purpose, The subcarrier frequency synthesizer 98 previously described in regard to figure 12B is now connected in place of the chroma oscillator quartz crystal at input lOS. It was poir.ted out earlier that the bandpass of the transponder receiver is tuned B0 as to pass only the thirty-two T-NET subcarriers and the ~ost TV

133~322 1 carrier frequency. Thu6 it doe6 not pass the 3.57954~ ~z chroma burst or the chroma subc~rrler. Conseguently tbe chroma burst detector, being gated to operate only durlng sp4cifiea portions of the horlzontal blank~ng lnterval, w~ ee lnstead a ¢ompos~e of s many T-NET data ~ubcarrier frequenclest depending upon whlch subcarrlers are being employed for down-link data transmission.
~ince freguency synthesizer 98 i~ tuned to a ~pecific subcarrier ~nd i~ $n~ectlng ~ignal ~nto the pba~e-lock loop 101, it and VC0 96 can be ~orced to lock on to only that speciflc down-link subcarr~er frequency. Consequently the re~erence frequency ~njected into I and Q detector 99 by VC0 96 cau6ec detector 99 to demodulate the in-phase and quadrature phase ~I ~ Q) digltal data components o~ that specific 6ubcarrier only.
Consequently the readily available find inexpensive ~color~
lS television integrsted circuit 81 can be u6ed to demodulate any one of the many guadrature modulated data subcarriers used in the instant invention. The output of detector 99 is connected to integrate and dump circuit 109 where a ~ynchronizing ~ignal based on the horizontal sync signal from 93 i8 used to accurately gate-out and-optimumly detect the T-NET digital data of the subcarrier which has been selected. The very powerful ~ost TV c~rrier component and sync signals are consequently used to effectively ~carry~ and thereby enhance T~NET transmi~sion reliability; i.e~
the synergetic modulation advantage~
~he transponder microprocessor 36 ~hown in figure 3 ~erves the purpose of coordinating the timing of the transponder's radio section6 as well as buffer storing and relaying message6 between it and the ~: ~?nion device (e.g. personal computer). It also ~33~3~2 1 performs certain hou~ekeeping functions sucb as recognizing which lncoming mes6ages it i8 to detect and pass on. It al80 helps the network control center coordinate overall T~ET 6ystem traffic by dynamically 6bifting to subchannel freguencies a6signed to it to s transm~t and receive on, either as directed by the network control center or as assigned by the companion device. The microrpocessor is conventional in its design and it~ progr G ing $~ relatlvely stralght forwardu Tran&ponder/Duplexer. The transponder duplexer 32 ~figure 10 12) permits sharing of the subscriber's existing TV antenna with t~e existing television receiver, the transponder'~ recei~er ~ection, and its transmitter ~ection. Its principal ~ob ic to isolate the transponder receiver and televis$on recel~er from the transm$tter section 80 that they will not be damaged ~hile ~t 15 transmits. Figure 13 Ehows one possible duplexer deslgn for isolating these receivers from the transmitter. The subscriber's s T~ antenna is connected to the transponder receiver and television receiver through a one-guarter wave length coaxial cable 114 which has at its output end a diode switch L16 that i8 2~ controlled through a radio frequency choke 118 by microprocessor 36. When the transponder is required to transmit a data pul~e, diode 116 is switched to a low impedance state by microprocessor 36 and this in effect short circuits the output end of coaxial cable 114 and cduses a reflected open circuit impedance at its 25 input end ~the left side in figure 131. Conseguently the RP
pulses generated by transmitter 38 see an open circuit at the input to-coax 114 and the signals are consequently routed on to the subscriber TV sntenna and radiated outward.

. ' 1 Qn the other hand,; when transmitter 38 is not transmitting, which i~ most of the time, signals coming into subscriber IV
antenna 12 pass through coax li4 and into ~ignal 6plitter 120 where they ~re routed both to the existing televi~ion receiver ~o that it mAy receive conventional televlsion programs and also to receiver 34, which is part of the transponder. Under these receiving condition~ transmitter 38 represents an open circuit ~nd it rejects the tncoming received signals.
Transponder duplexer 32 of figure 13 is only ~ne o~ several ~o method~ which can be used. For example, devices referred to as microwave circulators comprlse a three port pass~ve nl~twork wh~ch can accomplish a comparable function and have the ~ddltional advantage of being broadband~
We shall now describe the ma~or components of a r~dio central office ~nd will emphasize the unigue and novel circuits which have been devised to practice the instant inven~:ionO
B~io Centr~l Office lBSnL. Figure 14 i~ an overall block diagram of a typical radio central officeO Antennas :!8 represent one of ~ plurality of directional antennas, each connected exclusively to a ~eparate sector receiver 122. Each uector receiver 122 covers the entire 6 MBz TV channel which bas been assigned to the T-NET system. For example, ten sntennas 28, each having an 18-degree bea.-~width, connected to ten receivers 122 will provide a 180-degree coverage. If each angular ~ector uses s$xteen of thirty-two subchannels in a system where odd numbered subchannels are used on one sector, even numbered used on the adiacent sector, and the odd number again used on the next adiacent sector . . ~ etc., then the arrangement would be as shown in figure 14. In that case, each sector receiver 122 would 133~3~2 1 reguire sixteen filters 124 and these are shown as divided into two ban~s of elght subchannel filters each; the bank of filters Qhown in the top row of flgure 14 cover subchannels 9 through 16 ~nd the lower row of eight filters cover channels 1 through 8.
~ach of the subchannel filters 124 ~8 connected to ~ts 6eparate digital interface circuit card 126 and they zre numbered in a corresponding manner.
. In one preferred embodiment eight of these subchannel filters and as~ociated digital interface circuit6 can be controlled from one ~ingle-board computer 128 and t~s i8 the reason that figure 14 shows two groups of eight subch~nne~
filter/digital interface circuit~ connected to a ~B~ single-board computer 128, and 8 more subchannel filter/digital interface circuits connected to a ~A~ single-board computer 121~. Thus sector ~1 receiver feeds sixteen 6ubchannel filters 124, sixteen digital interface circuit cards 126, and two ~ingle-~)oard computer6 128. If a T-NET system had ten sec~or antennas and ten associated sector receivers, there would be a total of three hundred sixty 6ubchannel filter~ 124 ~nd digital interface cards 2~ 126 and twenty single-board computers 128.
Protocol computer 130 (figure 14) collects the data from all ~ingle-board ~omputers 128 and reformats and buffer ~tores' it as necessary and then transmits it through trunkline 26 to network control center 2 ~shown in figure 1). That trunkline may be any one of several commonly used links, such as a microwave link illustrated figure 14.
Display and I/0 133 shown in figure 14 is a computer monitor and input/output ~I/0) device which may be employed to input the } " -range address of the many subscribers who sign up for this communication service. It may also be used for overhead functions such as monitoring the activity of specific digital interface circuits 126, single board computers 128, or for trouble shooting purposes.
RCO/Sector Receiver. Figure 15 shows the block diagram of a typical sector receiver subsystem. Antenna 28 detects up-link signals from subscriber transponders and connects them to bonopass filter 134 and adjacent channel rejection filter 136.
These filters suppress most of the video components of the Host television signal and other interference to the level where they will not overwhelm the subscriber signals. It should be pointed out that typical television transmitters have an effective radiated power (ERP) ranging from 25,000 to two million watts or more and are consequently much more powerful than the subscriber signals. Incoming signals are further amplified in 138 and down converted in first mixer 140. Intermediate frequency (IF) filters 142 and amplifier 144 have approximately 6.0 MHz bandwidth and provide a sharp attenuation of all signals lying outside its bandwidth.
The output of amplifier 144 is connected to a second down converter mixer 146 and this is followed by a second IF filter 148 and amplifier 150. The second IF is 21 MHz and it also has 6.0 MHz bandwidth. All the thirty-two subchannels of a T-NET
system are emcompassed within this bandwidth. The output of sector receiver 122 is connected in parallel to a bank of filters 124; one filter is required for each of the thirty-two subcarriers used in the sector. Included within these bandpass filters is a detector so that the output of each filter is the -133~322 1 subcarrier baseband with analog data, i.e. it is t~e sum of all digital pulses transmitted by transponders in the sector. It has already been noted that each of the bandpass filter~detector assembl~es for a 6ector could consi~t of ~ixteen subchannels ~o s one sector would only operate on either odd channels or even ch~nnel~ in order to provide frequency re-use from ~ector to ~ector.
~ JDigital Interface. Each of the subchannel ~ilters 124 has connected to $ts output a digital interface circuit card and a block diagram o~ that card 18 shown ~n figure 16. ~he purpose of the digital interface card i8 to create range gateE~ ~t th-proper time delay representing the distance to each o~' the many subscribers operating on that subchannel 80 a8 to detect pulse from them and forward the data to its companion single board lS computer 128 (figure 14). We shall now explain the nc,vel aspects of the block diagram in figure 16. All circuits of digital interface card 126 are interconnected to other assemblies throuqh a ~tandard multi bus 156. For example, it has already been pointed out that tbere will be eight digital interface cards for each single board computer 128 and these will be ~nterconnected through multibus 156. Display and I/0 device 133 may also be - connected through that multibusO
In one mode of operation the display and I/0 device 133 (figure 14) is used to input the range address of a new subscriber based on known or computed range to that subscriber and this information will be ~logged~ into subscriber range address memory 160 through channel address decoder 158 and data bu8 157. At the same time, the strength of that subscriber 1 signal will be either measured or est~mated and that lnformation will be input to ~ubscriber ~mplitude ~ignature memory 166 in like manner. Thus the range and amplitude of each subscriber transponder w111 be held in memory 160 and 166 respect$vely. ~he ~nalog pulses from 6ubchannel filter 124 representing $ncoming data from each transponder i8 connected to integrate and dump analog circuit 164.
The operatlon of digital interface card 126 i8 repetitive and triggered Znto operation by the vertical and horizontal ~ync pul~es of the ~ost TV ~tation as detected by a separate receiver using antenna 27 (flgure 3). These ~ync pulses are connected to addres~ generator~ 174 and 176. ~pon this triggering, ~ubscriber ~ddress counter 176 begins to count upward to qenerat~ addre~ses ~n a series of steps, each step being proportional to the distance the H sync pulses have propagated outward from the te;levision station as it sweeps across the countryside. In other words 6ubscriber address counter 176 will have developed a count which i8 equal to the distance from the television station l~o the instant position of the propagating ~ost TV horizontaL blanking interval.
Subscriber address counter 176 i~ connected to the subscriber range address memory 160 and amplitude memory 166 and if those memory locations hold a subscriber, that fact is caused to trigger gate generator 162 and A/D 168. Thus ~ comparison is constantly being made by range address gate generator 162 to see if any subscriber lives at the currently developed address count:
if there is, range gate address generator 162 generates a range gate which enables integrate and dump analog circuit 164 to accept and integrate the pulse from that specific transponder at .~

133~2 1 that 6pecific range address. ~t the end of a five microsecond integrate period the pulse from that transponder iB
lnstantaneously compared in co~parator 170 against a threshold level wh$ch has been established by digital-to-analog converter s 168, which ln turn i~ dependent upon the expected strength of that subscrlber. Based on the result of analog comparator 170 determination is made as to whether there is a logic ~1 transmission or a logic ~0~ (no transmission) from the transponaer at that specific rnnge address. ~t can b~
appreciated that these compari~ons are done on a microsecond by ~crosecond basi~ and in accordance with a prearrange~ schedule depending upon which subscribers are logged in memory and what thelr distance ls from the Host tel~vision transm~tter. ~t can be further appreciated that pulses from many subscriber~ are all time interleaved and must be sorted out; that i8 the job of double buffer demux 172.
The double buffer demux 172 circuit has connected to it an address generator (counter) 174 which is triggered into operation by the Host TV vertical and horizontal sync pulses and it first generates a coarse time division component of subscriber address (i~e. the ~pecific H-sync pulses that the transponder has been assigned to operate on) and a second fine time division address based on the range to each subscriber. Demux 172 also has as input the output of comparator 170 which comprises digital pulses from each of the many subscribers assigned to that subch~nnel.
The double buffer demux 172 sorts out these time interleaved transponder pulses and reorganizes them into data files in which the data from each transponder is grouped together with the --'~
,~

1~3932~

1 tranponder's address an,d placed into a buffer storage location.
~hat buffer storage ~8 per$odically ~dumped~ ~nto the multibus for transfer to the single-board computer 128. The ~ingle-board computer 128 also receives like-data from seven other digital interface cards as shown in figure 14. The single-board computer groups al~ of this $nformation and forwards it to protocol computer 130 where ~t is properly queued with the output of many other single board computers and forwarded over a trunklink 26 to the notwork control center as ~reviously esplained.
~a~ y Deslgn, ~e shall now describe the application of o the T-NET 6ystem to cable TV ~CATV). CATV degign englneers have found that a problem exists when many subscrlbers are connected to a coaxial cable for reverse transmissions ~rom ~ubscrlbers to a central location. This problem is due to the fact that each of the subscribers contributes a finite amount of no~se ;~nd the cumulative effect of all of this noise seriously reduces each of their signal-to-noise ratiosr and perhaps also the down-link TV
program. This problem i8 self defeating in that increasing the power of each subscriber does not offer any solution because that also increases theiE cumulative noise. The instant invention solves this problem because the T-NET tran ponders only transmit pulses and these pul~es only exist at time intervals which are distinct and separate for each transponder. Therefore their cumulati~e effect is negligibleO
2~ Figure 17 illustrates a cable TV application of the T-NET
system. A transponder 14 operates substantially in the ~ame way as described in the preceding sections of this specification.
TY and data signals from the coaxial cable are connected through transponder 14 and CATV tuner 182 to TV receiver 112. The data , ~ .

133~322 1 outpu~ of transponder 14 i8 connected directly to TV receiver 112 -- ~
to provide interactive television operation. The talk-back feature could be through hand-held remote controller 18.
Transponder 14 (figure 17) detects the down-link data and s~nc pulses of the 80st TV ~tation 10 signal injected at CATV
~ead-End~ 177 and receives in the same manner already described.
The transponder 14 up-link reply pulses are sent through coaxial cable 183 and are collected at a multiplexed repeater 180 which could be located within the existing cable TV ~mplifier boxes 181 which are typ~cally spaced at intervals less than one mile. The multiplexed repeater 180 can be designed to function essentially as a multiplexed pulse transponder (somewhat like 14) so as to collect and retransmit up-link signals detected by 179. These are appropriately synchronized to the local TV signal and radiated through antenna 184. Usually there would not be more than a few dozen transponders 14 connected between cable TV
amplifier boxes. Their ~range address~ is determined by the Host TV-to-subscriber indirect distance which is the combined cable and over-the-air effective distance. The design of tllese zo ~ultiplex transponders (repeaters) will be obvious to those skilled in the art after studying the several drawings and discussion presented in this specification and duly observing the 8-sync requirements on the cable TV signal because it is off-set from the over-the-air TV signal H-syncO
The radio central office detect~ and process the5e semi CATV
signals in much the same manner as it already processes trans-ponder replies from purely over-the-air subscribers. The output o~ multip~ex repeater 180 would in fact appear like interleaved pulses from several dozen transponders. The fact that these particular CATV trasnponders operate partly through a coaxial cable would be transparent to the radio central office. A

~ '' 13393?2 1 similar arr2ngement could be used in large office buildings which use a coaxial cable and common antenna.
Veh~cle r~catlon neS~n. We shall now d~scuss a vehicle location ~pplicat~on of tbe T'NET ~ystem (figur~ 18). It has been pointed out that the range to each fixed locatlon ~ubscriber represents its ~range address- and this information is kept $n memory in each radio central office. On the other hand, if the transponder i8 a portable device or in a vehicle, then lts range will initially be unknown. Specific subchannels cal. be dedicated to operate only wlth such moving transponders~
When a precise determinat~on of the transponder location i~
desired, the T~NET system can ~e designed to provide for detection of up-link signals by at least two central receivers labeled ~1 and ~2 in figure 18. The position and diEitance between these two central receivers wil~ be precisely surveyed to establish a fixed baseline from which the position of each transponder can be accurately computed based on precise ~easurement of the range from the transponder to central receivers ~1 and ~2. Such computations are well known and commonly employed in radio navigation~ Figure 18 shows such an operation in which vehicle 186 detects ~ost television data and sync signals through mobile antenna 188 and connects those signals to RF modem 14. The demodulated signals from modt~ 14 are connected to vocoder 194 and to computer/monitor 196.
Up-link data pulses from modem 14 are detected by both central receiver ~1 and ~2 through their antennas 128. Since the range to the transponder is unknown initially, a series of sequential range gates is generated by central receiver ~1 and t2 and each of these gates is examined in sequence to find where ,~ .

133~3'~2 1 pulses are ~eing rece~ved. When this i8 determined, a pair of range gates, called an ~early~ ~nd ~late~ gate in radar terminology~ are positioned around the received pul6es 80 as to track ~t as the vehicle moves. Such pulse acquisition and tracking techn~ques are well known in the art of radar circuit des~gn. The range information whlch 18 thus measured 18 c~ ~cated from central receiver ~2 to central receiver ~1 where a navigation computation algorithm can be installed in a conventional computer to solve the triangulation problem to precisely locate and track vehicle 186. That positio~
~nformation can be forwarded to the network control center and/or to a ~08t such as a vehicle dispatcher. ~ndeed, the navigat~on computer could be installed at the Host computer, if that were more convenient.
1~ The vocoder 194 (figure 18) is intended to be a voice-to-digital and digital-to-voice converter which takes the output of microphone 190 and digitizes it'so that it may be sent through RF
modem 14. Likewise~ the digitized output of modem 14 can be converted to voice signals and transmitted through speaker 192.
This provides a means of verbal communication through RF modem 14. If RF modem 14 operates at 1200 baud, then it is too slow for direct digitized voice transmissions, however, microprocessor 36 in RF modem 14 can buffer-store and thus time-stretch and compress the 1200 baud digitized voice information in such a manner as to make it intelligible, although it may not permit effective real-time dialog between two speakers because of the time delay. This is referred to herein as ~slow-voice~ or ~voice messaging~. On the other hand, RP modem 14 could be designed to 13'~3 ;~J2 1 tran6m$t at a rate up to 15,734 baud and this i~ sufflciently fast to provide real-time voice transmissions thro gh a vocoder 194, if this were de6irable.
n~w~-n~nk Co-ch~nel ~dul~tor. We ~hall now descrlbe the manner in which digital data may be ~ent co-chAnnel 6imultaneously with a regula~ televlsion program without interfering with it. This is referred to as co-c~An~el mult$plesed data ~nd video in figure 19. It has already been ~xplained that the object of thi6 te~hnique ~8 to superimpose digital data onto the regular television video on each of the 525 l~nes of a TV frame and then, on the succeeding frame, to 6uperimpose the same digital data, but inverted, ~o that at each corresonding element (pixtel~ of the TV picture the data i8 fir6t added and then 6ubtracted 80 as to become invisible. It wa~
po~nted out that this could be done throughout the entire TV
picture a~ a 6acrifice in p~cture quality in motion segments, or it can be restrained to only those portions of the picture that convey fixed scene~
Figure 19 shows the block diagram of a system of this invention for sending digital data only in the fixed ~cene portion of TV pictures. The digital data to be transmitted is buffer-stored in digital memory 200 and read out from that 6torage device at prescribed times and ~ent through amplifier 202 to a split channel to provide data and inverted data into 6witch 206. One frame of 525 lines of regular TV video is connected to A/D converter 208 where it is digitized and stored in memory 210.
The video from a following frame of 525 lines is compared pixtel-by-pixtel in 214 against the corresponding information stored from the previous frame in order to detect where differences ~3393~2 1 (motion) ex~st- Where the second and f~rst ~rame picture elements are identical, it ls assumed to represent a fixed scene ~egment and in that event an enable c~ ~nd is sent by 214 to buffer store 200 and digital data is output from that buffer and i8 connected to summing circuit 212. Switch 206 use TV V-sync to reverse its pos$tion every TV frame 80 that the data is first added in one frame and ~ubtracted in a subsequent frame.
In effect comparator 214 is constantly comparing the output of the current TV frame against the previous TV frame in order to fin~ fixed scene locations 80 that it can transmit data in those ~egments. The output of 214, be$ng the curr~nt frame, is D/A
converted in 216 so as to restore the original analog video which is sent to summing circuit 212 where the data is adde/3 to it.
-The output of summing circuit 212 represents video pll~s and minus data and it is sent to the regular TY transmitt~r for transmission to TV viewers and to transponders specifically designed to detect the data portion of the TV signal. Such data ~ ~
receivers could operate essentially like present day Teletext receiver~ but it would include circuit which take advantage of the redundant transmission (i.e. Data + Data) for more reliable detectio~.
~ -NET/Cellular Radio In~a~atio~. The vehicle location capabilities of the T-NET system can be used to advantage to init~ate and coordinate the hand-off of cellular radio telephone 2s subscribers. Figure 20 illustrates such an application. Two T-NET radio central offices labeled RC0 tl and RC0 t2 are located with respect to cellular system 218 so that they may determine the position of any vehicle 222. Although the hexagon shape 220 6~

3 ~ 2 l defining the various cellular lim~ts ~re useful in popular description~ of the ~dea that cellular radio is partitioned into ~ndividual cell~, it i~ clear in actual practice the geometry of any ~pecific cell may take any arbitr~ry ~hape such as 22~. This is due to the fact that the only $nformation available to the cellular radio system as to the position of vehicle 222 ~8 $ts ~ignal strength. ~Signal strength is ~ot a reliable lndicator of vehicle position because it varies frsm time to time and because o~ local reflections from buildings, other vehicles and for other physical reasons. On the other hand, using a relat~vely simple .
radio survey, a geometr$c area such as 22~ can be found where reliable transmi~sion wlth all vehicle~ $n that area can be e~tablished and maintained. Hany areas ~uch f~ 224 c~ln be found ~o that complete coverage of the entire ~ervice area ~18 can be assured. In a practical world those areas 224 constil:ute the real cells.
Consequently if one has independent means such as a T-NET
system to determine in which cell 224 a ~ubscriber is located, then the problem of handingooff vehicle 222 as it woves from cell-to-cell become5 ~ relatively simple computer function. This is also a relatively ~imple process for the T-NET vehicle location mode to accomplish It would also occupy very little of ~ts traffic capabity. Furthermore, a specialized T-NET
transponder could be designed and built for this function alone 2s to reduce its cost and increases $ts reliability in th$s - operating mode.
While the invention has been particularly ~hown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that var$ous .~

~33~ 322 1 changes in form and details may be made therein wit~out departing from the ~pirit and scope of the invention.

Claims (105)

In the Claims

1. A bidirectional wireless digital communication system comprising a broadcast station for transmitting a video signal at least including horizontal blanking intervals and associated horizontal sync signals on a video carrier:
broadcast means for controllably transmitting downlink digital data signals:
a plurality of subscriber receiver-transmitters, each subscriber receiver-transmitter having a subscriber receiving means for receiving said video signal and detecting said horizontal sync signals, and for receiving and detecting said downlink digital data signals, each subscriber receiver-transmitter also having a subscriber transmitting means coupled to said subscriber receiving means for transmitting uplink digital data signals only during at least some of the horizontal blanking intervals of the received video signal; and at least one central receiver, each said central receiver being a means for receiving and detecting said uplink digital data signals transmitted by each subscriber transmitting means.

2. The system of Claim 1 wherein said broadcast means includes means for modulating said downlink digital data on a downlink data carrier having a substantially fixed relationship to and different from said video carrier prior to transmitting said downlink digital data signals.

3. The system of Claim 2 wherein said video carrier is the video carrier of a first conventional television channel.

4. The system of Claim 3 wherein said downlink data carrier is in the frequency band assigned to a conventional television channel adjacent said first conventional television channel.

5. The system of Claim 4 wherein said broadcast means includes means for modulating downlink digital data on a plurality of downlink data carriers, each of said downlink data carriers being different from the others and within the frequency band assigned to a conventional television channel adjacent said first conventional television channel.

6. The system of Claim 5 wherein said downlink data carriers are sufficiently separated in frequency whereby the modulated downlink digital data on each downlink data carrier may be received and uniquely detected in a subscriber receiver, at least in part by frequency discrimination.

7. The system of Claim 6 wherein the modulated downlink digital data is quadrature amplitude modulated on said downlink data carriers.

8. The system of Claim 5 wherein multiple bits of downlink digital data are transmitted on each downlink data carrier during at least some of the horizontal blanking intervals, and wherein the baud rate during the horizontal blanking intervals is a harmonic of the repetition rate of the horizontal sync signals, whereby bit clocks may be synthesized from the horizontal sync signals.

9. The system of Claim 1 wherein each subscriber transmitting means includes means for modulating uplink digital data on an uplink data carrier having a substantially fixed relationship to and different from said video carrier prior to transmitting uplink digital data signals.

10. The system of Claim 9 wherein said video carrier is the video carrier of a first conventional television channel.

11. The system of Claim 10 wherein said uplink data carrier is in the frequency band assigned to a conventional television channel adjacent said first conventional television channel and wherein said modulated uplink digital data is transmitted by a subscriber transmitting means only in at least some of the horizontal blanking intervals of said video signals as received by the respective said subscriber receiving means.

12. The system of Claim 1 wherein said video carrier is the video carrier of a first conventional television channel and wherein at least some of said subscriber transmitting means include means for modulating uplink digital data on a different one of a plurality of uplink data carriers, each of said plurality of uplink data carriers being a different frequency from the others and within the frequency band assigned to a conventional television channel adjacent said first conventional television channel.

;

13. The system of Claim 12 wherein said uplink data carriers are sufficiently separated in frequency whereby the modulated uplink digital data on each uplink data carrier may be received and uniquely detected in said central receiver, at least in part, by frequency discrimination.

14. The system of Claim 13 wherein the modulated uplink digital data is quadrature amplitude modulated by said uplink data carriers.

15. The system of Claim 12 wherein multiple bits of uplink digital data are transmitted on each uplink data carrier during at least some of the horizontal blanking intervals, and wherein the baud rate during the horizontal blanking intervals is a harmonic of the repetition rate of the horizontal sync signals, whereby bit clocks may be synthesized from the horizontal sync signals.

16. The system of Claim 12 wherein said broadcast means includes means for modulating downlink digital data on a plurality of downlink data carriers, each of said downlink data carriers being different from the others and within the frequency band assigned to a conventional television channel adjacent said first conventional television channel.

17. The system of Claim 16 wherein at least some of said downlink data carriers and at least some of said uplink data carriers have the same carrier frequency

18. The system of Claim 17 wherein said broadcast means transmits downlink digital data on a downlink data carrier during a horizontal blanking interval in which a subscriber transmitting means transmits uplink digital data on an uplink data carrier of the same frequency as the last named downlink data carrier, said downlink and uplink data signals being distinguishable by said at least one central receiver, at least in part, by the difference in arrival times of the transmitted signals.

19. The system of Claim 1 wherein said broadcast means is physically independent of said television broadcast station, and includes receiving means for receiving said video signal from said television broadcast station and detecting said horizontal sync signals to determine when said broadcast means is to transmit digital data signals.

20. The system of Claim 1 wherein said broadcast means is coupled to said television broadcast station, whereby said television broadcast station broadcasts through its antenna a video signal at least including horizontal blanking intervals and associated horizontal sync signals, at least some of said blanking intervals including a digital data signal.

21. The system of Claim 1 wherein said at least one central receiver is positioned approximately between said television broadcast station and said plurality of subscriber receiver-transmitters.

22. The system of Claim 1 wherein said television broadcast station transmits a video signal comprising regular television programming, said broadcast means being coupled to said ';' ~ '. ~ ~

television broadcast station so that said television broadcast station broadcasts through its antenna regular television programming and downlink digital data, said downlink digital data being transmitted as video information by adding said downlink digital data to at least part of the video signal during one video frame and subtracted from the corresponding part of the video signal during the next video frame.

23. The system of Claim 22 wherein the parts of the video signal to which downlink digital data are added and to which downlink digital data are subtracted are portions of the video signal which are substantially unchanged during the two successive video frames.

24. The system of Claim 1 wherein said television broadcast station antenna is substantially colocated with the broadcast antenna of at least one more television channel, and wherein the video signals of each of said substantially colocated channels are synchronized so that the horizontal blanking signals thereof coincide in time.

25. A system in accordance with Claim 1 for two way paging wherein said subscriber receiver-transmitters are mobile.

26. A bidirectional wireless digital communication system comprising a television broadcast station for transmitting ordinary television programming including vertical sync and horizontal sync signals and associated blanking intervals on a preassigned television channel and associated video carrier;

broadcast means for controllably transmitting down digital data signals:
a plurality of subscriber receiver-transmitters distributed about an area within the broadcast range of said television broadcast station and said broadcast means, each subscriber receiver-transmitter having a subscriber receiving means for receiving said video signal and detecting sync signals, and for receiving and detecting said downlink digital data signals, each subscriber receiver transmitter having modulating means for modulating uplink digital data signals on an uplink carrier of a frequency within the frequency band of a television channel adjacent said preassigned television channel, each subscriber transmitting means coupled to said subscriber receiving means for transmitting the. modulated uplink digital data signals during at least some of the blanking intervals of the received video signal; and a plurality of central receivers, each central receiver being located and having a directional antenna to predominately receive the modulated uplink digital data signals from a respective subarea within said area, each said central receiver being a means for receiving and detecting said uplink digital data signals transmitted by the respective subscriber transmitting means within the respective subarea.

27. The system of Claim 26 wherein each subscriber transmitting means includes means for modulating uplink digital data on an uplink data carrier having a substantially fixed relationship to and different from said video carrier prior to transmitting uplink digital data signals.

28. The system of Claim 27 wherein said uplink data carrier is in the frequency band assigned to a conventional television channel adjacent said first conventional television channel and wherein said modulated uplink digital data is transmitted by a subscriber transmitting means only in at least some of the blanking intervals of said ordinary television programming as received by the respective said subscriber receiving means.

29. The system of Claim 26 wherein at least some of said subscriber transmitting means include means for modulating uplink digital data on a different one of a plurality of uplink data carriers, each of said plurality of uplink data carriers being a different frequency from the others and within the frequency band assigned to a conventional television channel adjacent said preassigned television channel.

30. The system of Claim 29 wherein said uplink data carriers are sufficiently separated in frequency whereby the modulated uplink digital data on each uplink data carrier may be received and uniquely detected in said central receiver, at least in part, by frequency discrimination.

31. The system of Claim 30 wherein the modulated uplink digital data is quadrature modulated by said uplink data carriers.

32. The system of Claim 30 wherein multiple bits of uplink digital data are transmitted on each uplink data carrier during at least some of the horizontal blanking intervals, and wherein the baud rate during the horizontal blanking intervals is a harmonic of the repetition rate of the horizontal sync signals, whereby bit clocks may be synthesized from the horizontal sync signals.

33. The system of Claim 30 wherein said broadcast means includes means for modulating downlink digital data on a plurality of downlink data carriers, each of said downlink data carriers being different from the others and within the frequency band assigned to a conventional television channel adjacent said first conventional television channel.

34. The system of Claim 33 wherein at least some of said downlink data carriers and at least some of said uplink data carriers have the same carrier frequency.

35. The system of Claim 34 wherein said broadcast means transmits downline digital data on a downlink data carrier during a horizontal blanking interval in which a subscriber transmitting means transmits uplink digital data on an uplink data carrier of the same frequency as the last named downlink data carrier, said downlink and uplink data signals being distinguishable by said at least one central receiver, at least in part, by the difference in arrival times of the time signals.

36. A system in accordance with Claim 1 for two way paging wherein said subscriber receiver-transmitters are mobile.

37. A system in accordance with Claim 26 for interactive television wherein said subscriber receiver-transmitters are coupled to a television receiver for displaying on a display the ordinary television programming and for also displaying downlink information, said subscriber receiver-transmitters including means for user entry of uplink digital data at the respective subscriber receiver-transmitter.

38. The system of Claim 26 wherein said subscriber receiving means and said subscriber transmitting means both share a television antenna with at least one ordinary television receiver.

39. A method of information communication within an area utilizing the frequency band of a television channel adjacent that of a television channel serving the same area and broadcasting ordinary television programming, including horizontal and vertical sync signals and associated blanking intervals, comprising the steps of.
a) modulating information on a carrier within the frequency band of the television channel adjacent that of the television channel broadcasting ordinary television programming;
b) transmitting the modulated information only during the horizontal blanking intervals of the ordinary television programming;
c) receiving the transmitted modulated information at a distant location and detecting the information therein.

40. The method of Claim 39 wherein in step (c), the information is detected in part by also detecting the horizontal sync signals of the ordinary television programming and referencing the detection thereto.

41. The method of Claim 40 wherein the detection of step (c) is referenced to both the horizontal sync signals of the ordinary television programming and the known distance between the position of transmitting of step (b) and the position of receiving of step (c).

42. The method of Claim 39 wherein instep (b) the transmission is from the same antenna as the television channel broadcasting ordinary programming material.

43. The method of Claim 39 wherein in step (b) the transmission is from a different antenna as the television channel broadcasting ordinary programming material.

44. The method of Claim 43 wherein is step (c), the transmitted modulated information is received using a directional antenna located approximately between the location of broadcast of the ordinary television programming and the location of the transmission of step (b).

45. A method of communicating information from a plurality of remote locations to a central location served by a television station broadcasting ordinary television programming in the frequency band of a first preassigned television channel, the television programming including horizontal and vertical sync signals and associated blanking intervals, comprising the steps of:
(i) at each remote location;

a) receiving the ordinary television programming and detecting the horizontal sync signals thereof, b) modulating the information to be communicated from each remote location onto a carrier having a frequency in the frequency band of a preassigned television channel adjacent the frequency band of the first preassigned television channel, c) transmitting the modulated information during at least some of the horizontal blanking intervals of the received television programming, and (ii) at at least one central location d) receiving the transmissions of step (c) and detecting the information therein.

46. The method of Claim 45 wherein the detection of step (d) is referenced to the horizontal sync signals of the ordinary television programming.

47. The method of Claim 46 wherein the information is an digital form, whereby the carrier of step (b) is a data carrier.

48. The method of Claim 47 wherein a plurality of data carriers are used, each having a frequency in the frequency band of a preassigned television channel adjacent the frequency band of the first preassigned television channel, each remote location modulating the data in step (b) on a specific one of the data carriers assigned thereto.

49. The method of Claim 48 wherein said data carriers are distributed throughout the frequency band of the respective television channel.

50. The method of Claim 48 wherein in step (b) the modulated information is transmitted only during specific horizontal blanking intervals as assigned to that remote location, whereby the transmission from different remote locations may be during different horizontal blanking intervals to provide a plurality of data channels.

51. The method of Claim 50 wherein the specific horizontal blanking intervals used for transmission in each remote location are determined with reference to the reoccurence of the vertical sync pulse of the ordinary television programming.

52. The method of Claim 51 wherein the detection of step (d) is further referenced in time to the known distances of transmission, whereby data may be modulated on the same data carrier at different remote locations in step (b) and transmitted during the same horizontal blanking intervals in step (c) and unambiguously detected in step (d) by range gating at the detector, thereby providing additional data channels differing in range.

53. The method of Claim 52 wherein a plurality of central locations are used for step (d) and wherein the reception of step (d) is by way of a directional antenna, whereby different central locations will receive and detect digital data from different groups of remote locations, providing still additional data channels.

54. The method of Claim 53 wherein said plurality of control locations are physically positioned approximately between the television station antenna and the group of remote locations served thereby.

55. The method of Claim 54 wherein in step (c) a plurality of data bits are transmitted from at least one remote location during at least some of the horizontal blanking intervals, and wherein the bit rate is a harmonic of the repetition rate of the horizontal sync signals, whereby the bit clocks at the remote locations and at the at least one central location may be synchronized by the ordinary television programming horizontal sync signal.

56. The method of Claim 54 wherein the modulation of step (b) is a quadrature modulation.

57. The method of Claim 45 wherein at each remote location, the transmission of step (c) is on a television antenna shared with at least one ordinary television receiver.

58. In an interactive television system, a method of communicating information from a plurality of remote receiver locations, each connected to a cable television system, to a central location, the cable television system receiving ordinary television programming over the air and providing the same over a cable having amplifiers at various points along the cable, each serving a plurality of remote locations, comprising the steps of:
(1) at each remote location;
a) modulating the information to be communicated from each remote location onto a carrier having a frequency in the frequency band of an unused cable television channel;
b) transmitting the modulated information on the cable only during at least some of the horizontal blanking intervals of an adjacent cable channel carrying ordinary television programming:

(ii) adjacent each cable amplifier c) detecting the horizontal sync signals of a television channel broadcasting ordinary television programming over the air d) transmitting the modulated information received on the cable over the air and in the frequency band of a television channel adjacent the channel referred to in step (c) only during at least some of the horizontal blanking intervals associated 5 with the detected horizontal sync signals of the channel referred to in step (c); end (iii) at at least one central location e) receiving the transmissions of step (d) and detecting the information therein.

59. A method of information communication within an area utilizing the frequency band of a television channel adjacent to as well as that of a television channel serving the same area and broadcasting ordinary television programming, the programming including horizontal and vertical sync signals and associated blanking intervals, comprising the steps of:
a) modulating information on a plurality of subcarriers;
b) modulating the modulated information of step (a) onto the carrier of the television channel broadcasting the ordinary television programming, the subcarriers causing the modulation of this step (b) to result in frequency components within the frequency band of the television channel adjacent that of the television channel broadcasting ordinary television programming;
c) broadcasting the modulated information only during the horizontal blanking intervals of the ordinary television programming;
d) receiving the transmitted modulated information at a distant location and detecting the information therein.

60. The method of Claim 59 wherein the modulation of step (b) also results in frequency components within the frequency band of the television channel broadcasting ordinary television programming.

61. The method of Claim 59 wherein the modulation of step (b) is a single sideband modulation.

62. A method of determining the location of a mobile unit within an area utilizing the frequency band of a television channel adjacent to or the came as that of a television channel serving the same area and broadcasting ordinary television programming, including horizontal and vertical sync signals and associated blanking intervals, comprising the steps of:
a) modulating identification information on a carrier within the frequency band of the television channel adjacent to or the same as that of the television channel broadcasting ordinary television programming;
b) transmitting the modulated information only during the horizontal blanking intervals of the ordinary television programming;

.

c) receiving the transmitted modulated information at the mobile unit and transponding the received information; and d) receiving the transponded information and the television programming at at least two different central locations and determining the location of the mobile unit by differences in the transmission times of the received signals.

63. The method of Claim 62 wherein a video carrier including horizontal and vertical sync signals and associated blanking intervals is broadcast during periods the ordinary television programming is not on the air.

64. A wireless digital communication system comprising:
a broadcast station for transmitting a video signal at least including blanking intervals on a video carrier;
broadcast means for controllably transmitting downlink signals on a second carrier; and a plurality of subscriber receiver-transmitters, each subscriber receiver-transmitter having a subscriber receiving means for receiving said video signal and detecting said blanking intervals, and for receiving and detecting said downlink signals, each subscriber receiver-transmitter also having a subscriber transmitting means coupled to said subscriber receiving means for transmitting uplink signals only during at least some of the blanking intervals of the received video signal; and at least one central receiver, each said central receiver being a means for receiving and detecting said uplink signals transmitted by each subscriber transmitting means.

65. The system of claim 64 wherein said broadcast means controllably transmits downlink signals during at least some of the blanking intervals.

66. The system of claim 65 wherein said blanking intervals are horizontal blanking intervals.

67. The system of claim 65 wherein said blanking intervals are vertical blanking intervals.

68. The system of claim 65 further comprising a plurality of broadcast means, wherein each said subscriber receiving means is distributed about an area within the broadcast range of said broadcast station and at least one of said broadcast means, and each said broadcast means has an antenna to predominantly transmit the downlink signals to the subscriber receiving means located within a respective sub-area of said area.

69. The system of claim 68 wherein said antenna is a directional antenna.

70. The system of claim 68 wherein said blanking intervals are horizontal blanking intervals.

71. The system of claim 68 wherein said blanking intervals are vertical blanking intervals.

72. The system of claim 70 wherein the video carrier is associated with a preassigned television channel, and said second carrier is within the same frequency band as the preassigned television channel.

73. The system of claim 71 wherein the video carrier is associated with a preassigned television channel, and said second carrier is within the same frequency band as the preassigned television channel.

74. The system of claim 70 wherein the video carrier is associated with a preassigned television channel, and said second carrier is within the frequency band of a television channel adjacent to the preassigned television channel.

75. The system of claim 71 wherein the video carrier is associated with a preassigned television channel, and said second carrier is within the frequency band of a television channel adjacent to the preassigned television channel.

76. The system of claim 64 wherein the video carrier is associated with a preassigned television channel, and each subscriber transmitting means includes means for transmitting uplink signals on a carrier within the same frequency band as the preassigned television channel.

77, The system of claim 76 wherein said blanking intervals are vertical blanking intervals.

78. The system of claim 76 wherein said blanking intervals are horizontal blanking intervals.

79. The system of claim 68 wherein each said broadcast means further comprises:
modulation means for forming said downlink signals by modulating information to be communicated by said second carrier and for providing an output responsive thereto; and transmitter means coupled to said modulation means for transmitting the output of said modulation means; and each said subscriber receiving means comprises:
means for receiving as a receiver signal a signal containing the video carrier transmitted by the broadcast station and the signal transmitted by said transmitter means;
a first demodulator for demodulating said receiver signal using the video carrier of the broadcast station as a reference to recover a signal corresponding to the signal transmitted by said transmitter means; and a second demodulator coupled to said first demodulator for demodulating the signal recovered by the first demodulator to recover the information to be communicated.

80. The system of claim 79 wherein said second carrier has a frequency in the frequency band of a television channel adjacent to a preassigned television channel associated with the video carrier.

81. The system of claim 79 wherein said second carrier has a frequency in the frequency band of a preassigned television channel associated with the video carrier.

82. The system of claim 68 wherein each said broadcast means further comprises:
modulation means for forming said downlink signals by modulating information to be communicated by said second carrier and for providing an output responsive thereto; and transmitter means coupled to said modulation means for transmitting the output of said modulation means; and each subscriber receiving means comprises:
means for receiving as a receiver signal the signal transmitted by said transmitter means; and a demodulator for demodulating the signal on the second carrier to recover the information to be communicated.

83. The system of claim 82 wherein said second carrier has a frequency in the frequency band of a television channel adjacent to a preassigned television channel associated with the video carrier.

84. The system of claim 82 wherein said second carrier has a frequency in the frequency band of o preassigned television channel associated with the video carrier.

85. In an interactive television system, a method of communicating information from a plurality of remote receiver locations, each connected to a cable television system, to a central location, the cable television system receiving ordinary television programming over the air and providing the same over a cable and having amplifiers at various points along the cable, each amplifier serving a plurality of remote locations, comprising the steps of:
(i) at each remote location:
a) modulating the information to be communicated from each remote location onto a carrier having a frequency in the frequency band of an unused cable television channel;

b) transmitting the modulated information on the cable;
(ii) adjacent each cable amplifier:

c) transmitting the modulated information received on the cable over the air only during at least some of the blanking intervals of a first television channel broadcasting ordinary television programming over the air and in the frequency band of a television channel adjacent said first television channel; and (iii) at at least one central location:
d) receiving the transmissions of step (c) and detecting the information therein.

86. A method of determining the location of a mobile unit within an area utilizing the frequency band of a television channel adjacent to or the same as that of a television channel serving the same area and broadcasting ordinary television programming, including blanking intervals, wherein the mobil unit communicates with a first base station in a first cell within said area, the method comprising the steps of:
a) modulating identification information on a carrier within the frequency band of a television channel adjacent to or the same as that of the television channel broadcasting ordinary television programming;
b) transmitting the modulated information only during the blanking intervals of the ordinary television programming;
c) receiving the transmitted modulated information at the mobile unit and transponding the received information;
d) receiving the transponded information and the television programming at at least two different central locations and determining the location of the mobile unit by the differences in the transmissions times of the received signals;

,, e) determining that the mobile unit has changed location from the first cell to a second cell within said area; and f) handing off the mobile unit to a second base station in the second cell.

87. The system of a claim 64 wherein said broadcast station transmits a video signal comprising regular television programming, said broadcast means being coupled to said television broadcast station so that said television broadcast station broadcasts through its antenna regular television programming and downlink digital data, said downlink digital data being transmitted as video information by adding said downlink digital data to at least part of the video signal during one video frame and by subtracting said downlink digital data from the corresponding part of the video signal during the next video frame.

88. The system of claim 87 wherein said downlink digital data is transmitted at a data rate equal to an odd multiple of one half the horizontal sync rate of the video signal.

89. The system of claim 64 wherein said subscriber receiver-transmitter further comprises:

an uplink buffer coupled to said subscriber transmitting means for temporarily storing said uplink signals and thereafter transferring said uplink signals to said subscriber transmitting means for transmission of said uplink signals only during predetermined time slots.

..

90. The system of claim 89 further comprising a radio central office, wherein said radio central office controls the timing of the storage and transfer of said uplink signals by said uplink buffer.

91. The system of claim 64 wherein said downlink signals are digital data signals, said broadcast means comprises means for modulating digital data on a plurality of downlink data carriers, each of said downlink data carriers being different from the others and within the frequency band assigned to a conventional television channel.

92. The system of claim 91 wherein the modulated downlink digital data is quadrature amplitude modulated on said downlink data carriers.

93. The system of claim 92 wherein said subscriber receiving means comprises a conventional chroma subcarrier demodulator tuned to demodulate one of said data subcarriers and detect sync signals.

94. The system of claim 65 further comprising a central receiver substation physically positioned between said central receiver and at least one of said subscriber transmitting means for continuous reception of said uplink signals during periods of simultaneous uplink and downlink transmission.

95. The system of claim 94 wherein the distance between central receiver substation and said central receiver is at least the spatial length of one of said blanking intervals.

96. In an interactive television system, a method of communicating information from a plurality of remote receiver locations, each connected to a cable television system, to a central location, the cable television system transmitting ordinary television programming and providing the same over a cable having amplifiers at various points along the cable, each amplifier serving a plurality of remote locations, comprising the steps of:
(i) at each remote location:
a) modulating the information to be communicated from each remote location onto a first radio frequency carrier;
b) transmitting the modulated first radio frequency carrier of step (a) on the cable;
(ii) adjacent at least one cable amplifier:
c) detecting and re-transmitting the modulated information received on the cable over the air on a second radio frequency carrier during at least some of the blanking intervals of a television channel broadcasting ordinary television programming over the air; and (iii) at at least one central location:
d) receiving the transmissions of step (c) and detecting the information therein.

97. The method of claim 96 wherein the first radio frequency carrier is an unused channel of the cable television system.

98. The method of claim 96 wherein at step (b) the information is transmitted only during the blanking intervals of at least one channel of the cable television system.

99. The method of claim 96 wherein at step (a) the information is modulated in a first polarity during one video frame, said information being modulated in an opposite polarity in a corresponding part of a subsequent video frame to produce visual cancellation.

100. The method of claim 97 wherein at step (a) the information is modulated in a first polarity during one video frame, said information being modulated in an opposite polarity in a corresponding part of a subsequent video frame to produce visual cancellation.

101. The method of claim 98 wherein at step (a) the information is modulated in a first polarity during one video frame, said information being modulated in an opposite polarity in a corresponding part of a subsequent video frame to produce visual cancellation.

102. The method of claim 96 wherein the second radio frequency carrier is in the channel of the television channel broadcasting ordinary television programming over the air.

103. The method of claim 96 wherein the second radio frequency carrier is in an unused channel adjacent the channel of the television channel broadcasting ordinary television programming over the air.

104. The method of claim 96 wherein said at least one cable amplifier of step (ii) is a last cable amplifier.

105. The method of claim 96 wherein the ordinary television programming transmitted by said cable television system over said cable is first received by said cable television system over the air.