US2481915A - Power line carrier wave communication system - Google Patents

Description

2 Sheets-*Sheet l R. C. EDSON ET AL POWER LINE CARRIER WAVE COMMUNICATIN SYSTEM Filed March 9, 1946 R. c. 5050A/ NVEWORSJ W 5ML/NG ATTKORNEV Sept. 13, 1949. R. c. EDsoN ET A1.

POWER LINE CARRIER WAVE COMMUNICATION SYSTEM Filed March 9, 1946 2 Sheets-Sheet 2 ATTORNEY atented Sept. 13, 1949 NITED s #PATENT @ENCE POWER LINE CARRIER WAVE COMMUNICA- 'rroN SYSTEM corporation of New York v u Y pplication March 9, 1946, Serial No. 653-;254

8 Claims.

The invention relatesto an electric Wave transmission system, and. particularly to an electric wave communication system .employing high requency carrier waves foratransmitting the .communication signals. .l t.

The invention isapplicable to acommunication system employing one .orrcnore high .frequency carrier channels to providetworway telephone service to telephonesubscribers .overa common transmission medium. I'n-,suoh asystem, the .common medium maybe a transmission line, for example, one used primarily `forsupplying low .frequency, high voltage .electric-power to a number of power consumers. in a powerdistribution network in a rural varea, or-,it-mayl comprise one or more wire and radio transmission links. .Each carrier channel may bentilizedfor providing twoway communication between telephone. subscribers associated with that-channel von a. party-line basis over the line or other common transmission medium through an individualcarrier terminal having an- ;associatedrncentral switching cnice which Amay be associated also with other carrier channels on the commonrnedium. Thercentral oihce may provide meansforfconnecting the carrier Atelephone subscribers- Astations associated with each carrier channel on the'common transmission medium' in communication relation with each other or with otherwtelephone subscribers stations yassociated :with: other carrier channels on the same medium orion-l outside systems.

The; inventionis specifically applicable to a carrier communication system of the general type described lia'bove in whichthree difierent high frequency 4electridwaves per channel are employed as carriers ,for bothsignaling and speech transmission over theline orother common transmission mediumisuch asis disclosed in the copending patent application of M` L. Almquist, Serial No. 625,083 filed October 27, 194i. In such ya system,transmissionfrom the carrier terminal to the carriertelephone subscriber associated with a `carrier channel on the line orrother common transmission medium is alwayson one of these kfrequencies,.-say-thev frequency f1, and transmissionjrom each--subscriber,'s` station on that carrierchannel over the common medium to the eentraloice carrier ftermin-al is on eitherl one of` two -otherqcarrrer frequencies, say frequency f2 or f3, dependingonythertypeof call. (Voice frequency4 transmission `is provided between the central office and-the.carrierjterminal On central ofce calls that i s -l Vcalls between-a carrier Isubscriber associated vitlii.,oneac :arrierA channel on the common medium and, thefcentnal cnice, or

outside subscriber'connected tothat oice, transmissionirom .the .carrier terminal to the carrier subscriber .is on carrierifrequency f1 and from the carrier sulzvscri-berto,.the;4 carrier terminal. on .carrier .frequencyja .Dnreverting orV party-line calls, .that is,fcallszbe'twe.er`1 twdcarrier subscribers on the same fcarriermhannel jon the common mediunnav calling..subscriber `transmits to the carrier terminalonetheacarrier.frequency f3, and receivesv from the; .-c'arriler.` .terminal lon carrier frequency f1, andthegcalled subscriber transmits to. thev carriergterminal -oncarrier frequency f2 and receives fnomthattermnal on carrier frequency-f1, means being-provided at each subscribers station for. Shifting from thel normal transmitting carrier frequency f2v to the alternate transmitting .Carrienfrequency f3 when the subscriber at .that station;.hasfinitated a. reverting call. In the systemiofnthe aforementioned patent application this frequency shiftingmeans is under control ofthe, calling subscriber, a nonlockne key being providedfineaoh, canriersubscrbers set for this pllpQS.

An object o the iinvention is to improve a carriersignaling systemgof'the general y ,three-carrier frequency. typedescribedabove, particularly from the standpointdfofgautomatic operationen revert- Calls, y

Another object lis .to Supply two-way carrier telephbnc` ,service eciently and economically over, acominontransmission ,medium1 such as a power ,distribution line, ,toa ,number of telephone subscribers, for example, to individual power consumers lserved byl--thesama power linein a rural area.A 1,. Y A

Other vobjects are to :provide two-way communictionbetweec any two, of. afnumbef ,0f Subscribers associated V-withfL the same5 carrier Achannel 0h a.emmonfraasmisson mdium@ a Darty-.line basis.; teamlid?. two-nay Communication between any subscriber associated withone carrier` channelforiy the common medium and another subscriber onanotherarrier channel on the Samemedium-:Om an. Outs. e, Sysm @Ver the common medium through-sacentral orifice; and to provideaconference connection between any two s ubscribersnonsthe same carrier channel on the common medium and,any. out side connection (voice or other carrier channel on the common medium); thrQueh-the-entra1-Simca.

In.A accordancewith the present invention, three-frequency carrieracomrnnnication system of the above-described generaltype 'isf mainlyftogpwvisaaitinerant@ trolmr ne and; Pirelli-0. ttmgolma level'mng Cap-S- his carrier transmitter to be energized so as to send out a carrier of the normal frequency f2 over the common medium to the carrier terminal where it operates switching arrangements to bring in the operator or dialing equipment in the associated central cnice, and to energize the carg rier transmitter at that terminal so that it sends the carrier frequency f1 over the common medium to operate busy signaling devices at all subscriber stations. The calling subscriber then places his call with the central oflice by sending out his voice or dialing call signals on his normal transmitted carrier f2 to the carrier terminal where they are demodulated to voice frequencies for transmission to the central office. The calling subscriber then hangs up,and the carrier f1 modulated with coded low frequency (-cycle) ringing current is sent out from the carrier terminal under control of the central oce operator or machine switching equipment until the called subscriber answers and sends out his normal carrier frequency f2 to energize the transmitter at the carrier terminal. The continuous carrier frequency f1 then sent out over the line from the carrier terminal operates switching equipment at the calling subscribers station to shift the tuning of the transmitter at the latter station from fz to fs. When ringing ceases, the calling subscriber lifts his telephone set from the switchhook to energize his transmitter which sends out the reverting carrier frequency f3, since the frequency f1 initiated by the called subscriber was on the line prior to this operation of the calling subscribers switchhook. The calling subscribers talking signals are transmitted on carrier f3 to the carrier terminal where they are demodulated to voice frequencies and remodulated on carrier f1 for transmission over the line to the called subscriber. The called subscribers talking signals are transmitted on the carrier f2 to the carrier terminal and are there demodulated to voice frequencies and are remodulated on carrier fi for transmission over the line to the calling subscriber.

If a continuous carrier wave of the frequency f1 is not present on the line when a subscriber operates his hook switch, the operation of the latter, in addition to energizing the transmitter at the station, also disables the switching equipment provided for shifting the transmitting carrier frequency so that the normal carrier of frequency f2 is sent out.

These and other features of the system of the invention will be brought out in the following detailed description when read in conjunction with the accompanying drawing, in which:

Figs. 1 and 2, when placed side by side with Fig. 1 at the left, show in block schematic form a power line carrier telephone system embodying the invention.

Fig. 3 shows schematically one type of terminating network which may be used on the power line in the system of l1"'igs. 1 and 2 to adapt it for `carrier transmission; Y

Fig. 4 shows schematically one type of filter network which could be used for properly couayireis" pling the carrier terminal to the power line in the system of Figs. 1 and 2, and for providing in addition a suitable termination on the carrier terminal end of the power line; and

Fig. 5 shows schematically one type of filter which could be used for coupling each of the subscriber stations to the power line in the system of the invention shown in Figs. 1 and 2.

The power line carrier telephone system in accordance with the invention shown in Figs. 1 and 2 includes a transmission line I primarily employed for transmission and distribution of low frequency, high voltage electric power and adapted for simultaneous use for high frequency carrier wave telephony. The line I is shown as a single phase line consisting of one phase conductor 2 and a neutral conductor 3, the latter being grounded at a number of spaced points, for example, at each pole. This type of line is coinmonly used in rural areas for transmitting high voltage, low frequency (G0-cycle) electric power to electric power consumers.

The portion of the power line I used for carrier transmission is isolated from the other portion of the line not so used by means of the carn rier choke coils 4 connected in series with the phase wire 2 at the two ends of the carrier section, as indicated, these coils being designed to offer a high series impedance to the carrier frequencies so as to minimize adverse transmission effects from the portion of the power line not used for carrier transmission, but a low series impedance to the low frequency (G0-cycle) power currents so as to enable the latter currents to be transmitted freely over the portion of the line used for carrier transmission to the power loads. Other suitable isolating or sectionalizing equipment may be employed in place of the choke coils, for example, fllters or tuned circuits.

To prevent reflections which might make it difiicult to detect the carrier frequency energy at various points in the line, it is desirable to terminate the ends of the portion of the line I employed for carrier transmission in approximately f characteristic impedance at the carrier frequencies used; similarly, the ends of all b-ranches (not shown) of the power line used for carrier transmission should be terminated in approximately characteristic impedance at the carrier frequencies used. The network 5 at the far end of the section of line I shown, for this purpose may be, for example, of the type illustrated schematically in Fig. 3. As shown in Fig. 3, it comprises a configuration of uncoupled inductances, condensers and a resistor forming a k-type half-section, band-pass filter network connected across the line, including a condenser 6 in one series arm, and a drainage impedance to ground which may be an inductance coil l, as shown, or a resistor, shunted by a protective spark gap 8, connected in series with the condenser 6 between the phase wire and the grounded neutral conductor. The condenser 6, of value in the order of 2,000 micromicrofarads, serves as a high impedance to the high voltage (7.2 kilovolts), low frequency (60 cycles) power currents transmitted over the line I, and the inductance 'l of value in the order of 10 millihenries, has a, relatively high impedance over the carrier frequency band.

A plurality of like subscriber two-way carrier telephone stations A, B, C associated with one carrier channel on the power line I, that is, all transmitting on the same carrier frequencies and receiving on the same carrier frequency, are bridged across that line at different points by aje'sieis" tile @mm1-er1 ltirangtn-efiis??,ifaria sa individual twoiway"carrier'terrnna statlon C'I formatearpower line I' esseri e, o'f-yaiue' in the pid'r-pr 1,000" to 2'oco-l i micromicrofaradsf,7 in seriesf witlfi "an inductance coil l shunted by"a"`prote tivf spark gapn,` clonnected irf'shunt' with the phasenwirerand neutral wire of the epowerline. An 'impedance transformer, 'band-pass filter networkil connects the subscribers' carrier telephonet set 'Y across "j the` inductance 'l'.4v The `network` I Ifis designed to'selectively transmit' a narrowba l"of frequencies, for example; a band 'extending mitting carrier' frequency is "185 'kilocycles vand the receiving carrier freduencyfisfl65 kilocycles, in each Asubs'cribers`carrie telephone set as in the' system to. bedescribede;Av andfals'o'v to vprevent band and thuswiu cause'biuy' asmaubridging loss on line" I. In ad'riiitioir'lf'owing tothe nature of a mid-seriesterminated"baiidpass lt'er such as shown, the Aimpedance ill beyry'hig'h (and largely" reactive)` outside t e pa's'sband and' thus channels Aexcept the onefto which; the 'associated subscriber set is assigned. If subscriber station is at the end' ofthe seculieri of line" I, Vor branch thereof, used for `cartrier;'trarisriiission, the'nein worl: ofthe type illustrated'n'figf in'place of the terminating resistor element"ofthe latter.

i fromieo'tc 20oy kilocycles, if' the normal' transmitting'- Acarrier frequency is 195 kilooycl`es the `reverting trans'-V The terminal couple'rarrangement ID also inay @rated in Fig. 4, wnichdifrers 'essentiauyfmm the subscriber'coupler" arrangement'of Eigj iin` that its impedance transformer; bandLpa'ss' filter' network I2 is designedtoselectively` transmita` wider frequency band (15()y to 485 kilocycles) and to present an impedance on th line side l suitable for terminating the"'carrie'r terrninal end ofthe portion yof power line' VI e'IiipIcSyed' for carrier transmission in approximatelycharacteristic im- `pedance at' the carrier" frequenciesus'edjso as to prevent reiiectionstendi'n'g to' jnali di'fticlt'the detection of the oarr'ierffrequency energy in carrier terminal CT. 'lhl'lsfitsfv 'use' inak'es"unneces-Y receiving'circuit" I 6` *i I 5 includes in order,

bin'dinca standard?lhafdtelepiione 'set i8' 'with the telephone receiver' Itifavrriiulator"aiidassofA ciated carrier oscillator forsupplying tothe modL ulator the carrier oscillations of the normal transmitting frequency f2 (195 kilocycles) or the re. verting transmitting car'rier4 frequency fa. ,(185 kilocycles), represented' by thej` box: 20. and the associated piezoelectric crystals YI and Y2 forY respectively tuning the carrie'rOscillator to the.v normal carrier frequency .f2 195 kilocycles) and' the reverting, carrier frequencyfa `.(1851kilocyfcies); the high frequency amplifier 21'; the transinitting band-pass filter 22; and the'plugl-,in resistance equalizer pad 23 feeding into'the line 'I through the coupler device 9.A The high fre; quency receiving circuit I5 includes in order, reading from left to right, the plug-in resistance equalizer pad 2li fed from the line 'I' througlrithe couplerdevice 9; the receiving'lbandlpas's filter.. 25; the high frequency amplierZS whichfrnay compriseY one or more vacuum'tube stages with.l interstage and output ltering" networks fortunf ing it to amplify properly the carrier signalsoverv the frequency range transmitted by the preceding band filter, and an automaticvolufne control'y circuit fi? associated with the amplier, which may be of any of the well 'known types, utilized for controlling the'gain 'of the amplifier to c0111-, v pensate for variations in the amplitude level of the received carrier signals due to variations in the less of the preceding section of line; the demcdulator 2B; and the telephone receiver I9 of the standard hand telephone set IBLv The modulator included within'the boX'Z in the transmitting circuit I5 may be of any suitwm cause 'negligibletridgihglessfof al1 carrier :i able unbalanced type, includ-ieg 0116. 01 more electron discharge tubes or other variableresistance devices, adapted for combining high fre. quency carrier oscillations supplied to its carrier input terminals with audio frequency signals ap.-`

Work `H my'b-con-et-dfda termatihg nef/ 9;. plied to its signal input terminals, so as to .pro-

duce signal side-band components as well as the unmodulated carrier component in its output. The carrier oscillator in loo-X 20 may also be of any suitable type, and may be .combined with the associated modulator in a single electron discharge tube in conventional manner. The demodulator 25 in receiving circuit I5' may be of any suitable type, for example, itmay comprise a varistor consisting' of a plurality of copper oxide or crystal rectifier elements connected in series between the output cf amplifier 23 and the telephone receiver I9. The transmitting band-pass filter 22 in the transmitting circuit I5 is de-V signed to selectively transmit the voice ksignalmodulated carrier waves (one or more signal sidebands) of the high frequenciesjz (195 kilocycles)A and fg (185 kilocycles) which may ,be ,applied` thereto from the output of the oscillator-modulator 2n, as well as the unmodulated. carrier` f requency components of these frequenciesand the band-pass filter 25 in the receiving circuitll isL designed to selectively transmit the voiceY signalmodulated carrier wave of the hi'ghffrequency; f1 (165 kilocycles), aswell astheunmodulated carrier of that frequency, received from the line I.

Each of the subscribers statiOnsA, B, C l also includes the telephonel'iook'switoh 29 Iassociated with the handset i8`; a suitable power sup-v ply 3i, such as a batteryor'rectified'ISO-cycle"alf ternating current power source, ia'daptdfor coril nection through switching contacts'onthe hooliff switch '29, V'when thehai'ids'tI' ernvo'vdffrom'f` the hook, to the modulatoros'cifllatforA Ziilandlt'he high* vfrequernzy amplieri 12 l "in "the transmitting .u

vcircuit it; a conventioaal'caiung 'uia'r 32" con!" www.

nected :in thek power supply of the modulatoroscillator and a normally closed monitor key 33 which Vmay be opened by the subscriber to open the` power supply circuiti (from battery 3|) to disable transmitting circuit |5 when he desires only to monitor (listen-in) on the transmission over the power line I'. Each of these stations also includes the electromagnetic switching relays R| and R2 controlled from the output of the demodulator 28 in receiving circuit, IG, and station signaling apparatus including a voice frequency ringer 34 and a busy signal lamp 35 controlled from theserelays in a manner which will be described in detail later in connection with a description of the operationfof the complete system.

The carrier terminal station CT comprises a transmitting circuit 3G and two receiving circuits 3`| and 38. The ouput of the transmitting circuit 36 and the inputs of the two receiving circuits 3'l and 38 are connected in parallel with each other and through the common coupler device I0 to the'power line I. The input of the transmitting circuit 36 and the outputs ofthe two receiving circuits 3l and 38 are ladapted for connection by means of the hybrid coil 39 and associated normal line balancing network 45 to the two-wire voice frequency line 4| leading to the central telephone switching oiiice CO for the subscriber stations on the power line. The central office CO, which may be of the manual or automatic type, may be utilized also for connecting the telephone subscriber stations on the power line to any of the telephone subscriber stations outside the carrier system in an associated telephone network. The central office CO is normally disconnected from the carrier terminal station CT due to the normal break in the line 4| provided through parallel connected, normally open switching contacts on the "control switching relays R3 and R4, respectively. The operation of either of these relays to close its contacts will enable the line 4| in its output so that the central cnice CO will be connected in transmission relation with the carrier terminal CT through the hybrid coil 39. An alternate line balancing network 44 is adapted to be connected to the network terminal of the hybrid coil 39 in place of the normal balancing network 43 by operation of the control relay R4 I when the carrier terminal CT is receiving carrier oscillations on the reverting frequency f3 (185 kilocycles) in the manner which will be described later in connection with the description of operation of the whole system.

The alternate balancing network 44 may comprise a single terminating resistor or other simple network such as to provide suitable unbalance of the hybrid coil 39 so that voice signals may be transmitted through the latter from the receiving circuit 38 to the transmitting circuit 36 on a reverting call, with as little loss and distortion as possible, and to provide substantially the same degree of loss for all frequencies in the voice frequency band. The normal balancing network 43, on the other hand, should comprise a more elaborate network suitable for providing adequate balance over the voice frequency range of any one of the different impedance lines which may be' connected to the terminal by the associated central office CO, and ifi-particular is designed to provide the best possible balance on long distance central office calls.

The transmitting circuit`36 of the carrier termlnal CT includes in order between the hybrid coil 39 and the -vcoupler devicev |07, reading from left to right, a `voice frequency amplifierY andra.

modulator with an associated oscillator for supplying thereto carrierv oscillations of the frequency f1 165 kilocycles), represented by the box 45; and an automatic voice frequency volume limiter 46 which may be of any of the well-known types, associated with the voice frequency amplifier for controlling the gain of the latter so as to limit the volume level of the Voice frequency signals applied to the modulator to that value which will provide adequate modulation on long distance incoming ca llsf4 from outside subscribers while preventingovjerloading of the modulator on incoming calls from nearby outside subscribers or subscriber stations on the power line; a high frequency amplifier 41 for amplifying the Waves in the output of the modulator; and the transmitting band-pass filter 48 designed to selectively transmit one or both signal side-band products as well as the unmodulated carrier component of the frequency f1 (165 kilocycles) produced in the output of the modulator. The voice frequency amplifier,l modulator and carrier oscillator included in the box 45 may comprise sept arate elements or may be combined in a single multigrid electron discharge tube as is wellknown in the art.

The receiving circuit 31 in the carrier terminal CT includes in order between the coupler device l0 and the hybrid coil 39, reading from right to left, the receiving band-pass lter 49 adapted for selectively transmitting the speech-modulated and unmodulated carrier waves of the normal frequency ,fz kilocycles) received over the line from the subscriber carrier stations; the high frequency amplifier 55, which may comprise one or more Vacuum tube amplifying stages with associated interstage and output band-pass iilters for tuning it over the frequency band transmitted by the. band-pass iilter 49; an automatic Volume control circuit 5| of any of the wellknown types associated with this amplifier for controlling its gain so asvto compensate for variations in the amplitude levels of the applied carrier signals due to loss variations on the power line I; and the demodulator 52 for demodulating the applied`high frequency modulated carrier waves to voice frequencies, which demodulator may be of any of the well-known types, such as one comprising copper-oxide or crystal rectiers.

The receiving circuit 38 includes in order between the coupler device l0 and the hybrid coil 39, reading from right to left, the receiving band- ,Y pass lter 53 adapted for selectively transmitting the speech modulated and unmodulated carrier waves of the reverting frequency f3 (185 kilocycles) which maybe received over the line from the subscriber carrier stations; a high frequency amplifier 54, which may comprise one or more vacuum tube stages tuned to amplify elnciently the band of frequencies transmitted by the band-pass filter 53; an automatic volume control 55, similar to the automatic Volume control circuit 5|, associated with amplifier 54; and the demodulator 55, similar to the demodulator 52 in the receiving circuit 31, for demodulating the applied high frequency signal waves to voice frequenc1es.

The output of the Vreceiving circuit 38 is normally disabled due to the normal break in the circuit connections between the demodulator 56 and the hybrid coil 39 provided by one set of normally open switchingcontacts of the unoperated relay R4. The switching relay R3 is conthat station, the operation of. the relay RI by the received continuous carrier of .frequency f1 will cause the associated control relay R2 to be operated from battery 3|,v and then locked operated from that battery through theoperated righthand front contacts. of. the latter-relay. The closure of the leftlhandcontacts of operated relay R2 will cause the .busy signal lamp 35 at the station to be lighted from battery 62. Thus, all other subscriber stationsB, C.

In a dial area,y the .dialtone automatically -transmitted over the voice frequency line lll from the central office CO to the carrierterminal CT in response to the closure of the voice frequency line 4I at thatterminal byrelay R3 controlled by the rectified carrierof .frequency f2 from the calling station A,.is applied by hybrid coil 39 to the input of the oscillator-modulator to modulate the outgoing carrier of frequency Vf1 (165 kilocycles) supplied to lineI. .In -a manual area, speech fromthe. operatorfat .the central office CO is transmitted overf-ftheyoice frequency line liI and applied .through-hybrid coil 39 to the oscillator-modulator .45` -to modulate the outgoing carrier of .frequencyjr .Atthe calling station A, the dial tone or operatorfs speech signalsY modulated on the carrier .f1-will pass through the coupler device 5lv andreceiving-circuit I6 at the calling subscribers station (station A) to the demodulator 28 in :which they'will. be demodulated to voice frequencies and-will be Vheard by the calling subscriberin histelephone receiver I9.

The subscriber atthecalling station A then talks into his transmitter. to tellthercentral oice operator to which outsidesubscribers station he wishes to be connected, or `operates his calling dial 32 in the usual manner to dial the number of the station being called.` In theformer case, As signals will be combined `inhis-modulatoroscillator 26 with the carrier oscillations of the normal frequency f2 (195 kilocycles) and the modulated carrier will be transmitted out over thelinel to the demodulator 52 inthe receivingcircuit 31 0f the 4carrier terminal station. CT.-as. previously described for the unmodulated.carrier.A The demodulated voice signalsofA. in `the output of the demodulator 52 will pass through the hybrid coil 39 and voice frequency lined! (maintained closed by the continued Voperationof -relay.R3) to the operator in the central oilce CO.-

When the subscriber at the. calling station A dials the number to be called on his dialing device 32, this operation .causes the. transmitted carrier of frequency f2 (195 kilocycles) to be interrupted in accordance` with the digits dialed. The control relay R3 associated with the demodulator 52 in the receiving .-circuit 31 at the carrier terminal CT follows thereceived 195 kilocycle pulses, the corresponding closures and releases of its left-hand contacts; operating to relay the dialing pulses onto. the central Yofce over the voice frequency line 4I where they direct the switches (not shown)..in.the.selection of the called party.

The` central oilce operator or themachine switching equipmentinjthe .central oiiice'calls the called subscriber. on the -associated network in the usual manner, and sets up a talking circuit between the called subscriber. and Vcalling subscriber A on the powerf line through thecentral office CO, so thattheycan-converse with each other. f. i

The voice-signalsfof the :called outside subscriber Will be transmitted at voice .frequencies froml the central oiiiceCO'vov'er'the Vvoice frequency line 4I to the carrier terminal station CT CPI are informed l:- Ythat the line I is being.. utilized on a call.

and will be impressed by the hybrid coil 39 on the transmitting circuit 36 at that terminal. In

- that circuit, the called subscribers voice signals will be ampliedland then modulated in the device 45, which is maintained energized by the continued operation of relay R6, on the carrier oscillations of the frequency f1 (165 kilocycles), and 'the modulated carrier wave will be amplified in Vthe high frequencyramplier 41 and passed out through the transmittingband filter 48'and the coupler device II) to the power line over which it Will be transmitted tothe calling subscribers station AA. At the latter station,'the voice-modulated carrier wave of frequency f1 (165 kilocycles) will pass through the coupler device() and the plug-in resistance pad 24 and will be selected by the receiving band filter 25 in the receiving circuit I6 at that station. The selected signalsl of the outside subscriber .will be amplified in amplifier 26, demodulated inthe demodulator 28 and will be heard by the subscriber A in `his telephone receiver I9. Y. f

The voice signals of the calling subscriber at station A will be transmitted from his station as modulations of his normal carrier frequency fz kilocycles) over the rline I to the demodulator 52 in the receiving circuit 31 of the carrier terminal CT inthe manner previously described for the unmodulated carrier. of that frequency. As voice signals in the output of the demodulator will be divided by the hybrid coil 39 between the voice frequency line 4I and the transmitting circuit 36 of the carrier terminal. The portion entering voice frequency line 4| will pass over that line through the central roce CO to the called subscribers line. The other portion of As voice signals will be modulated in the transmitting circuit I6 on the transmitted carrier Wave of the frequency f1 (165 kilocycles), and the resulting voice modulated carrier will be returned over the power line I. to the calling subscriber's station where it will be selected, amplied, demodulated and heard in its receiver I 9 as side tone. f

At the end of the'call, the subscriber at station A will hang his handset I8 up on hook switch 29 `causing the opening of switching contacts in the latter to disconnect the power supply 3| from the modulator-oscillator 26 and high frequency amplier 2 I. This will stop the transmission of the carrier frequency f2 (195 kilocycles) over the line I. At the carrier terminal CT, the cessation of the received carrier f2 will cause the relay R3 to release to open the voice frequency line closure at the terminal, and the subsequent release of the relay R6 will remove the energizing power supply from the apparatus in the transmitting circuit 36 to stop the transmission of the outgoing carrier frequency f1 and will return the other circuits in the carrier terminal to normal.

Operation on a central office call originated through the central o'ce On a central office call originated through the central office CO to one of the subscriber stations A, B, C on the power line I, the operator or machine switching equipment in that oice applies coded ringing current, say of 20 cycles per second, between one side of the voice frequency line 4I and ground. At the carrier terminal station CT, this current is rectified in the rectifier 51 causing the operation of the associated relay R5, while the supply of ringing current continues, to

carrelages :connect an-.-.energizing. circuit from battery'ill'fto .thenvoice :frequency amplifier yoscillator-modulator w15 and '.the`1high frequency amplier' 4l in the `transmittingcircuit :3.5 at the terminal condition- I.ing that transmitting circuit to send out amplied carrier oscillations .of the frequency ,f1 .(165 kilocycles) through ithe :coupler device Iii to the line; and also to connect the voice irequencyline BI to f the'finputo'f the oscillator-modulator 45 lsothat `the oodedjZO-'cycle ringing signals from .the cen- 10 tral nfce will fbesupplied to the modulator to "modulate, the carrier of' frequency f1' (165 kilocycles) transmitted out` over the line I` during the'duration of the applied ringing. tone. The

-carrier'sofzfrequency f1 '(165 kilocycles) modu- 15 "lated With-.theZO-cycle ringing signals is diverted through the coupler devices 9 into the receiving circuit I6 'of'. all subscribers stationsv A, B, Cv on the power line I. In the receiving circuit'vl at veach subscribers station this ringing signal- 20 ymodulated carrierfr (165I kilocycles) will be passed through the pad 24 and the receiving band iilter "25, amplied iin amplier 26 and demodulated'in'demolulator 428 causing the operation of the associated -control r'elay- RI at the 20-cyc1e 25 "ratawhichinturn will lcause the voice frequency yringer 34 at "theistation to be operated at the 20- cyclezcoded rate. AA .circuit for operating' the ringerJ 34 isclosed by charging the condenser 63 in series with the ringer 34 when relay RI is oper- 30 ated. The condenser 63` is discharged through the ringe1t34`during the periods in which the relayRI is released.

Thesubscriber at the called station on the pow- Iremovingthe'handset I'from the hook switchV 29 at hisv station. f' ThisA will `cause the modulatoroscillator`2c and :highV frequency amplifier 2l in his 'transmitting circuit `I5 to be energized from battery 3i througlroperated switching contacts 40 onY the'iho'ok switch 2Q, andthe operating circuit .for the'control relayRZ from battery 3i to be opened bythe opening of other associated switching .contacts on the hook switch 25. If the relay therelay through its locking contact, so as to connect lthe 185-kilocycle crystal Y2 in the ire- 'quency-determining#circuit of the modulatoroscillator 20Tin placeV of 'the normal 195-kilocycle crystalYI l, the yreverting carrier frequency fa (185 -kilocy'clesl may be-1nomentarily applied to the poweriline through the transmitting circuit I5 and coupler device 9. This Vmomentary spurtof carrier `frequency f3f (185 kilocycles) will have no eiectV on the switching circuits at the carrier ter- `155 rminal CT` inasmuch as theV switching relay R4 associated with the demodulator 5E in the receiving circuit" 3B of' that terminal which selects that frequency,cannot beoperated due to the break in its energizing circuit through the normallyfm open `switching contacts on the control relay R3. i The 4latter-willunot be' operated in the absence of 'the supply ofthe normal carrier frequency f2 (195 line I from the-output of the oscilltaor-modulator 2t in`transmitting circuit i5 of the called subscribers` station.

At the carrier terminalCT, Vthis carrier wave of frequency f2 ('195y kilocycles) will be diverted' from 75 `er `line;recognizinglhis coded ring, answers by 35 v R2 lis'oper'ateclon-'the ringing current supplied to 45 thexlinea throughf. the `*coupler device Iainto :the

carriert'erminal' CTand. will'be selected' bythe -ilter 119 sin; the receiving circuit 31 therein. The

-selected carrier fziwillzbe ampliiedin'amplier 50 and rectied demodulator 52 causing the oper- Vationcoftherassociated relay R3lin the -manner vlpreviously;describedttov provide a: closure. of the .voiceifr'equency line Alito the central oiceLCO, f andtozoperate the control relay RB. The operation vof Ithe relayl R6. causes:- vthe `modulator-oscil- .Llator Iarid -1 voice i frequency amplifier l apparatus (box: 45) indthe transmitting. circuit 351at the, teraminal tocbe. energized from batteryl 6I sothata `scriberslstationsfA, B, C ,.eXcepttheHcalled subsciiberls.` station, ,the received carrierlofr fre- -quency f1 .willfoperate the relay R2.. which in turn :Will causethe busy .lampfSL-atthe station :to be lighted.

...The closure of the voice frequency line-Mat the carrier. 'terminaljCTin responseJ to. the operation :ofthe-control'relayRS. by the. rectied carrier of the; normal. frequency .f2 (195 .kilocycles) will trip the ringingfromthe. centraloiiice. The outside subscriber and the called carrier-subscriber then can communicate with each other over the talkingpaths-which are set yup through the 4central cnice CO.

1When` thecall is completed, and the calledV subscriber-'on thelpower 'linehangs up his handsetfon the-hook; his carrier transmitting vapparatus isdeenergized? `byfthel openingof certain switching contactson the hook switch 29 vbreaking the energizing 'circuitf from battery 3 i and transmission yof the-*normal carrier frequency f2 (195 kilocycles) -overfthe'power lineA I ceases. This causes the re- 'layfR tobe'releasedat the carrier terminal CT. "Ihe release ofthe-relay R3 in lturn causes the release-of the relay'RS, and `the resultant opening of the voice frequency line 4I to the central olce CO transmitsa disconnect signal to that oui-ce.

The relay R51' at the carrier terminal CT isa slow-release relay. During the releasing time of this relay, the relay'PlZ in the called subscribers stationwill vmomentarily operate to light the busy lampiathis-station. 'With the release of the relay R6 at the carrier terminal CT, the apparatus `in thetransmitting circuit at that terminal is delenergized so that the carrier f1 (165kilocycles) Yis removed from vthefline I. "Therefore, the operated ARI vand R2 relays in all of the subscribers stations' A, B, C will Irelease and the busy signal lamps' at these stations-will be extinguished.

Ifa subscriberat'one'of the stations A,B, `C on'the power line. when connected through Vacomrnon battery manual central oice should ash his switchhook. the* circuit operation will be asiollowsr On the rst hang-upof his handset I8, thecarrier transmitting apparatus at his station :will 'bedeenergized This will cause the relay R3 at vthe'carriierV terminal CT tobe released vso as to disconnect operating battery from the relay R6 at that terminal. The relay R5; howevenis slow releasing. ATherefore, the carrier apparatus-in the transmittingl circuit 36 at 'the carrier terminal CT -willbel maintained energized 4to maintain transrnis-sion' of the carrier f1 (165 kilocycles) vover the line: I rfor a long enough time to permit the relay R2 at the subscribers station to be operated and locked so asto revert the transmitting frequency ofthe subscribersstationfrom f2 (195 kilocycles) -to ,f3 "(1851: kilocycles) dueE rto thesubstitution of '15 the crystal Y2'for the crystal YI in the frequencydetermining circuit of the carrier oscillator at that station. This, however, will not result in the operation of the relay R4 at the carrier terminal CT by the received reverting frequency f3, because :the relay R3 at that terminal is already released and its contacts provide a break in the energizing circuit for relay RII from the demodulator 56. When the relay R3 released, it also opened the line loop 4I to the central office CO giving a signal to the operator at that oillce. This condition will be maintained until the relay R5 at the carrier terminal completely releases to deenergize the carrier transmitter at the terminal and remove the outgoing carrier f1 (165 kilocycles) from the line I thus releasing the relay R2 at the carrier subscribers station. The release of the relay R2 at the subscribers. station changes the carrier frequency transmitted from that station from fs (185 kilocycles) to f2 (195 kilocycles) and this frequency is transmitted provided the switchhook is again operated. This causes the relay R3 at the carrier terminal CT to be again operated to close the loop to the central oice CO and thus signal the operator. This operation should tend to make all flashes from the subscriber slow in character.

Reverting call-between carrier subscribers in the same channel on the power Zine To make an inter-party line or reverting call, say to a subscriber at station B, the subscriber at station A will remove his handset i3 from the hook switch 29 causing the transmission apparatus 2D and 2I in his transmitting circuit I5 to be energized so as to transmit his normal transmitting carrier f2 (kilocycles) to the line I The subscriber will then either dial or pass the listed number to the operator at the central office CO by talking in his transmitter to modulate his transmitted carrier of frequency ,f2 (195 kilocycles) in the manner previously described in connection with a central ofce call originated by the subscriber.

The calling subscriber at station A will then hang up, and the operator or machine switching equipment at the central office CO will apply coded Eil-cycle ringing current to the voice frequency line `6I which, in the manner previously described for a central oce call, will cause the carrier transmitter at the carrier terminal CT to be energized to send carrier of the frequency f1 (165 kilocycles) out over the line modulated with the coded -cycle ringing current from the central oce. The carrier f1 modulated with 20- cycle ringing current will be received and detected in the receiving circuits of all of the subscribers sets on the line I causing the operation of the control relay RI at all of these stations to produce operation of the ringer 34 in accordance with the code as previously described.

When the called subscriber at station B answers his coded ring by removing his handset I8 from the switchhook 29, the carrier apparatus in his transmitting circuit I5 is energized so as to send out his normal carrier frequency f2 (195 kilocycles) to the line I and over that line to the carrier terminal CT where it will cause the operation of the relay R3 associated with the demod-ulator 52 in the receiving circuit 3l' which selects the normal carrier frequency. The closure of the line loop 4I to the central ofce CO at the carrier terminal CT by operation of the relay R3 operates to trip the ringing from that office. The operation of the relay R3 at the carrier terminal also causes the operation of the relay R6 which energizes the carrier apparatus in the transmitting circuit 36 at that terminal so that the carrier frequency fi kilocycles) is sent out through the coupler device I9 over the line I. During the ringing phase of the call, the' signaling operations are as described above for a central ofce call originated through the central omce CO.

When the calling subscriber at station A notes that the ringing has stopped and that the called party has answered, indicated by the lighting up of the formers busy signal lamp 35, the calling subscriber at station A again removes his handset I8 from the hook switch 29 causing the oscillator-modulator 29 and high frequency amplifier 2l in the transmitting circuit I5 at his station to be energized from battery 3I through operated switching contacts on the hook switch. At this time, the relay R2 at the calling subscribers station is operated and locked from battery 3i through its operated right-hand front contact due to the previous operation of the relay RI in response to the rectied continuous carrier of the frequency fr (165 kilocycles) received over the line I from the carrier terminal CT. The operated relay R2 through its right-hand back contacts causes the 195-kilocycle crystal YI to be removed from the frequency determining circuit of the modulator-oscillator 29 and the 135- kilocycle crystal Y2 to be substituted therefor in that circuit. Thus, the energized transmitting circuit I5 at the subscribers station A will send out continuously a carrier wave of the reverting frequency f3 (185 kilocycles) while the handset I8 remains olf the hook. This carrier wave will be amplied in the high frequency amplier 2| and will pass through the transmitting band lter 22, plug-in pad 23 and coupler device 9 to the line I over which it Will be transmitted to the carrier terminal CT.

At the carrier terminal CT, the received carrier Wave of the frequency f3 (185 kilocycles) will pass through the coupler device I0 and will be selected by the receiving band filter 53 in the receiving circuit 38. The selected carrier will be amplified by the amplier 54 and be rectified in the demodulator 56. The rectified output of the demodulator 56 will cause the operation of the associated relay R4 which can be operated at this time because the relay R3 which controls the energizing circuit of the latter relay has been previously operated in response to the rectified carrier wave of the frequency fz (195 kilocycles) transmitted from the called subscribers station B, the two relays R3 and RA providing joint holding paths for the called and calling subscribers, respectively. Each relay R3 and Ril by operating provides a separate closure of the voice frequency loop 4I to the central oflice CO and holds relay R6 operated from batteries 58 and 59, respectively. The operation of the relay R4 also closes the connection from the output of the demodulator 56 in the receiving circuit 38 to the hybrid coil 39, and connects the auxiliary balancing network 44 suitable for use on a reverting call to the network terminals of hybrid coil 39 in place of the normal balancing network 4D used on central office calls. Thus a direct talking connection between the calling subscriber A and the called subscriber B is provided through the carrier terminal CT.

The calling subscriber at station A by talking into his transmitter I1 modulates the carrier or" the reverting frequency fa kilocycles) in his oscillator-modulator 20, and the voice-modulated 17 carrier wave of this frequency in amplified form is transmitted through the transmitting circuit l and the coupling device 9 to the power line I and over that line to the demodulator 56 in the receiving circuit 38 at the carrier terminal CT in the manner previously described for the unmodulated carrier wave of that frequency. The demodulated voice signals of subscriber A in the output of the demodulator 56 will be transmitted through the partially unbalanced hybrid coil 39 to the transmitting circuit 36 at the carrier terminal CT, in which it will be remodulated on the carrier frequency f1 (165 kilocycles) in the modulator-oscillator 45 held energized by operated relay R6. The voice-modulated carrier wave of the frequency f1 will then be amplified in the amplifier 41 and will pass out through the transmitting band filter 48 and the coupler device I6 to the power line I over which it will be transmitted to subscriber station B. At the subscriber station B, the voice modulated carrier of frequency f1 will be diverted through the coupier device 9 and equalizer pad 24 into the receiving circuit I6 of the station in which it will be selected by the receiving band filter am- 1 plii'led by the amplifier 26 and passed to the demodulator 28 in which As voice signals will be demodulated and will be heard by subscriber B in his receiver I9.

Similarly, the speech signals from the called subscriber B at his station modulates a carrier wave of the normal frequency f2 (195 kilocycles) in the transmitting oscillator-modulator 26, and the modulated carrier wave will be amplified by high frequency amplifier 2| and transmitted through transmitting band lter 22, plug-in pad 23 and coupler device 9 to the power line I over which it will be transmitted to the carrier terminal CT. At the carrier terminal CT, the carrier wave of frequency fz (195 kilocycles) modulated with subscriber Bs voice signals, will be diverted through the coupler device I0 into the receiving circuit 31 in which it will be selected by the band filter 49, amplified by the amplifier 50 and demodulated to voice frequencies in the demodulator 52. Bs voice signals will pass from the output of demodulator 52 through the partially unbalanced hybrid coil 39 to the transmitting circuit 36 of the carrier terminal in which they will be remodulated on the transmitted carrier wave of the frequency f1 (165 kilocycles) in the oscillator-modulator 45, and the voicemodulated carrier will be passed out in the amplied form to the power line I over which it will be transmitted to the calling station A. At the subscribers station A, the carrier wave of the frequency f1 modulated with Bs voice signals will be diverted through the coupler device 9 and equalizer pad 24' into the receiving circuit I6 in which it will be selected by filter 25, amplified by amplifier 26 and demodulated by demodulator 28, and the demodulated voice signals of B will be heard by subscriber A in his receiver I9.

When the call between subscribers stations A and B is completed, both subscribers will hang up their handsets I8 which will automatically deenergize the modulator-oscillator 26 and high frequency amplifier 2| in each transmitting circuit I5, stopping the transmission from the station of the carrier oscillations of the frequencies f3 and f2, respectively. The cessation in the supply of the carrier waves of the frequencies f2 and f3 at the carrier terminal CT will result in the release of the switching relays R3, R4 and R6 at that terminal, returning the circuit at that 18 terminal to the normal condition shown with the carrier transmitter deenergized and the nor'- mal balancing network 46 associated with the hybrid coil 39. The deenergization of the transmitting circuit at the carrier terminal cutting off the transmission of the carrier frequency f1 (165 kilocycles) will result in the release of the switching relays RI at subscribers stations A and B. The consequent release of relay R2 at the calling subscribers station A will extinguish the busy signal lamp 35 and will cause the crystal Y2 to be disconnected from the frequency-determining circuit of the modulator-oscillator 20 and the crystal YI to be substituted therefor so as to revert the modulator-oscillator 20 at that station to its normal tuning (f2-195 kilocycles).

If, during a reverting or central office call, a third carrier subscriber removes his handset I8 from the swtchhook, the latters transmitter will send out carrier oscillations of the reverting frequency f3 (185 kilocycles) over the line I because of the presence of carrier oscillations f1 on that line, holding the relay R2 at the third subscribers' station locked operated. This will cause beat notes and distortion of the speech between the other parties-engaged in the call, since the transmitted carrier frequency of the third subscriber will in most cases be slightly different from that of thesubscriber previously using the carrier frequency-fs. In spite of this distortion interference, transmission will probably be suiiiciently good to provide a means for explaining the emergency condition which would require this type of break-in by a certain subscriber. Such a breakin will not release the circuit, and transmission will return to normal as soon as the third party hangs up. The operator can always break in on either a central office or a reverting call if requiredby an emergency.

The invention has been described as applied to only one carrier channel on the power line I, but it is apparent that a number of different channels may be operated on the same line by the assignment of three dierent carrier frequencies for each channel, in which case separate carrier terminals associated with the same or different central switching offices would be employed for each individual channel. The apparatus and circuit arrangements at the subscribers stations and carrier terminals associated with the additional carrier channels may be identical with those in the system as described above and illustrated in the drawings, except for the changes required in the carrier oscillators and band lters due to the use of different carrier frequencies for the several channels. Other changes in the circuits illustrated and described which are within the spirit and the scope of the invention will occur to persons skilled in the art.

What is claimed is:

1. In a carrier communication system, a transmission medium and stations coupled to said medium, including a carrier terminal station and a plurality of carrier subscriber stations, said carrier terminal station including a transmitter for transmitting communication signals to said medium on a carrier wave of one frequency and two receivers for respectively detecting communication signals received over said medium on a carrier wave of a second frequency and a carrier wave of a third frequency, each of said carrier subscriber stations including a transmitter normally conditioned for transmitting to said medium on a carrier wave of said second frequency only,V switching meansresponsive to applied carrier waves of said one frequency received over said medium from said carrier terminal station for conditioning the transmitter at the local subscriber station for transmitting to said medium on a carrier wave of said third frequency only and other switching means for preventing operation of the first switching means if said local subscriber station is transmitting to said medium on a carrier wave of said second frequency at the time of arrival of a carrier wave of said one frequency thereat over said medium.

2. In a carrier communication system, a transmission medium and a plurality of stations coupled to said medium, including a carrier terminal station and a plurality of carrier subscriber stations, said carrier terminal station including a transmitter for transmitting communication signals on carrier waves of one frequency to said medium for transmission thereover to all of said carrier subscriber stations and two receivers for respectively detecting communication signals received over said medium on a carrier wave of a second and a third frequency, each of said carrier subscriber stations including a transmitter normally conditioned for operation under control of the subscriber at the station to transmit communication signals to said medium on a carrier wave of said second frequency only, switching means responsive to carrier waves of said one frequency received over said medium to condition the transmitter at the local subscriber station for transmitting to said medium on a carrier wave of said third frequency only anda second switching means operating to disable the first switching means only if the transmitter at the station is made operative to transmit carrier waves of said second carrier frequency to said medium prior to arrival of the carrier waves of said one frequency at the subscriber station.

3. In combination in a carrier communication system, a transmission medium and stations including a carrier terminal station and a plurality of carrier subscriber stations, coupled to said medium, said carrier terminal station including a transmitter for sending communication signals on a carrier wave of one frequency out over said medium to said subscriber stations, and two receivers for detecting communication sig-l nals received over said medium from said subscriber stations on a carrier wave of a second frequency and a third frequency, respectively, each of said subscriber stations including a receiver for detecting communication signals re` ceived over said medium on a carrier wave of said one frequency and a transmitter for transmittingcommunication signals over said medium to said carrier terminal either on a carrier wave of said second frequency or a carrier wave of said third frequency, the latter transmitter being normally conditioned for operation under control of the local subscriber to transmit on a carrier wave of said second frequency only, relay switching means responsive to the wave energy in the output of the local receiver when a carrier wave of said one frequency is received at the station over said medium for conditioning the transmitter at the station for operation under control of the local subscriber to transmit over said medium on a carrier wave of said third frequency only and other switching means controlled by the local subscriber when he seizes control of the local transmitter, only in the absence of operation of the first switching means by carrier wave energy of said one frequency previously received at the -station over said medium, for preventing operation of said first switching means ln response to carrier wave energy of said one frequency subsequently received over said medium.

4. In a carrier communication system, a transmission line, stations coupled to said line including a different group of carrier subscriber stations and an individual carrier terminal station for each carrier channel thereon and a central switching ofce associated with the carrier terminal stations, the individual carrier terminal station for one carrier channel including a normally de-energized signal transmitter which, when energized, is operative to send out communication signals on a carrier wave of one frequency over said line to said subscriber stations and two receivers for detecting communication signals received over said line on a carrier wave of a second frequency or on a carrier wave of a third frequency, respectively, each of the subscriber stations for said one carrier channel including a receiver for detecting communication signals received over said line on a carrier wave of said one frequency, a transmitter normally adapted under control of the subscriber at the station for sending out communication signals over said line on a carrier wave of said second frequency only, relay switching means responsive to the wave output of the local receiver when carrier wave energy of said one frequency is received at the station over said line to condition the transmitter at the station for operation under control of the subscriber thereat to send out communication signals over said line on a carrier wave of said third frequency and means also under control of the subscriber at the station forv eifectively disabling said switching means if he causes the transmitter at the station to be rendered operative prior to the arrival at the station over said line of carrier waves of said one frequency.

'5. The system of claim 4 in which said carrier terminal station for said one carrier channel also includes means responsive to ringing signals received from said central oftice or carrier waves of said second frequency received over said line from one of said subscriber stations for said one channel for energizing the local transmitter means responsive to carrier waves of said third frequency received over said line for holding the transmitter in the energized condition.

6. In a carrier communication system, a main transmission line, a plurality of stations coupled to said line including an individual carrier terminal station and a different group of carrier subscriber stations associated with each carrier channel on said line, a central switching oice for said stations on said line connected lthereto through the carrier terminal stations, and adapted also for connecting outside subscriber communications stations on other lines in communication relation with the carrier subscriber stations in each group on said main line through its individual carrier terminal station, each of the carrier subscriber stations in the group assigned to one carrier channel including a receiver operative to detect communication signals received at the station over said main line modulated on a carrier wave of a frequency f1, a transmitter normally conditioned under control of the subscriber at the station for sending out communication signals modulated on a carrier wave of a second frequency f2 over said main line to the carrier terminal station for that channel on calls between that station and said central oiiice or an outside subscriber station connected through said office, switching means responsive to a continuous carrier wave of the frequency f1 received over said main line from the latter carrier terminal station only if the transmitter at the subscriber station at the time is not transmitting to said main line on a carrier Wave of the frequency f2, to condition that transmitter for sending out under control of the local subscriber communication signals modulated on a carrier wave of a third frequency over said main line, for a reverting call between the local subscriber station and another carrier subscriber station in the same group on the main line initiated at the local station, the carrier terminal station for said one carrier channel including a transmitter operative under control of call vsignals from said central oice or in response to a carrier wave of the frequency f2 received over said main line from one of the carrier subscriber stations associated with said one channel,

to send out a carrier Wave of the frequency f1 over said main line to the carrier subscriber stations, two receivers for respectively selectively receiving the carrier Waves of the frequency fz and fa incoming over said main line and for demodulating the communication signals therefrom and means to supply the demodulated signals to said central oice and to the transmitter in the carrier terminal station for remodulation on the carrier wave of the frequency f1 sent out thereby over said line to said carrier subscriber stations.

7. The system of claim 6, in which said carrier terminal station for said one carrier channel includes means for causing the signal output of the receiver thereof for selectively receiving carrier waves of the frequency f3 to be connected to said central office and to the transmitter at that station only when carrier waves of both frequencies fz and f3 are being received over said line at said carrier terminal station.

8. The system of claim 6, in which said central ofce is connected to the transmitter and two receivers at the carrier terminal station for said one carrier channel through a voice frequency line, a hybrid coil and an associated normal line balancing network suitable for providing adequate balance of any of said other lines which may be connected in communication relation through the central oce with the carrier subscriber stations on said main line, and said carrier terminal station for said one channel includes switching means responsive to a carrier wave of said third frequency received thereat over said main line for substituting an auxiliary balancing network for said normal network, said auxiliary balancing network being suitable for providing a desired amount of unbalance of said hybrid coil so as to enable a suiicient amount of the demodulated signal energy output of said receiver for selectively receiving carrier waves of said frequency f3 to be transmitted through said hybrid coil to the transmitter at the carrier terminal station on a reverting call.

ROBERT C. EDSON. JOHN W. EMLING.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,705,891 Arnold Mar. 19, 1929 1,773,613 Clark Aug. 19, 1930 2,289,048 Sandalls July 7, 1942 2,292,222 Haigis Aug. 4, 1942 2,430,471 Lang Nov. 11, 1947 OTHER REFERENCES Technical Manual 11-600 pages 8; 67-68 March 3, 1943.

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