WO1999053692A1

WO1999053692A1 - Communication apparatus with modulators and oscillators

Info

Publication number: WO1999053692A1
Application number: PCT/US1999/008007
Authority: WO
Inventors: Terry W. Bush
Original assignee: Trilithic, Inc.
Priority date: 1998-04-13
Filing date: 1999-04-13
Publication date: 1999-10-21
Also published as: AU3491799A

Abstract

A communication method and apparatus comprise providing at least one source of program material, at least one carrier frequency signal source (...11, 13, 15, 17, ..., 11', 13', 17', ..., 11'', 13'', 17'',...) and at least one first modulator (...20, 22, 24, 26,..., 20', 22', 26', ..., 20'', 22'',...) for modulating the at least one carrier frequency signal source (...11, 13, 15, 17, ..., 11', 13', 17', ..., 11'', 13'', 17'',...) with the at least one program material source (... 10, 12, 14, 16, ..., 10', 12', 16', ..., 10'', 12'', 16'', ...). the at least one first modulator (...20, 22, 24, 26, ..., 20', 22', 26', ..., 20'', 22'',...) has a first input, a second input and a first output. At least one terminal apparatus (...p, ...q, ...r, ...60, ...62, ...64, ...p', ...q', ...r',...) is provided for demodulating program material-modulated carrier frequency signals. The at least one program material source (...10, 12, 14, 16, ..., 10', 12', 16', ..., 10'', 12'', 16'', ...) is coupled to the at least one first input. The at least one carrier frequency signal source (...11, 13, 15, 17, ..., 11', 13', 17', ..., 11'', 13'', 17'', ...) is coupled to the at least one second input. The at least one first output is coupled through a device (40) to the at least one terminal apparatus (...p, ...q, ...r, ...60, ...62, ...64, ...p', ...q', ...r',...). First (42, 42') and second (44, 44') sources of signals have first and second frequencies, respectively. The first and second frequency signals are alternately coupled to the device (40). The first output terminal is coupled to the device (40). An output of the device (40) is coupled to the at least one terminal apparatus (...p, ...q, ...r, ...60, ...62, ...64, ...p', ..q', ...r', ...).

Description

-1- COMMUNICATION APPARATUS WITH MODULATORS AND OSCILLATORS

Background of the Invention

This invention relates to a communication method and apparatus. It is disclosed in the context of a signaling method and apparatus for CATV systems, but is believed to be useful in other applications as well.

The use of amplitude modulation to monitor and/or control certain aspects of the operation of CATV systems is known. There are for example the systems and methods illustrated and described in U.S. Patent 5,608,428 and co- pending U.S.S.N. 08/690,122 filed July 31, 1996. This application is related to those applications, and the disclosures of those applications are incorporated in this application by reference.

Disclosure of the Invention According to one aspect of the invention, a communication system comprises at least one source of program material, at least one carrier frequency signal source and at least one first modulator for modulating the at least one carrier frequency signal source with the at least one program material source. The at least one first modulator has a first input, a second input and a first output. At least one terminal apparatus is provided for demodulating program material-modulated carrier frequency signals. The at least one program material source is coupled to the at least one first input. The at least one carrier frequency signal source is coupled to the at least one second input. The at least one first output is coupled to the at least one terminal apparatus through a device. First and second sources of signals having first and second frequencies, respectively, are provided. A controller alternately couples one of the first and second frequency signals to the device. An output of the device is coupled to the at least one terminal apparatus.

According to another aspect of the invention, a communication system comprises at least one source of program material, at least one carrier frequency signal source and at least one first modulator for modulating the at least one carrier frequency signal source with the at least one program material source. The at least one first modulator has a first input, a second input and a first output. At least one -2- terminal apparatus is provided for demodulating program material-modulated carrier frequency signals. The at least one program material source is coupled to the at least one first input. The at least one carrier frequency signal source is coupled to the at least one second input. The at least one program material source is coupled to the at least one first input through a device having at least third and fourth inputs and a second output. First and second sources of signals have first and second frequencies, respectively. A controller alternately provides one of the first and second frequency signals to the device. The second output is coupled to the first input. The first output is coupled to the at least one terminal apparatus. Illustratively according to this aspect of the invention, the device comprises a device for coupling the at least one first audio modulator to the at least one terminal apparatus.

According to another aspect of the invention, a communication system comprises at least one source of program material, at least one carrier frequency signal source and at least one first modulator for modulating the at least one carrier frequency signal source with the at least one program material source. The at least one first modulator has a first input, a second input and a first output. At least one terminal apparatus is provided for demodulating program material-modulated carrier frequency signals. The at least one program material source is coupled to the at least one first input. The at least one carrier frequency signal source is coupled to the at least one second input. The at least carrier frequency signal source is coupled to the at least one first input through a device having at least third and fourth inputs and a second output. First and second sources of signals have first and second frequencies, respectively. A controller alternately provides one of the first and second frequency signals to the device. The second output is coupled to the first input. The first output is coupled to the at least one first modulator.

Illustratively according to this aspect of the invention, the device comprises a device for coupling the at least one first audio carrier frequency signal source to the at least one first audio modulator. Illustratively according to these aspects of the invention, the controller comprises a controller for encoding an instruction in the alternate coupling of the first and second sources of signals to the device. -3-

According to another aspect of the invention, a communication method comprises providing at least one source of program material, at least one carrier frequency signal source and at least one first modulator for modulating the at least one carrier frequency signal source with the at least one program material source. The at least one first modulator has a first input, a second input and a first output. At least one terminal apparatus is provided for demodulating program material-modulated carrier frequency signals. The at least one program material source is coupled to the at least one first input. The at least one carrier frequency signal source is coupled to the at least one second input. The at least one first output is coupled through a device to the at least one terminal apparatus. First and second sources of signals having first and second frequencies, respectively, are alternately coupled to the device. The first output terminal is coupled to the device. An output of the device is coupled to the at least one terminal apparatus.

According to another aspect of the invention, a communication method comprises providing at least one source of program material, at least one carrier frequency signal source and at least one first modulator for modulating the at least one carrier frequency signal source with the at least one program material source. The at least one first modulator has a first input, a second input and a first output. At least one terminal apparatus is provided for demodulating program material-modulated carrier frequency signals. The at least one program material source is coupled to the at least one first input through a device having at least third and fourth inputs and a second output. The at least one carrier frequency signal source is coupled to the at least one second input. The at least one first output is coupled to the at least one terminal apparatus. First and second sources of signals having first and second frequencies, respectively, are alternately coupled to the device. The second output is coupled to the first input. The first output is coupled to the at least one terminal apparatus.

According to another aspect of the invention, a communication method comprises providing at least one source of program material, at least one carrier frequency signal source and at least one first modulator for modulating the at least one carrier frequency signal source with the at least one program material source. The at least one first modulator has a first input, a second input and a first output. At least -4- one terminal apparatus is provided for demodulating program material-modulated carrier frequency signals. The at least one carrier frequency signal source is coupled to the at least one first input through a device having at least third and fourth inputs and a second output. The at least one program material source is coupled to the at least one second input. The at least one first output is coupled to the at least one terminal apparatus. First and second sources of signals have first and second frequencies, respectively. The first and second frequency signals are alternately coupled to the device. The second output is coupled to the first input. The first output is coupled to the at least one terminal apparatus. Illustratively according to this aspect of the invention, the device comprises a device having at least third and fourth inputs and a second output for coupling the at least one carrier frequency signal source to the at least one first input.

Illustratively according to the several aspects of the invention, alternately coupling one of the first and second frequency signals to the device comprises encoding an instruction in the alternate coupling of the first and second frequency signals to the device.

Additionally illustratively according to the several aspects of the invention, the at least one terminal apparatus comprises at least one terminal apparatus for detecting the first and second frequencies, discriminating between them, and decoding the instruction based upon such discrimination.

Further illustratively according to the several aspects of the invention, the at least one terminal apparatus comprises at least one terminal apparatus including automatic gain control (AGC) means having an AGC bandwidth.

Illustratively according to the several aspects of the invention, the first and second sources of signals comprise first and second sources of signals having frequencies in the AGC bandwidth.

Additionally illustratively according to the several aspect of the invention, the first and second signals having frequencies in the AGC bandwidth comprise first and second signals having frequencies less than about 35 Hz. Further illustratively according to the several aspects of the invention, the terminal apparatus comprises a television receiver. -5-

Illustratively according to the several aspects of the invention, the at least one source of program material comprises at least one source of video program material and at least one source of complementary audio program material, the at least one carrier frequency signal source comprises at least one video carrier frequency signal source and at least one complementary audio carrier frequency signal source, and the at least one first modulator comprises at least one first video modulator for modulating the at least one video carrier frequency signal source with the at least one source of video program material and at least one first audio modulator for modulating the at least one complementary audio carrier frequency signal source with the at least one source of complementary audio program material.

Additionally Illustratively according to the several aspects of the invention, the device comprises a device for coupling the at least one first audio modulator to the at least one terminal apparatus.

Brief Description of the Drawings

The invention may best be understood by referring to the following description and accompanying drawings which illustrate the invention. In the drawings:

Fig. 1 illustrates a highly simplified block diagram of a system constructed according to the present invention;

Fig. 2 illustrates a highly simplified block diagram of another system constructed according to the present invention;

Fig. 3 illustrates a highly simplified block diagram of another system constructed according to the present invention; and Fig. 4 illustrates a highly simplified block diagram of another system constructed according to the present invention.

Detailed Descriptions of Illustrative Embodiments

Referring now particularly to Fig. 1, a CATV system 8 includes several program material sources . . . 10, 12, 14, . . . 16 . . . such as satellite links, VCR's and the like at a head end 18 of the CATV system 8. Sources . . . 10, 12, 14, . . . 16 . . . are coupled to input ports of respective modulators . . . 20, 22, 24, . . . 26 . . . . The -6- modulators modulate the source . . . 10, 12, 14, . . . 16 . . . signals onto channel carrier frequencies provided from channel carrier frequency oscillators . . . 11, 13, 15, . . . 17, . . ., respectively, for placement on a CATV circuit 29. The output ports of channel modulators . . . 20, 22, 24, . . . are coupled through respective power couplers . . . 30, 32, 34, . . . to circuit 29. The output port of an arbitrary one, 26, of the channel modulators, however, is coupled to an input port of a variable attenuator, or amplitude modulator, 40. Two oscillators 42, 44 have output ports 46, 48 on which appear signals having distinct frequencies of, for example, 1 OHz and 20Hz. These signals are coupled from ports 46, 48 to input ports of a combiner 50, the output port of which is coupled to a control input port 52 of amplitude modulator 40.

An output port of amplitude modulator 40 is coupled through a power coupler 54 to circuit 29. In this way, the signals from oscillators 42, 44 amplitude modulate the channel m carrier and are placed on circuit 29 alternately and under the control of a system controller 58, without occupying otherwise revenue-producing bandwidth of the circuit. The system controller will typically include a personal computer (PC) or mainframe computer, depending upon the requirements of a particular CATV system 8. The two modulating frequencies provided by oscillators 42, 44 permit addresses, instructions and data to be send in binary code in a well known manner. The percentage modulation produced by amplitude modulator 40 in the output signal from modulator 26 is very low, typically under 20%, which results in only about 2dB or so of modulation of the source 16 signal applied to circuit 29. Because the modulating frequencies are within the automatic gain control (AGC) bandwidth of all of the subscribers' television receivers . . . 60, . . . 62, . . . 64, . . . along the circuit 29, those frequencies are not detectable in the programs reproduced on receivers . . . 60, . . . 62, . . . 64, . . .. The subscriber terminal apparatus . . . p, . . . q, . . . r, . . . are separately addressable by the system controller 58. Addresses and messages can be sent through the circuit 29 by frequency shift keying (fsk) the amplitude modulating frequency between the output frequency of oscillator 42 and the output frequency of oscillator 44. Because the output frequencies of oscillators 42, 44, are quite low

(recall that these frequencies are within the AGC bandwidth of receivers . . . 60, . . . 62, . . . 64, . . . ) the data rate of a system so configured is also quite low. There are -7- different ways in which this limitation can be addressed. For example, higher frequencies can be chosen for oscillators 42, 44. However, if these frequencies are outside the receivers' . . . 60, . . . 62, . . . 64, . . . AGC bandwidths, they have the potential to affect adversely the reproduction by receivers . . . 60, . . . 62, . . . 64, . . . of the programs modulated on the various channel carriers. Consequently, higher frequencies from oscillators 42, 44, while they increase the data rate for signals sent by this technique, also have to be carefully separated by the terminal apparatus . . . p, . . . q, . . . r, . . . from the signals passed to their respective receivers . . . 60, . . . 62, . . . 64, . . . for display, in order to prevent the oscillator 42, 44 frequencies from interfering with the video display or audio program reproduction by receivers . . . 60, . . . 62, . . . 64, . . . .

Another way to achieve higher data rates than can be achieved using oscillators 42, 44 having output frequencies outside the AGC bandwidth, while at the same time reducing the likelihood of interference by the oscillator 42, 44 output frequencies with the video display or audio program reproduction, is illustrated in Fig. 2. CATV system 8' includes several video program material sources . . . 10', 12', . . . 16' . . . and audio program material sources . . . 10", 12", . . . 16" . . . such as satellite links, VCR's and the like at a head end 18' of the CATV system 8'. Sources . . . 10', 12', . . . 16' . . . 10", 12", . . . 16" . . . are coupled to input ports of respective modulators . . . 20', 22', . . . 26' 20", 22", . . . 26" The modulators modulate the source . . . 10', 12', . . . 16' 10", 12", . . . 16" . . . signals onto respective channel intermediate frequencies provided from channel intermediate frequency oscillators . . . 11', 13', . . . 17', . . . 11", 13", . . . 17", . . ., respectively, for placement on a CATV circuit 29'. The output ports of channel modulators . . . 20', 22', 20", 22", . . ., respectively, are combined in respective combiners/output frequency converters. . . 21, 23, . . . and coupled through respective power couplers . . . 30, 32, . . . to circuit 29'. The output port of an arbitrary one, 26", of the audio channel modulators, however, is coupled to an input port of a variable attenuator, or amplitude modulator, 40. Two oscillators 42', 44' have output ports 46', 48' on which appear signals having distinct frequencies of, for example, 25 KHZ and 30 Khz. These signals are coupled from ports 46', 48' to input ports of a combiner 50', the output port of which is coupled to a control input port 52 of amplitude modulator 40. The signal -8- from the output port of modulator 40 is combined in a combiner/output frequency converter 27 with the modulated video signal appearing at the output port of channel video modulator 26' and coupled through a power coupler 36 to circuit 29'. The oscillator 42', 44' output frequencies in this embodiment are above the upper limit of the audio program material which lies in, for example, a 50 Hz-20KHz band. The oscillator 42', 44' output fsk frequencies can readily be separated from the audio program frequencies and from each other, for example, by appropriately designed filters in terminal apparatus . . . p', . . . q', . . . r', . . . and detected to provide the binary address and/or data stream. Referring now particularly to Fig. 3, a CATV system 308 includes several program material sources . . . 310, 312, 314, . . . 316 . . . such as satellite links, VCR's and the like at a head end 318 of the CATV system 308. Sources . . .

310, 312, 314, . . . are coupled to input ports of respective modulators . . .320, 322, 324, . . .. The modulators modulate the source . . . 310, 312, 314, . . . signals onto channel carrier frequencies provided from channel carrier frequency oscillators . . .

31 1, 313, 315, . . ., respectively, for placement on a CATV circuit 329. The output ports of channel modulators . . . 320, 322, 324, . . . are coupled through respective power couplers . . . 330, 332, 334, . . . to circuit 329. The output port of an arbitrary one, 316, of the program material sources is coupled to an input port 343 of a combiner 350. Two oscillators 342, 344 have output ports 346, 348 on which appear signals having distinct frequencies of, for example, the channel carrier frequency f_c of a channel m carrier frequency oscillator 317 plus 4.5 MHz minus 25 KHz, and f_c plus 4.5 MHz minus 30KHz. These signals are alternately coupled under the control of a system controller 358 from ports 346, 348 to input ports of combiner 350. The output port of combiner 350 is coupled to a control input port 352 of a channel modulator 326. Channel carrier frequency oscillator 317 is coupled to a carrier frequency input port of channel modulator 326.

An output port of channel modulator 326 is coupled through a power coupler 354 to circuit 329. In this way, the signals from oscillators 342, 344 are combined with the channel m program material and are placed on circuit 329 alternately and under the control of system controller 358. The system controller will typically include a personal computer (PC) or mainframe computer, depending upon -9- the requirements of a particular CATV system 308. The two modulating frequencies provided by oscillators 342, 344 permit addresses, instructions and data to be send in binary code in a well known manner. The frequencies of oscillators 342, 344 must lie in positions in the program material bandwidth where they will not disrupt the playback of information to the viewers of television receivers . . . 360, . . . 362, . . . 364, . . . along the system 308. The subscriber terminal apparatus . . . 361, . . . 363, . . . 365, . . . are separately addressable by the system controller 358 and are capable of removing the f_c plus 4.5 MHz minus 25 KHz, and f_c plus 4.5 MHz minus 30KHz signalling frequencies from the signals coupled to their respective receivers . . . 360, . . . 362, . . . 364, Addresses and messages can be sent through the circuit 329 by frequency shift keying (fsk) the signalling frequency between the output frequency of oscillator 342 and the output frequency of oscillator 344.

Referring now particularly to Fig. 4, a CATV system 408 includes several video program material sources . . . 410, 412, . . . 416 . . . and audio program material sources . . . 410', 412', . . . 416' . . . such as satellite links, VCR's and the like at a head end 418 of the CATV system 408. Sources . . . 410, 412, . . . 416 . . . 410', 412', . . . 416' . . . are coupled to input ports of respective video and audio modulators

. . . 420, 422, . . . 426 420', 422', . . . The modulators modulate the source . . .

410, 412, . . . 416 410', 412', . . . signals onto respective channel video and audio intermediate frequencies provided from oscillators . . . 411, 413, . . . 417, . . . 411', 413', . . ., respectively, for placement on a CATV circuit 429. The output ports of modulators . . . 420, 422, 420', 422', . . ., respectively, are combined in respective combiners/output frequency converters. . . 421, 423, . . . and coupled through respective power couplers . . . 430, 432, . . . to circuit 429. Two oscillators 442, 444 have output ports 446, 448 on which appear signals having distinct frequencies of, for example, 25 KHZ and 30 Khz. These signals are coupled from ports 446, 448 to input ports of a combiner 450, the output port of which is coupled to a control input port 452 of an amplitude modulator 440. The output port of an arbitrary one, 417', of the audio intermediate frequency oscillators is coupled to another input port of amplitude modulator 440. The intermediate frequency output of oscillator 417' is amplitude modulated in modulator 440 alternately by the frequency of oscillator 442 and the frequency of oscillator 444 -10- under the control of a system controller 458. System controller 458 illustratively includes a personal computer (PC) or mainframe computer, depending upon the requirements of a particular CATV system 408 The signal from the output port of modulator 440 is coupled to an input port of an audio modulator 426'. The audio to combine with the video from video program material source 416 is provided to another input port of audio modulator 426'. The thus-modulated audio is combined in a combiner/output frequency converter 427 with the modulated video signal appearing at the output port of channel video modulator 426 and coupled through a power coupler 436 to circuit 429. The oscillator 442, 444 output frequencies in this embodiment alternately amplitude modulate the audio carrier frequency provided by oscillator 417'. The oscillator 442, 444 output fsk frequencies can readily be separated from the audio program frequencies and from each other by appropriately designed terminal apparatus . . . 461, . . . 463, . . . 465, . . . associated with respective television receivers . . . 460, . . . 462, . . . 464, . . . and detected to provide the binary address and/or data stream.

Claims

-1 1- Claims:

1. A communication system comprising at least one source of program material, at least one carrier frequency signal source and at least one first modulator for modulating the at least one carrier frequency signal source with the at least one program material source, the at least one first modulator having a first input, a second input and a first output, and at least one terminal apparatus for demodulating program material-modulated carrier frequency signals, the at least one program material source being coupled to the at least one first input, the at least one carrier frequency signal source being coupled to the at least one second input, the at least one first output being coupled to the at least one terminal apparatus through a device, first and second sources of signals having first and second frequencies, respectively, a controller for alternately coupling one of the first and second frequency signals to the device, an output of the device being coupled to the at least one terminal apparatus.

2. The apparatus of claim 1 wherein the controller comprises a controller for encoding an instruction in the alternate coupling of the first and second sources of signals to the device.

3. The apparatus of claim 2 wherein the terminal apparatus comprises apparatus for detecting the first and second frequencies, discriminating between them, and decoding the instruction based upon such discrimination.

4. The apparatus of claim 1 wherein the at least one terminal apparatus includes at least one terminal apparatus having an automatic gain control (AGC) bandwidth.

5. The apparatus of claim 4 wherein the first and second sources of signals comprise first and second sources of signals having frequencies in the AGC bandwidth.

6. The apparatus of claim 5 wherein the controller comprises a controller for encoding an instruction in the alternate coupling of the first and second frequency signals to the device.

7. The apparatus of claim 6 wherein the terminal apparatus comprises apparatus for detecting the first and second frequencies, discriminating between them, and decoding the instruction based upon such discrimination. -12-

8. The apparatus of claim 7 wherein the terminal apparatus comprises a television receiver.

9. The apparatus of claim 8 wherein the first and second sources of signals comprise first and second oscillators having output signal frequencies less than about 35 Hz.

10. The apparatus of claim 1 wherein the at least one source of program material comprises at least one source of video program material and at least one source of complementary audio program material, the at least one carrier frequency signal source comprises at least one video carrier frequency signal source and at least one complementary audio carrier frequency signal source, and the at least one first modulator comprises at least one first video modulator for modulating the at least one video carrier frequency signal source with the at least one source of video program material and at least one first audio modulator for modulating the at least one complementary audio carrier frequency signal source with the at least one source of complementary audio program material.

11. The apparatus of claim 10 wherein the device comprises a device for coupling the at least one first audio modulator to the at least one terminal apparatus.

12. The apparatus of claim 11 wherein the controller comprises a controller for encoding an instruction in the alternate coupling of the first and second frequency signals to the device.

13. The apparatus of claim 12 wherein the terminal apparatus comprises apparatus for detecting the first and second frequencies, discriminating between them, and decoding the instruction based upon such discrimination.

14. A communication method comprising providing at least one source of program material, at least one carrier frequency signal source and at least one first modulator for modulating the at least one carrier frequency signal source with the at least one program material source, the at least one first modulator having a first input, a second input and a first output, and at least one terminal apparatus for demodulating program material -modulated carrier frequency signals, coupling the at least one program material source to the at least one first input, coupling the at least one carrier frequency signal source to the at least one second input, and coupling the -13- at least one first output through a device to the at least one terminal apparatus, providing first and second sources of signals having first and second frequencies, respectively, alternately coupling one of the first and second frequency signals to the device, coupling the first output terminal to the device, and coupling an output of the device to the at least one terminal apparatus.

15. The method of claim 14 wherein alternately coupling one of the first and second frequency signals to the device comprises encoding an instruction in the alternate coupling of the first and second frequency signals to the device.

16. The method of claim 15 wherein providing at least one terminal apparatus comprises providing at least one terminal apparatus for detecting the first and second frequencies, discriminating between them, and decoding the instruction based upon such discrimination.

17. The method of claim 14 wherein providing at least one terminal apparatus comprises providing at least one terminal apparatus having an automatic gain control (AGC) bandwidth.

18. The method of claim 17 wherein providing first and second frequency signals comprises providing first and second signals having frequencies in the AGC bandwidth.

19. The method of claim 18 wherein alternately coupling one of the first and second frequency signals to the device comprises encoding an instruction in the alternate coupling of the first and second sources of signals to the device.

20. The method of claim 19 wherein providing terminal apparatus comprises providing apparatus for detecting the first and second frequencies, discriminating between them, and decoding the instruction based upon such discrimination.

21. The method of claim 20 wherein providing terminal apparatus comprises providing a television receiver.

22. The method of claim 21 wherein providing first and second signals having frequencies in the AGC bandwidth comprise providing first and second signals having frequencies less than about 35 Hz.

23. The method of claim 14 wherein providing at least one source of program material comprises providing at least one source of video program -14- material and at least one source of complementary audio program material, providing at least one carrier frequency signal source comprises providing at least one video carrier frequency signal source and at least one complementary audio carrier frequency signal source, and providing at least one first modulator comprises providing at least one first video modulator for modulating the at least one video carrier frequency signal source with the at least one source of video program material and at least one first audio modulator for modulating the at least one complementary audio carrier frequency signal source with the at least one source of complementary audio program material.

24. The method of claim 23 wherein providing the device comprises providing a device for coupling the at least one first audio modulator to the at least one terminal apparatus.

25. The method of claim 24 wherein alternately coupling one of the first and second frequency signals to the device comprises encoding an instruction in the alternate coupling of the first and second frequency signals to the device.

26. The method of claim 25 wherein providing terminal apparatus comprises providing apparatus for detecting the first and second frequencies, discriminating between them, and decoding the instruction based upon such discrimination.

27. A communication system comprising at least one source of program material, at least one carrier frequency signal source and at least one first modulator for modulating the at least one carrier frequency signal source with the at least one program material source, the at least one first modulator having a first input, a second input and a first output, and at least one terminal apparatus for demodulating program material-modulated carrier frequency signals, the at least one program material source being coupled to the at least one first input, the at least one carrier frequency signal source being coupled to the at least one second input, the at least one program material source being coupled to the at least one first input through a device having at least third and fourth inputs and a second output, first and second sources of signals having first and second frequencies, respectively, a controller for alternately providing one of the first and second frequency signals to the device, the second -15- output being coupled to the first input, and the first output being coupled to the at least one terminal apparatus.

28. The apparatus of claim 27 wherein the controller comprises a controller for encoding an instruction in the alternate coupling of the first and second frequency signals to the device.

29. The apparatus of claim 28 wherein the terminal apparatus comprises apparatus for detecting the first and second frequencies, discriminating between them, and decoding the instruction based upon such discrimination.

30. The apparatus of claim 27 wherein the at least one terminal apparatus includes automatic gain control (AGC) means having an AGC bandwidth.

31. The apparatus of claim 30 wherein the first and second frequency signals comprise first and second signals having frequencies in the AGC bandwidth.

32. The apparatus of claim 31 wherein the controller comprises a controller for encoding an instruction in the alternate coupling of the first and second frequency signals to the third input.

33. The apparatus of claim 32 wherein the terminal apparatus comprises apparatus for detecting the first and second frequencies, discriminating between them, and decoding the instruction based upon such discrimination.

34. The apparatus of claim 33 wherein the terminal apparatus comprises a television receiver.

35. The apparatus of claim 34 wherein the first and second frequency signals having frequencies in the AGC bandwidth comprise first and second frequency signals having frequencies less than about 35 Hz.

36. The apparatus of claim 27 wherein the at least one source of program material comprises at least one source of video program material and at least one source of complementary audio program material, the at least one carrier frequency signal source comprises at least one video carrier frequency signal source and at least one complementary audio carrier frequency signal source, and the at least one first modulator comprises at least one first video modulator for modulating the at least one video carrier frequency signal source with the at least one source of video program material and at least one first audio modulator for modulating the at least one -16- complementary audio carrier frequency signal source with the at least one source of complementary audio program material.

37. The apparatus of claim 36 wherein the device comprises a device for coupling the at least one first audio modulator to the at least one terminal apparatus.

38. The apparatus of claim 37 wherein the controller comprises a controller for encoding an instruction in the alternate provision of the first and second frequency signals to the device.

39. The apparatus of claim 38 wherein the terminal apparatus comprises apparatus for detecting the first and second frequencies, discriminating between them, and decoding the instruction based upon such discrimination.

40. A communication method comprising providing at least one source of program material, at least one carrier frequency signal source and at least one first modulator for modulating the at least one carrier frequency signal source with the at least one program material source, the at least one first modulator having a first input, a second input and a first output, and at least one terminal apparatus for demodulating program material-modulated carrier frequency signals, coupling the at least one program material source to the at least one first input through a device having at least third and fourth inputs and a second output, coupling the at least one carrier frequency signal source to the at least one second input, and coupling the at least one first output to the at least one terminal apparatus, providing first and second sources of signals having first and second frequencies, respectively, alternately coupling one of the first and second frequency signals to the device, coupling the second output to the first input, and coupling the first output to the at least one terminal apparatus.

41. The method of claim 40 wherein alternately coupling one of the first and second frequency signals to the device comprises encoding an instruction in the alternate coupling of the first and second frequency signals to the device.

42. The method of claim 41 wherein providing at least one terminal apparatus comprises providing at least one terminal apparatus for detecting the first and second frequencies, discriminating between them, and decoding the instruction based upon such discrimination. -17-

43. The method of claim 40 wherein providing at least one terminal apparatus comprises providing at least one terminal apparatus including automatic gain control (AGC) means having an AGC bandwidth.

44. The method of claim 43 wherein providing first and second sources of signals comprises providing first and second sources of signals having frequencies in the AGC bandwidth.

45. The method of claim 44 wherein alternately coupling one of the first and second frequency signals to the device comprises encoding an instruction in the alternate coupling of the first and second frequency signals to the device.

46. The method of claim 45 wherein providing terminal apparatus comprises providing apparatus for detecting the first and second frequencies, discriminating between them, and decoding the instruction based upon such discrimination.

47. The method of claim 46 wherein providing terminal apparatus comprises providing a television receiver.

48. The method of claim 47 wherein providing first and second sources of signals having frequencies in the AGC bandwidth comprise providing first and second frequency signals having frequencies less than about 35 Hz.

49. The method of claim 40 wherein providing at least one source of program material comprises providing at least one source of video program material and at least one source of complementary audio program material, providing at least one carrier frequency signal source comprises providing at least one video carrier frequency signal source and at least one complementary audio carrier frequency signal source, and providing at least one first modulator comprises providing at least one first video modulator for modulating the at least one video carrier frequency signal source with the at least one source of video program material and at least one first audio modulator for modulating the at least one complementary audio carrier frequency signal source with the at least one source of complementary audio program material.

50. The method of claim 49 wherein alternately coupling one of the first and second frequency signals to the device comprises encoding an instruction in the alternate coupling of the first and second frequency signals to the device. -18-

51. The method of claim 50 wherein providing terminal apparatus comprises providing apparatus for detecting the first and second frequencies, discriminating between them, and decoding the instruction based upon such discrimination.

52. A communication system comprising at least one source of program material, at least one carrier frequency signal source and at least one first modulator for modulating the at least one carrier frequency signal source with the at least one program material source, the at least one first modulator having a first input, a second input and a first output, and at least one terminal apparatus for demodulating program material-modulated carrier frequency signals, the at least one program material source being coupled to the at least one first input, the at least one carrier frequency signal source being coupled to the at least one second input, the at least carrier frequency signal source being coupled to the at least one first input through a device having at least third and fourth inputs and a second output, first and second sources of signals having first and second frequencies, respectively, a controller for alternately providing one of the first and second frequency signals to the device, the second output being coupled to the first input, and the first output being coupled to the at least one first modulator.

53. The apparatus of claim 52 wherein the controller comprises a controller for encoding an instruction in the alternate coupling of the first and second frequency signals to the device.

54. The apparatus of claim 53 wherein the terminal apparatus comprises apparatus for detecting the first and second frequencies, discriminating between them, and decoding the instruction based upon such discrimination.

55. The apparatus of claim 52 wherein the at least one terminal apparatus includes automatic gain control (AGC) means having an AGC bandwidth.

56. The apparatus of claim 55 wherein the first and second frequency signals comprise first and second signals having frequencies in the AGC bandwidth.

57. The apparatus of claim 56 wherein the controller comprises a controller for encoding an instruction in the alternate providing of the first and second frequency signals to the device. -19-

58. The apparatus of claim 57 wherein the terminal apparatus comprises a television receiver.

59. The apparatus of claim 58 wherein the first and second frequency signals having frequencies in the AGC bandwidth comprise first and second frequency signals having frequencies less than about 35 Hz.

60. The apparatus of claim 52 wherein the at least one source of program material comprises at least one source of video program material and at least one source of complementary audio program material, the at least one carrier frequency signal source comprises at least one video carrier frequency signal source and at least one complementary audio carrier frequency signal source, and the at least one first modulator comprises at least one first video modulator for modulating the at least one video carrier frequency signal source with the at least one source of video program material and at least one first audio modulator for modulating the at least one complementary audio carrier frequency signal source with the at least one source of complementary audio program material.

61. The apparatus of claim 60 wherein the device comprises a device for coupling the at least one first audio carrier frequency signal source to the at least one first audio modulator.

62. The apparatus of claim 61 wherein the controller comprises a controller for encoding an instruction in the alternate provision of the first and second frequency signals to the device.

63. The apparatus of claim 62 wherein the terminal apparatus comprises apparatus for detecting the first and second frequencies, discriminating between them, and decoding the instruction based upon such discrimination.

64. A communication method comprising providing at least one source of program material, at least one carrier frequency signal source and at least one first modulator for modulating the at least one carrier frequency signal source with the at least one program material source, the at least one first modulator having a first input, a second input and a first output, and at least one terminal apparatus for demodulating program material-modulated carrier frequency signals, coupling the at least one carrier frequency signal source to the at least one first input through a device having at least third and fourth inputs and a second output, coupling the at least one -20- program material source to the at least one second input, and coupling the at least one first output to the at least one terminal apparatus, providing first and second sources of signals having first and second frequencies, respectively, alternately coupling one of the first and second frequency signals to the device, coupling the second output to the first input, and coupling the first output to the at least one terminal apparatus.

65. The method of claim 64 wherein providing at least one terminal apparatus comprises providing at least one terminal apparatus for detecting the first and second frequencies, discriminating between them, and decoding the instruction based upon such discrimination.

66. The method of claim 64 wherein providing at least one terminal apparatus comprises providing at least one terminal apparatus including automatic gain control (AGC) means having an AGC bandwidth.

67. The method of claim 66 wherein providing first and second sources of signals comprises providing first and second sources of signals having frequencies in the AGC bandwidth.

68. The method of claim 67 wherein alternately coupling one of the first and second frequency signals to the device comprises encoding an instruction in the alternate coupling of the first and second frequency signals to the device.

69. The method of claim 68 wherein providing terminal apparatus comprises providing apparatus for detecting the first and second frequencies, discriminating between them, and decoding the instruction based upon such discrimination.

70. The method of claim 69 wherein providing terminal apparatus comprises providing a television receiver.

71. The method of claim 70 wherein providing first and second sources of signals having frequencies in the AGC bandwidth comprise providing first and second frequency signals having frequencies less than about 35 Hz.

72. The method of claim 64 wherein providing at least one source of program material comprises providing at least one source of video program material and at least one source of complementary audio program material, providing at least one carrier frequency signal source comprises providing at least one video carrier frequency signal source and at least one complementary audio carrier -21- frequency signal source, and providing at least one first modulator comprises providing at least one first video modulator for modulating the at least one video carrier frequency signal source with the at least one source of video program material and at least one first audio modulator for modulating the at least one complementary audio carrier frequency signal source with the at least one source of complementary audio program material.

73. The method of claim 72 wherein alternately coupling one of the first and second frequency signals to the device comprises encoding an instruction in the alternate coupling of the first and second frequency signals to the device.

74. The method of claim 73 wherein providing terminal apparatus comprises providing apparatus for detecting the first and second frequencies, discriminating between them, and decoding the instruction based upon such discrimination.