System for selectively receiving either 4-channel or- 2-channel stereophonic broadcastings
United States Patent [1 1 Mawake et a1.  
 1 1 Apr. 29, 1975 221 Filed: Jan. 17,1973  
  21 Appl.No.:324.561  
 [58] Field o1Search.... 179/15 ET, 1 GO. 100.4 ST, l79/100.l TD  
 [56] References Cited UNITED STATES PATENTS 3.679.832 7/1972 Halpern 179/15 BT 3,708,623 H1973 Dorrcn 179/15 BT 3,787,629 l/l974 Limbcrg i 179/15 BT OTH ER PUBLICATIONS Ouadrasonics on the Air by Feldman Audio Magazine Jan. 1970.  
 The Quart Broadcasting System by Gerzon Audio Magazine Sept. 1970.  
 Compatible FM Broadcasting of Panoramic Sound by Gibson et al. Journal ABS Dec. 72.  
 Primary Examiner-Kathleen H. Claffy Assistant Examiner-Thomas D&#39;Amico [57] ABSTRACT The invention relates to a system for selectively receiving either 4-channel 2-channel stereophonic broadcastings. The 4-channel signal is a composite of stereophonic signals comprising a main channel signal, a pilot sub-carrier, a first sub-channel signal. a second sub-channel signal and a composite signal of 2- channel stereophonic signals comprising a main channel signal, a pilot sub-carrier. and a first sub-channel signal. Transmission and reception are accomplished by inserting and superimposing, with respect to the composite signal of the Z-channel stereophonic signals, a modulated wave which is obtained by modulat ing a carrier wave of a frequency which is four times that of the pilot sub-carrier with a program signal differing from the Z-channel signals in a band corresponding to the second sub-channel signal of the composite signal of the 4-channel stereophonic signals.  
 6 Claims, 6 Drawing Figures use MOD  
  l 30 MAIN r CARRIER osc 26 27 SWITCHING PULSE 118 FREQ 4 SEN cm GEN MOD I 20 1 uglcea I 19 25 MAIN ost:  
  1 SYSTEM FOR SELECTIVELY RECEIVING EITHER 4-CI-IANNEL OR- Z-CI-IANNEL STEREOPHONIC BROADCASTINGS BACKGROUND OF THE INVENTION This invention relates generally to a transmitting and receiving system of stereophonic broadcastings and more particularly to a system capable of receiving twochannel stereophonic broadcasts and four-channel stereophonic broadcastings by switching from one to the other.  
  Heretofore, various sytems have been proposed for stereophonic broadcasting systems. In general, information signals obtained from a large number of sound sources are transmitted as signals of a plurality of channels. Each channel is reproduced on the receiving side by means of a plurality of loudspeakers, to impart a stereophonic or three-dimensional sound sensation to the listeners.  
  A system representative of these known systems is a 2-channel stereophonic broadcasting system which depends on a carrier-suppression AM-FM system. In the operation, this system, simultaneously transmits: a sum signal (L R) comprising left (L) and right (R) channel signals, a pilot sub-carrier of a 19 KHz frequency which is slightly higher than the upper limit frequency of the sum signal band; and an amplitude-modulated wave of the difference signal (L R) of a two-channel signal in a form resulting from the suppression of a carrier wave of a frequency of 38 KHz which is twice that of the pilot sub-carrier, and a signal wave generated by frequency modulating a carrier wave of a 67 KHz frequency with a signal (which may be a program signal of a different kind) other than the above mentioned stereophonic signal. These signals occupy respective divided frequency bands.  
  A frequency modulated wave of a signal other than the stereophonic signal may be obtained. The frequency of the stereophonic carrier wave is 67 KHz and has no higher-harmonic relationship with respect to the frequency of 38 KHz of the carrier wave of the amplitude-modulated wave. For this reason, a large number of standard oscillators are necessary as carrier-wave sources on the transmission side. On the receiving side, there are difficulties such as the generation of much interference noise in the received signals.  
  A so-called Dorren system which is 4-channel stereophonic broadcasting system is compatible with the 2- channel stereophonic broadcasting system depending on a carrier suppression AM-FM system which has recently been practiced in an experimental broadcasting in the U.S.A.  
 SUMMARY OF THE INVENTION It is a general object of the invention to provide a new and useful transmitting and receiving system capable of receiving either this 4-channel stereophonic broadcasting signal or a ZChannel stereophonic broadcasting including program signals of different kinds.  
  Another object of the invention is to provide a transmitting and receiving system of 2-channel 4-channel stereophonic broadcasting changeover type wherein, in 2-channel stereophonic broadcasting, the frequency of a carrier wave modulated by program signals of different kinds for multiplexing by frequency division is made equal to the frequency of a higher harmonic of a pilot subcarrier. In the system according to the present invention, a large number of standard oscillators are not required as carrier wave sources on the transmitting side. Furthermore, on the receiving side, there is no large amount of interference noise.  
  Still another object of the invention is to provide a system capable of positively accomplishing the above stated Z-channel 4-channel stereophonic broadcasting changeover. In this case, the program signals of different kinds are amplitude modulated for multiplexing on the 2-channel stereophonic broadcasting.  
  A further object of the invention is to provide a system wherein it is possible to separate automatically and receive a 4-channel stereophonic broadcasting and a 2-channel stereophonic broadcasting wherein program signals of different kinds are multiplexed without the necessity of providing any changeover means.  
  Further objects and features of the invention will be apparent from the following detailed description when read in conjunction with the accompanying drawings, throughout which like parts are designated by like reference numerals.  
 BRIEF DESCRIPTION OF THE DRAWINGS In the drawings:  
  FIG. 1 is a block diagram showing one embodiment of transmitting apparatus of a system according to the invention;  
  FIGS. 2A and 2B are graphical representations respectively indicating the frequency spectrums of a 4- channel stereophonic broadcasting signal and of a 2- channel stereophonic broadcasting signal resulting from the multiplexing of different program signals;  
  FIG. 3 is a block diagram showing one embodiment of receiving apparatus for receiving signals transmitted by the transmitting section shown in FIG. 1;  
  FIG. 4 is a block diagram showing another embodiment of transmitting apparatus of a system according to the invention; and  
  FIG. 5 is a block diagram showing one embodiment of receiving apparatus for receiving signals transmitted by the transmitting apparatus shown in FIG. 4.  
 DETAILED DESCRIPTION One embodiment of transmitting apparatus of the system of the present invention is illustrated in FIG. 1. Left-front and left-rear channel signals LF and LR are supplied to a 76 KHZ switching circuit 15 through input terminals 11 and 12, respectively. Right-front and right-rear channel signals RF and RR are introduced to a KI-Iz switching circuit 16 through input terminals 13 and 14. The switching circuits l5 and 16 receive switching pulses from a switching pulse generating circuit 20. A signal of I52 KHz frequency from a main oscillator I9 is formed into these switching pulses of 76 KHz, with successively lagged switching phases.  
  Signals which have been successively switched and time divided by the 76 KHz pulses are supplied from the switching circuits l5 and 16 respectively to a 38 KHz switching circuit I7, where they are alternately switched by two switching pulses of mutually opposite phase supplied by the switching pulse generating circuit 20. These switching pulses are then sent out to a terminal 18.  
  The switching circuits l5, l6 and I7 constitute a switching multiplex circuit (time-division multiplex circuit). The composite signal C appearing at the output terminal 18 may be represented by the following equation, when the signals applied to terminals 11 through 14 are the 4-channel signals LF, LR, RF, and RR.  
 C A+B sin wt D cos out C sin 2mt The frequency spectrum of this composite signal C (FIG. 2A) comprises a direct wave of the signal A as a main channel signal, amplitude-modulated waves of signals B and D resulting from the suppression of the carrier wave of 38 KHz as a first sub channel signal, and amplitude-modulated signals of the signal C resulting from the suppression of the carrier wave of 76 KHz as a second sub-channel signal.  
  When the signals applied to the input tenninals 11 through 14 are Z-channel signals LF LR L and RF RR R, the composite signal C&#39; appearing at the output terminal 18 may be represented by the following equation.  
 CI&#34;): (L R) sinmt The frequency spectrum of this composite signal C&#39; (FIG. 28) comprises a direct wave of the signal (L R) as a main channel signal and amplitudemodulated signals of the (L R) signal resulting from the suppression of the carrier wave of 38 KHz as a first sub-channel signal.  
  As is apparent from a comparison of FIGS. 2A and 28, a 2-channel signal component does not exist as a second sub-channel signal in the case illustrated in FIG. 28. For 2-channel signal broadcasting, this vacant second sub-channel signal band has superimposed thereon a modulated wave produced by modulating a 76 KHz carrier wave with program signals of different kinds.  
  A pilot signal generator 21 operates when supplied with a signal from the main oscillator 19, to generate a pilot sub-carrier of 19 KHz and send this signal to an adder 22. The pilot sub-carrier of 19 KHz added in the adder 22 to the signal from the terminal 18 corresponds to a position indicated by the letters fp in FIGS. 2A and 2B.  
  A switch 23 is connected between the terminal 18 and the adder 22. Its stationary contact point a is connected directly to the terminal 18. The stationary contact point b is connected through an attenuator 24 to the terminal 18. This attenuator 24 prevents overmodulation when, during multiplexing of 2-channel stereophonic signals and program signals of different kinds. The level of the 2-channel stereophonic signal component is lowered to frequency modulate the main carrier wave with the composite signal.  
  If the signals applied to the input terminals 11 through 14 are 4-channel signals LF, LR, RF, and RR, the movable contact of the switch 23 is connected with the contact point a The output signal of the switching circuit 17 is supplied directly to the adder 22. If the signals applied to the input terminals 11 through 14 are 2-channel signals L(=LF=LR) and R(=RF, RR), the selector contact of the switch 23 is connected to the contact point 12,, whereby the output signal of the switching circuit 17 is supplied by way of the attenuator 24 to the adder 22.  
  A carrier oscillator 25 generates a 76 KHz carrier wave when a signal is supplied thereto from the main oscillator 19. A frequency modulator 26 (such as a serrasoid modulator, for example) modulates this carrier. Program signals of different kinds are applied as a modulation wave through a terminal 27 to the frequency modulator 26.  
  Between the frequency modulator 26 and the adder 22, there is connected a switch 28 having a movable contact which is changed over in intercoupled relationship with the movable contact of the aforedescribed switch 23. This switch 28 has an unused idle contact point 0 and a contact point b connected to the frequency modulator 25. The movable contact of the switch 28 is switched to the contact point a when the movable contact of the switch 23 is changed over to the contact point a,, and when the movable contact of the switch 23 is changed over to its contact point b,, that of the switch 28 is switched to its contact point b Accordingly, if the input signal is a Z-channel signal, the movable contact of the switch 28 is switched to its contact point b A frequency-modulated wave is obtained by frequency modulating the 76 KHZ carrier wave with program signals of different kinds, supplied to the adder 22. The frequency spectrum of this frequency-modulated wave is as indicated in the second sub-channel signal zone in FIG. 2B. It is apparent from a comparison of FIGS. 2A and 2B that the 76 KHz carrier wave component is large, in this case.  
  The output of the adder 22 is applied as a modulating wave to a frequency modulator 29, where a carrier wave from a main carrier wave oscillator 30 is frequency modulated. The resulting output signal of this frequency modulator is led out through an output terminal 31 and transmitted from a transmitting antenna (not shown).  
  The signal thus transmitted is received by receiving apparatus one embodiment of which described below with reference to FIG. 3. The radio signal is received by a receiving antenna 40 and passed through a circuit 41 including a high frequency amplifier, a frequency converter, an intermediate frequency amplifier, and other parts, to a frequency demodulator 42. A compos ite signal is produced on the output side of this frequency demodulator.  
  This composite signal is supplied to a narrow bandpass filter 43, where the frequency components in the vicinity of 76 KHz are filtered. The resulting signal is supplied to a switch control circuit 44. If there are different kinds of programs in the above mentioned composite signal, there is a large 76 KHz carrier wave component, as indicated in FIG. 2B. For this reason, an output of relatively high level is obtained from the filter 43.  
  If the composite signal is a 4-channel stereophonic signal, the 76 KHz carrier wave component is suppressed. The output signal of the filter 43 is nonexistent, or its level is extremely low. The switch control circuit 44 operates in accordance with the output signal of the filter 43 to form a control signal for switching. The movable contacts of switches 45 and 46 are activated by the output control signal of the switch control circuit 44. For the switches 45 and 46, suitable switching means such as relays and electronic switches can be used.  
  The switch 45 has an unused stationary contact point a and a stationary contact point b connected to the output side of the frequency demodulator 42. The switch 46 has a stationary contact point a, connected to the output side of the frequency demodulator 42 and a contact point b connected to the output side of a low-pass filter 47. This filter 47 has a filtering characteristic such that the main channel signal and the first sub-channel signal of the composite signal are in passing bands, while the second sub-channel signal is in a non-passing band.  
  Furthermore, the composite output signal of the frequency demodulator 42 is supplied to a narrow bandpass filter 48, where a pilot signal of 19 KHz is filtered. This pilot signal is supplied to a generator 49 for generating 38 KHz switching pulses and to a generator 50 for generating 76 KHz switching pulses.  
  The movable contact of the switch 46 is connected to a switching circuit 51, which carries out a switching operation in response to switching pulses of 38 KHz from the switching-pulse generator 49. The output side of this switching circuit 51 is connected to switching circuits 52 and 53, which respectively are activated by 76 KHz switching pulses from the switching pulse generator S0 to carry out switching.  
  When the composite signal obtained from the frequency demodulator 42 is a signal corresponding to a 4-channel stereophonic signal, the movable contacts of the switches 45 and 46 are switched to their contact points a and a.,, respectively. When the composite signal obtained from the frequency demodulator 42 is a multiplexed signal comprising different kinds of program signals and a 2-channel stereophonic signal, the movable contacts of the switches 45 and 46 are respec&#39; tively switched to their contact points b and b.,.  
  If the received signal contains different kinds of program signals, the movable contact of the switch 45 is switched to its contact point b The signal passing through the switch 45 is supplied to a band-pass filter 54. The signal of the second sub-channel band is in a state wherein it has been frequency modulated by program signals of different kinds which have been filtered out in the filter 54 is supplied to a frequency demodulator 55. The different kinds of program signals thus demodulated by the frequency demodulator is led out through an output terminal 60.  
  When the apparatus is in the state wherein program signals of different kinds are being obtained from the terminal 60, the high-level output signal produced by the switch control circuit 44 responsive to the 76 KHz signal component derived by the narrow band-pass filter 43 causes the movable contact of the switch 46 to make contact with the contact point [7,. The demodulated signals of the frequency demodulator 42 are such that only the main channel signal and the first subchannel signal component pass through the low-pass filter 47. The switch 46 is supplied to the switching circuit 51 for demodulation and further to the switching circuits 52 and 53. As a result of the demodulation due to the switching circuits 51, 52, and 53 for demodulation, a signal LF LF L is produced as an output at terminals 56 and 57. A signal RF RR R is produced as output at terminals 58 and 59.  
  When the receiving apparatus is in the state wherein it is receiving a 4-channel stereophonic broadcasting,  
 a 76 KHz signal component is not derived from the narrow band-pass filter 43. The output signal of the switch control circuit 44 is of low level. Consequently, the movable contacts of the switches 45 and 46 are switched to their contact points 0 and 1 The signal supply to the circuit for demodulation and reproduction of the different kinds of program signals is interrupted. At the same time, the output signal of the frequency demodulator 42, is supplied without passing through the low-pass filter 47, to the switching circuits 51, 52, and 53 for demodulation. From the output terminals 56, S7, 58, and 59, are obtained 4-channel signals LF, LR, RF, and RR.  
  By the practice of the present invention, as described above, selective switching can be effected for receiving a 4-channel stereophonic broadcasting system compatible with a 2-channel stereophonic broadcasting system depending on a carrier-suppression AM-FM system. When the switching occurs, the system may receive from a broadcasting system 2-channel stereophonic program signals, different kinds of which are so transmitted after being frequency divided and multiplexed at the transmission band position of a second subchannel signal in the 4-channel stereophonic broadcasting system. Accordingly, on the transmission side, only a small number of standard oscillators is required. On the receiving side, the receiving circuit is automatically switched in accordance with the broadcasting system to be received. Absolutely no noise is generated responsive to the presence of other signals in the reproduced signal of the stereophonic signal and in the reproduced signal of different kinds of program signals. Another advantageous feature of this system is that the frequency of the carrier wave modulated with different kinds of program signals is equal to the frequency (a quadrupled frequency in the instant embodiment) of a higher harmonic of the frequency of the pilot subcarrier. For this reason, there is no possibility of interference noise mutually between signals.  
  As is known, if the level of the frequency component of the carrier wave becomes low in a frequencymodulated wave, in a state other than that of no modulation, the frequency component of the carrier wave position becomes zero in some cases. In the foregoing embodiment, a frequency component in the vicinity of four times the frequency value of a pilot carrier exists only if a frequency-modulated wave, due to different kinds of program signals, is in the second sub-channel band of the composite signal. This frequency component is extracted and used to carry out automatic switching of the above mentioned circuits. For the frequency-modulated wave described above, the above mentioned automatic circuit switching operation of the receiver is apt to become unreliable.  
  As one measure to solve this problem, an amplitude modulator is used in place of the frequency modulator 26 in the transmitting apparatus illustrated in FIG. 1. An amplitude demodulator is used in place of the frequency demodulator 55 in the receiving apparatus shown in FIG. 3. In this case, it becomes possible to switch selectively between a 4-channel stereophonic broadcasting system compatible with a Z-channel stereophonic broadcasting system depending on a carriersuppression AM-FM system and a broadcasting system wherein, with respect to a 2-channel stereophonic broadcasting system depending on a carriersuppression AM-FM system, different kinds of program signals are transmitted after being frequency divided and multiplexed in the form of an amplitude modulated wave in the transmission band position of the second sub-channel signal in a 4-channel stereophonic broadcasting system.  
  On the receiver side, the switching of the receiving circuits is automatically carried out in accordance with the presence or absence of a carrier wave in the second sub-channel signal band. In this connection, it will be obvious that a 76 KHz carrier wave is used for the amplitude modulation wave of the program of different kinds, and carrier suppression is not carried out.  
  In the system of this embodiment, a carrier wave is always present in the second sub-channel signal band. This gives rise to a problem. Unless the entire level of the composite signal is lowered, the frequency modulation due to the composite signal with respect to the main carrier wave becomes overmodulation. In this case, there is a poor signal-to-noise ratio at the time of Z-channel stereophonic broadcasting including program signals of different kinds. In addition, a circuit for automatic switching is necessary in the receiver in each of the above described embodiments. Consequently, the circuit organization of the receiver becomes complicated and expensive.  
  These difficulties can be overcome by a system as described below with respect to one embodiment as illustrated in FIGS. 4 and 5.  
  FIG. 4 is a block diagram indicating the organization of the transmitting apparatus of the system, in which parts which are the same as or equivalent to those of the embodiment shown in FIG. 1 are designated by like reference numerals and characters, and detailed description thereof will not be repeated.  
  In this transmitter, a main oscillator 70 generates a l9 KHz frequency signal. This signal is supplied as a pilot sub-carrier directly to the adder 22 and, at the same time, to a frequency multiplier 72 within a switching pulse generating circuit 71. The frequency signal which has thus been doubled in the frequency multiplier 72 is supplied to a 38 KHz switching pulse generating circuit 74 and to a frequency multiplier 73. The frequency signal which has been further doubled by the frequency multiplier 73 is supplied to a 76 KHZ switching pulse generating circuit 75 and to a 90 phase shifter 76.  
  The switching pulse generating circuits 74 and 7S supply respective switching pulse trains having specifi cally required repetition cyclic periods and initial phases, respectively, to the switching circuits l5, l6, and I7. Thereupon, these circuits operate similarly as in the embodiment illustrated in FIG. I to switch the 4- channel signals LF, LR, RF, and RR applied through the input terminals 11 through 14 and to send out timedivided, multiplexed signals.  
  The output phase-shift signal of the 90 phase shifter 76 is supplied as a carrier wave to a ring modulator &#39;77 and there subjected to balanced amplitude modulation by a modulation signal of programs of different kinds from the terminal 27. This carrier wave supplied to the ring modulator 77 has a frequency which is 4 times that of the pilot sub-carrier. Moreover, the carrier has a phase difference of 90 relative to the phase of the carrier wave of the second sub-channel signal used at the time of 4channel stereophonic signal transmission.  
  Accordingly, if the 4-channel signals applied to the input terminals ll through 14 are signals LF, LR, RF,  
 and RR, the movable contacts of the switches 23 and 28 are respectively in contact with their contact points a, and a and the composite signal C obtained on the output side of the adder 22 with the transmitter in this state can be represented by the following equation.  
  4 (at C sin 20:! +asin If the 4-channel signals applied to the input terminals 11 through 14 are LF=LR=L and RF=RR=R, the movable contacts of the switches 23 and 28 are respectively connected to the contact points b, and h The composite signal C&#39; obtained on the output side of the adder 22, with the transmitter in this state, can be represented by the following equation.  
  The signals occupying the second sub-channel signal band mutually have a 90 phase difference.  
  A difference of 90 exists between the phases of carrier waves (switching pulses) used in the demodulation of the second sub-channel signals in the composite signals C and C&#39; represented by the above two equations. For this reason, when the composite signal C is received by a receiver for 4-channel stereophonic broadcastings different kinds of program signals do not become mixed, as interference within the stereophonic signals to generate noise, whereby only true and normal 2 channel stereophonic signals are reproduced.  
  Furthermore, when 4-channel stereophonic broadcasting is received in a receiving apparatus for different program signals, the apparatus is held in a state wherein there is no demodulation output. Hence, two kinds of broadcast signals can be received merely by adding a receiving apparatus for different program signals into a receiver for 4-channel stereophonic broadcastings. In this case, it is not necessary to provide an automatic switching device for preventing interference mixing.  
  One embodiment of receiving apparatus for receiving radio signals transmitted by the transmitting apparatus shown in FIG. 4 is illustrated by block diagram in FIG. 5. In FIG. 5, parts which are the same as or equivalent to those of the embodiment shown in FIG. 3 are designated by like reference numerals, and detailed descrip tion thereof will not be repeated.  
  The output signal of the frequency demodulator 42 is supplied to the switching circuit 51 and, at the same time, to a narrow band-pass filter and a band-pass filter 85. A 19 KHZ pilot sub-carrier is thus filtered by the filter 80. After passing through an amplifier circuit 8], it is doubled by a multiplier 82 and supplied to a multiplier 83 and to a 38 KHz switching pulse generating circuit 84. The resulting signal has been further doubled by the multiplier 83 to a frequency of 76 KHz and is supplied to a phase shifter 87 and to a switching pulse generating circuit 88. The output switching pulses of the switching pulse generating circuit 84 are supplied to the switching circuit 51. The output switching pulses of the switching pulse generating circuit 88 are supplied to the switching circuits S2 and 53.  
  A signal of the second sub-channel signal band part, which has been filtered by the band-pass filter 85, is  
 supplied to a 76 KHZ switching circuit 86. A signal passed through the 90 phase shifier 87 is applied to this switching circuit 86.  
  In the receiver of the above described organization, a 4-channel stereophonic broadcast depending on the so-called Dorren system is being received. The four channel signals are demodulated by the switching operations of the switching circuits 51, 52, and 53. Four channel signals LF, LR, RF, and RR are individually led out of the terminals 56, 57, 58, and 59. In this case, no output appears at the output terminal 60.  
  If the receiver is receiving a Z-channel stereophonic broadcasting wherein difierent program signals are multiplexed, the demodulation action due to the switching operations of the switching circuits 51, 52, and 53 causes a signal L to appear as an output at the output terminals 56 and 57 and a signal R to appear as output at the output terminals 58 and 59. in this case, different program signals are produced as output at the output terminal 60 responsive to the demodulation action due to the switching operation of the switching circuit 86.  
  The system of the instant embodiment affords automatic separation and reception of a 4-channel stereophonic broadcasting depending on the so-called Dorren system and a Z-channel stereophonic broadcasting wherein diffrent program signals are multiplexed. There is no need for any provision of any means whatsoever for changing over or switching therebetween. Furthermore, since the signal of the second subchannel signal band is a carrier-suppression, amplitude modulation wave, transmission is possible with a better signal-to-noise ratio state of the broadcasting wave which is FM transmitted than the system of the above described embodiment. Since those skilled in the art will perceive modifications, the appended claims are to be construed to cover all equivalent structures falling within the scope and the spirit of the invention.  
 What we claim is:  
  l. A system for transmitting either 4-channel signals or Z-channel signals comprising:  
 main oscillator means for generating a pilot signal having a predetermined frequency;  
 first oscillator means responsive to the pilot signal for generating a first signal having a frequency which is twice that of the pilot signal;  
 second oscillator means responsive to the first signal for generating a second signal having a frequency which is four times that of the pilot signal; composing means;  
 means responsive to the first and second signals and to first, second, third and fourth channel signals for operating said composing means to produce a first composite signal including a first main channel signal, a first suppressed carrier subchannel signal and a second suppressed carrier subchannel signal, said first main channel signal being a sum signal of the first, second, third and fourth channel signals, said first suppressed carrier subchannel signal having a first carrier of a frequency equal to twice the predetermined frequency and having a first modulating signal modulated on the first carrier and a second modulating signal modulated in quadrature with the first carrier, said first modulating signal being the first channel signal plus the second channel signal minus the third channel signal minus the fourth channel signal, said second modulating signal being the first channel signal minus the second channel signal minus the third channel signal plus the fourth channel signal, said second suppressed carrier subchannel signal having a second carrier of a frequency equal to four times the predetermined frequency and having a third modulating signal modulated on the second carrier, said third modulating signal being the first channel signal minus the second channel signal plus the third channel signal minus the fourth channel signal;  
 means for also operating said composing means responsive to the first and second signals and to fifth and sixth channel signals for producing a second composite signal including a second main channel signal and a third suppressed carrier subchannel signal, said second main channel signal being a sum signal of the fifth and sixth channel signals, said third suppressed carrier subchannel signal having a third carrier of a frequency equal to twice the predetermined frequency and having a fourth modulating signal modulated on the third carrier, said fourth modulating signal being a difference signal between the fifth and sixth channel signals; modulating means for modulating the second signal with a program signal which is different from the fifth and sixth channel signals and for producing a non-suppressed carrier subchannel signal;  
 first switching means for passing the non-suppressed carrier subchannel signal from said modulating means responsive to the second composite signal produced by said composing means and for interrupting the non-suppressed carrier subchannel signal responsive to the first composite signal produced by said composing means;  
 means for adding the output signal of said composing means, the pilot signal and the output signal of said switching means to produce 4-channel signals responsive to the first composite signal produced by said composing means and to produce the 2- channel signals responsive to the second composite signal and the program signal produced by said composing means; and  
 means for transmitting the output signal of said adding means.  
  2. A receiving system for selectively reproducing either 4-channel signals or Z-channel signal, said 4- channel signals including a first main channel signal, a pilot signal having a predetermined frequency, a first suppressed carrier subchannel signal and a second suppressed carrier subchannel signal, said first main channel signal being a sum signal of first, second, third and fourth channel signals, said first suppressed carrier subchannel signal having a first carrier of a frequency equal to twice the predetermined frequency and having a first modulating signal modulated on the first carrier and a second modulating signal modulated in quadrature with the first carrier, said first modulating signal being the first channel signal plus the second channel signal minus the third channel signal minus the fourth channel signal, said second modulating signal being the first channel signal minus the second channel signal minus the third channel signal plus the fourth channel signal, said second suppressed carrier subchannel sig nal having a second carrier of a frequency equal to four times the predetermined frequency and having a third modulating signal modulated on the second carrier, said third modulating signal being the first channel signal minus the second channel signal plus the third channel signal minus the fourth channel signal; said 2- channel signals including a second main channel signal, the pilot signal, a third suppressed carrier subchannel signal and a non-suppressed carrier subchannel signal, said second main channel signal being a sum signal of fifth and sixth channel signals, said third suppressed carrier subchannel signal having a third carrier of a frequency equal to twice the predetermined frequency and having a fourth modulating signal modulated on the third carrier, said fourth modulating signal being a difference signal between the fifth and sixth channel signals, said non-suppressed carrier subchannel signal having a fourth carrier which frequency is equal to four times the predetermined frequency and having a program signal modulated on the fourth carrier; said receiving system comprising:  
 first filter means responsive to the 4-channel signals and the 2-channel signals for passing the pilot signal; first generator means responsive to the output signal of said first filter means for generating a signal having a frequency which is twice the frequency of the pilot signal; second generator means responsive to the output signal of said first filter means for generating a signal having a frequency which is four times the frequency of the pilot signal; second filter means responsive to the 2-channel signals for passing the carrier of the non-suppressed carrier subchannel signal; control signal generating means responsive to the output signal of said second filter means for producing a control signal during the time while the carrier of the non&#39;suppressed carrier subchannel signal is present; third filter means responsive to the 4-channel signals and the 2-channel signals for passing the first main channel signal and the first suppressed carrier sub channel signal among the 4-channel signals, and the second main channel signal and the third suppressed carrier subchannel signal among the 2- channel signals respectively; demodulator means responsive to the 2-channel signals for reproducing the program signal; first switching means responsive to the control signal for passing the Z-channel signals to said demodulator means responsive to the control signal and for interrupting the passage of any 4-channel signals to said demodulator responsive to an absence of the control signal; second switching means responsive to the control signal for passing the output signal of said third filter means responsive to the control signal and for bypassing said third filter means responsive to an absence of the control signal; and separating means responsive to the output signal of said second switching means and the output signals of the first and second generator means for separating the first, second, third and fourth channel signals during the time while the 4-channel signals are present, and for separating the fifth and sixth channel signals during the time while the Z-channel signals are present. 3. A system for transmitting either 4-channel signals or Z-channel signals comprising:  
 main oscillator means for generating a pilot signal having a predetermined frequency;  
 first oscillator means responsive to the pilot signal for generating a first signal having a frequency which is twice that of the pilot signal;  
 second oscillator means responsive to the first signal for generating a second signal having a frequency which is four times that of the pilot signal;  
 composing means responsive to the first and second signals and to first, second, third and fourth channel signals for producing a first composite signal including a first main channel signal, a first suppressed carrier subchannel signal and a second suppressed carrier subchannel signal, said first main channel signal being a sum signal of the first, second, third and fourth channel signals, said first suppressed carrier subchannel signal having a first carrier of a frequency equal to twice the predetermined frequency and having a first modulating sig nal modulated on the first carrier and a second modulating signal modulated in quadrature with the first carrier, said first modulating signal being the first channel signal plus the second channel signal minus the third channel signal minus the fourth channel signal, said second modulating signal being the first channel signal minus the second channel signal minus the third channel signal plus the fourth channel signal, said second suppressed carrier subchannel signal having a second carrier of a frequency equal to four times the predetermined frequency and having a third modulating signal modulated on the second carrier, said third modulating signal being the first channel signal minus the sec ond channel signal plus the third channel signal minus the fourth channel signal;  
 means for also operating said composing means responsive to the first and second signals and to fifth and sixth channel signals to produce a second composite signal including a second main channel signal and a third suppressed carrier subchannel signal, said second main channel signal being a sum signal of the fifth and sixth channel signal, said third suppressed carrier subchannel signal having a third carrier of a frequency equal to twice the predetermined frequency and having a fourth modulating signal modulated on the third carrier, said fourth modulating signal being a difference signal between the fifth and sixth channel signals;  
 phase-shifting means responsive to the second signal for phase-shifting the second signal by modulating means for modulating the output signal of said phase-shifting means with a program signal which is different from the fifth and sixth channel signals to produce a modulated signal;  
 switching means for passing the modulated signal from said modulating means responsive to the second composite signal produced by said composing means and for interrupting the modulating means responsive to the first composite signal produced by said composing means;  
 means for adding the output signal of said composing means, the pilot signal, and the output signal of said switching means so that 4-channel signals are produced responsive to the first composite signal produced by said composing means and 2-channel signals including the program signal are produced responsive to the second composite signal produced by said composing means; and  
 means for transmitting the output signal of said add ing means.  
  4. A receiving system for selectively reproducing either 4-channel signals or 2-channel signals, said 4- channel signals including a first main channel signal, a pilot signal having a predetermined frequency, a first suppressed carrier subchannel signal and a second suppressed carrier subchannel signal, said first main channel signal being a sum signal of first, second, third and fourth channel signals, said first suppressed carrier subchannel signal having a first carrier of a frequency equal to twice the predetermined frequency and having a first modulating signal modulated on the first carrier and a second modulating signal modulated in quadrature with the first carrier, said first modulating signal being the first channel signal plus the second channel signal minus the third channel signal minus the fourth channel signal, said second modulating signal being the first channel signal minus the second channel signal minus the third channel signal plus the fourth channel signal, said second suppressed carrier subchannel signal having a second carrier of a frequency equal to four times the predetermined frequency and having a third modulating signal modulated on the second carrier, said third modulating signal being the first channel signal minus the second channel signal plus the third channel signal minus the fourth channel signal; said 2- channel signals including a second main channel signal, the pilot signal, a third suppressed carrier subchannel signal and a fourth subchannel signal, said second main channel signal being a sum signal of fifth and sixth channel signals, said third suppressed carrier subchannel signal having a third carrier of a frequency equal to twice the predetermined frequency and having a fourth modulating signal modulated on the third carrier, said fourth modulating signal being a difference signal between the fifth and sixth channel signals, said fourth subchannel signal having a fourth carrier of a frequency equal to four times the predetermined frequency and which phase is in quadrature with that of the second carrier, and having a program signal modulated on the fourth carrier; said receiving system comprising:  
 first filter means responsive to the 4-channel signals and the 2-channel signals for passing the pilot signal;  
 first generator means responsive to the output signal of said first filter means for generating a signal having a frequency which is twice the frequency of the pilot signal;  
 second generator means responsive to the output signal of said first generator means for generating a signal having a frequency which is twice the frequency of the output signal of said first generator means;  
 second filter means for passing the second suppressed carrier subchannel among the 4-channel signals and for passing the fourth subchannel among the 2-channel signals;  
 phase-shifting means responsive to the output signal of said second generator means for phase-shifting the output signal of said second generator means by demodulator means responsive to the output signal of said phase-shifting means and the output signal of said second filter means for reproducing the program signal; and  
 separating means responsive to the 4-channel signals and the output signals of said first and second generator means for separating the first, second, third and fourth channel signals responsive to the 4- channel signals, and responsive to the 2-channel signals and the output signals of said first and second generator means for separating the fifth and sixth channel signals responsive to the Z-channel signals.  
  5. A transmitting system as defined in claim 1 further comprising attenuator means for attenuating the level of the output signal of said composing means; and second switching means cooperative with said first switching means for passing the output signal of said attenuator means during the time when said composing means produces the second composite signal and for bypassing the attenuator means during the time when said composing means produces the first composite signal; said adding means adding the output signal of said second switching means, the second signal, and the output signal of said switching means.  
  6. A transmitting system as defined in claim 3 further comprising attenuator means for attenuating the level of the output signal of said composing means; and second switching means cooperating with said switching means for passing the output signal of said attenuator means during the time while said composing means produces the second composite signal and for bypassing the attenuator means during the time while said composing means produces the first composite signal; said adding means adding the output signal of said second switching means, the second signal and the output signal of said switching means.