System for recording and/or reproducing four channel signals on a record disc

Abstract

This system records four channel signals on or reproduces them from a record disc. The four channel signals are combined into sum signals and difference signals, respectively, each composed of two signals. Each difference signal is converted into an angle modulated wave by modulating a carrier from a single oscillator and is multiplexed with each sum signal. The four channel signals picked up from the record disc are reproduced and sounded respectively from four speakers placed at predetermined positions.

What I claim is: 
     
       1. A system for recording four channel signals in a single groove cut on a record disc, said system comprising signal source means for supplying four separate channel signals, matrix circuit means for composing a first sum signal and a first difference signal responsive to the first and second channel signals and also a second sum signal and a second difference signal responsive to the third and fourth channel signals of the four separate channel signals, single oscillator means for generating a carrier wave having a predetermined frequency, first modulator means for angle modulating said carrier wave with the first difference signal, second modulator means for angle modulating said carrier wave with the second difference signal, each of said modulator means comprising means whereby the phase angle deviation of said carrier wave is between 0.3 and 3 radians .Iadd.for at least those frequencies which are higher than a predetermined frequency when the level of each of said difference signals applied respectively to said two modulator means is a standard level.Iaddend., the lower frequency limit of the carrier wave in the output of each of said modulator means being higher than the upper frequency limit of the first and second sum signals, first mixer means for mixing and multiplexing the first sum signal with the output signal of the first modulator means, second mixer means for mixing and multiplexing the second sum signal with the output signal of the second modulator means, and means for simultaneously recording the output signal of said first mixer means on one wall of a single groove of the record disc and recording the output signal of said second mixer means on the other wall of the groove. 
     
     
       2. A system for reproducing the four channel signals recorded by the apparatus of the system of claim 1, said reproducing system comprising first separator means for separating the first sum signal and the angle modulated signal which is modulated with the first difference signal, said first sum signal and the angle modulated signal being electrical signals generated in response to the signals recorded on the one wall of the groove of the record disc, second separator means for separating the second sum signal and the angle modulated signal which is modulated with the second difference signal, said second sum signal and the angle modulated signal being electrical signals generated in response to the signals recorded on the other wall of the groove of the record disc, first demodulator means for demodulating the angle modulated signal separated by the first separator means and thus reproducing the first difference signal, second demodulator means for demodulating the angle modulated signal separated by the second separator means and thus reproducing the second difference signal, second matrix circuit means responsive to the separated first sum signal and the reproduced first difference signal for separately reproducing the original first and second channel signals, and third matrix circuit means responsive to the separated second sum signal .Iadd.and the reproduced second difference signal .Iaddend.for separately reproducing the original third and fourth channel signals. 
     
     
       3. A system for reproducing the four channel signals recorded by the apparatus of the system according to claim 1, said reproducing system comprising pickup means for picking up a composite signal responsive to different signals recorded on the two sidewalls of a record groove, first separator means for separating the first sum signal and the angle modulated signal which is modulated with the first difference signal, said first sum signal and the angle modulated signal being electrical signals generated by the pickup means in response to the signals recorded on the one wall of the groove of the record disc, second separator means for separating the second sum signal and the angle modulated signal which is modulated with the second difference signal, said second sum signal and the angle modulated signal being electrical signals generated by the pickup means in response to the signals recorded on the other wall of the groove of the record disc, first demodulator means for demodulating the angle modulated signal separated by the first separator means and thus reproducing the first difference signal, second demodulator means for demodulating the angle modulated signal separated by the second separator means and thus reproducing the second difference signal, second matrix circuit means responsive to the separated first sum signal and the reproduced first difference signal for separately reproducing the original first and second channel signals, and third matrix circuit means responsive to the separated second sum signal and the reproduced second difference signal for separately reproducing the original third and fourth channel signals. 
     
     
       4. A system for reproducing the four channel signals recorded by the apparatus of the system of claim 1, said reproducing system comprising pickup means for generating first electrical signals responsive to the signals recorded on the one wall of the groove and other electrical signals responsive to the signals recorded on the other wall of the groove, first separator means for separating the first sum signal and the angle modulated signal which is modulated with the first difference signal, second separator means for separating the second sum signal and the angle modulated signal which is modulated with the second difference signal, first demodulator means for demodulating the angle modulated signal separated by the first separator means and thus reproducing the first difference signal, second demodulator means for demodulating the angle modulated signal separated by the second separator means and thus reproducing the second difference signal, second matrix circuit means responsive to the separated first sum signal and the reproduced first difference signal for separately reproducing the original first and second channel signals, third matrix circuit means responsive to the separated second sum signal and the reproduced second difference signal for separately reproducing the original third and fourth channel signals, and means comprising four speakers for individually generating sound respectively responsive to the four reproduced channel signals. 
     
     
       5. The reproducing system as defined in claim 4, wherein said four speakers are arranged with respect to the listener so that the first speaker is placed at the front and on one side, the second speaker at the rear and on the one side, the third speaker at the front and on the other side, and the fourth speaker at the rear and on the other side. 
     
     
       6. A system for recording four channel signals in a single groove cut on a record disc, said system comprising signal source means for supplying four separate channel signals, matrix circuit means for composing a first sum signal and a first difference signal responsive to the first and second channel signals and also a second sum signal and a second difference signal responsive to the third and fourth channel signals of the four separate channel signals, single oscillator means for generating a carrier wave having a predetermined frequency, first modulator means for angle modulating said carrier wave with the first difference signal, second modulator means for angle modulating said carrier wave with the second difference signal, each of said modulator means comprising means whereby the phase angle deviation of said carrier wave is between 0.3 and 3 radians for frequencies which are included in the four channel signals, the lower frequency limit of the carrier wave in the output of each of said modulator means being higher than the upper frequency limit of the first and second sum signals, first mixer means for mixing and multiplexing the first sum signal with the output signal of the first modulator means, second mixer means for mixing and multiplexing the second sum signal with the output signal of the second modulator means, and means for simultaneously recording the output signal of said first mixer means on one wall of a single groove of the record disc and recording the output signal of said second mixer means on the other wall of the groove. 
     
     
       7. A system for recording four channel signals in a single groove cut on a record disc, said system comprising signal source means for supplying four separate channel signals, matrix circuit means for composing a first sum signal and a first difference signal responsive to the first and second channel signals and also a second sum signal and a second difference signal responsive to the third and fourth channel signals of the four separate channel signals, single oscillator means for generating a carrier wave having a predetermined frequency, first modulator means for angle modulating said carrier wave with the first difference signal, second modulator means for angle modulating said carrier wave with the second difference signal, each of said modulator means comprising means whereby the phase angle deviation of said carrier wave is between 0.3 and 3 radians for frequencies in at least the vicinity of or more than 800 Hz of the difference signals, the lower frequency limit of the carrier wave in the output of each of said modulator means being higher than the upper frequency limit of the first and second sum signals, first mixer means for mixing and multiplexing the first sum signal with the output signal of the first modulator means, second mixer means for mixing and multiplexing the second sum signal with the output signal of the second modulator means, and means for simultaneously recording the output signal of said first mixer mean on one wall of a single groove of the record disc and recording the output signal of said second mixer means on the other wall of the groove. 
     
     
       8. A system for recording four channel signals on a record disc, said system comprising a signal source means for supplying four separate channel signals, matrix circuit means for composing a first sum signal and a first difference signal responsive to the first and second channel signals and a second sum signal and a second difference signal responsive to the third and fourth channel signals among the four separate channel signals, single oscillator means for oscillating to provide a signal having a predetermined frequency, frequency divider means operated responsive to the oscillator signal of predetermined frequency for dividing the frequency thereof, means responsive to the output of the frequency divider means for generating a saw-tooth waveform signal, first serrasoid modulator means for angle-modulating said saw-tooth waveform signal with the first difference signal, second serrasoid modulator means for angle-modulating said saw-tooth waveform signal with the second difference signal, first frequency multiplier means for multiplying the frequency of the output signal of the first serrasoid modulator means, second frequency multiplier means for multiplying the frequency of the output signal of the second serrasoid modulator means, first mixer means responsive to the output signal of the first frequency multiplier means and the output signal of the single oscillator means for producing a signal having frequencies equal to the difference between the frequencies of the output signal of the first frequency multiplier means and the predetermined frequency of the single oscillator means, second mixer means responsive to the output signal of the second frequency multiplier means and the output signal of the single oscillator means for producing a signal having frequencies equal to the difference between the frequencies of the output signal of the second frequency multiplier means and the predetermined frequency of the single oscillator means, means whereby the carrier frequencies of the output signals of said first and second mixer means have frequencies in a recordable frequency range of the record disc, the lower frequency limit of said carrier wave being higher than the upper frequency limit of the first and second sum signals, third mixer means for mixing and multiplexing the first sum signal with the output signal of the first mixer means, fourth mixer means for mixing and multiplexing the second sum signal with the output signal of the second mixer means, and means for simultaneously recording the output signal of the third mixer means on one wall of a single groove of the record disc and the output signal of the fourth mixer means on the other wall of the groove. 
     
     
       9. The system of claim 8 wherein the phase angle deviation of the carrier wave of each of the output signals of said first and second mixer means is between 0.3 and 3 radians. 
     
     
       10. A system for recording four channel signals on a record disc, said system comprising signal source means for supplying four separate channel signals, matrix circuit means for composing a first sum signal and a first difference signal responsive to the first and second channel signals and a second sum signal and a second difference signal responsive to the third and fourth channel signals among the four separate channel signals, first equalizer means for receiving the first difference signal, second equalizer means for receiving the second difference signal, said first and second equalizer means having a characteristic frequency response which increases in proportion to an increase of frequency at a slope of 6 dB/octave, single oscillator means for generating a carrier wave having a predetermined frequency, first modulator means for frequency modulating said carrier wave responsive to the output signal of the first equalizer means, second modulator means for frequency modulating said carrier wave responsive to the output signal of the second equalizer means, the phase angle deviation of the modulated carrier wave being between 0.3 and 3 radians in each output of said first and second modulator means, and the lower frequency limit of said modulated carrier wave being higher than the upper frequency limit of said first and second sum signals, first mixer means for mixing and multiplexing the first sum signal with the output signal of the first modulator means, second mixer means for mixing and multiplexing the second sum signal with the output signal of the second modulator means, and means for simultaneously recording the output signal of said first mixer means on one wall of a single groove of the record disc and the output signal of said second mixer means on the other wall of the groove. 
     
     
       11. A system for reproducing the four channel signals recorded by the system of claim 10, said reproducing system comprising first separator means for separating the first sum signal and the frequency modulated signal which is modulated with the first difference signal, said first sum signal and the angle modulated signal being electrical signals generated in response to the signals recorded on the one wall of the groove of the record disc, second separator means for separating the second sum signal and the frequency modulated signal which is modulated with the second difference signal, said second sum signal and the angle modulated signal being electrical signals generated in response to the signals recorded on the other wall of the groove of the record disc, first demodulator means for demodulating the frequency modulated signal separated by the first separator means and thus reproducing the first difference signal, second demodulator means for demodulating the frequency modulated signal separated by the second separator means and thus reproducing the second difference signal, first equalizer means for receiving the demodulated first difference signal, second equalizer means receiving the demodulated second difference signal, said first and second equalizer means having a characteristic frequency response which decreases in proportion to an increase of frequency at a slope of 6 dB/octave, second matrix circuit means responsive to the separated first sum signal and the output signal of the first equalizer means for separately reproducing the original first and second channel signals, and third matrix circuit means responsive to the separated second sum signal and the output signal of the second equalizer means for separately reproducing the original third and fourth channel signals. 
     
     
       12. The reproducing system as defined in claim 11 wherein said first and second equalizer means have a falling slope frequency characteristic of 6 dB/octave in a frequency range which is higher than a predetermined frequency and a flat characteristic in a frequency range which is lower than said predetermined frequency. 
     
     
       13. The reproducing system as defined in claim 12 wherein said predetermined frequency is a selected certain frequency within a range of 100 to 2,000 Hz. 
     
     
       14. An apparatus for recording four channel signals in a single groove cut on a record disc, said apparatus comprising signal source means for supplying four separate channel signals, matrix circuit means for composing a first sum signal and a first difference signal responsive to the first and second channel signals and also a second sum signal and a second difference signal responsive to the third and fourth channel signals of the four separate channel signals, single oscillator means for generating a carrier wave having a predetermined frequency, first modulator means for angle modulating said carrier wave with the first difference signal, second modulator means for angle modulating said carrier wave with the second difference signal, each of said modulator means comprising means whereby the phase angle deviation of said carrier wave is between 0.3 and 3 radians, the lower frequency limit of the carrier wave in the output of each of said modulator means being higher than the upper frequency limit of the first and second sum signals, first mixer means for mixing and multiplexing the first sum signal with the output signal of the first modulator means, second mixer means for mixing and multiplexing the second sum signal with the output signal of the second modulator means, and means for simultaneously recording the output signal of said first mixer means on one wall of a single groove of the record disc and recording the output signal of said second mixer means on the other wall of the groove. 
     
     
       15. A system for recording four channel signals in a single groove cut on a record disc, said system comprising signal souce means for supplying four separate channel signals, matrix circuit means for .[.comprising.]. .Iadd.composing .Iaddend.a first sum signal and a first difference signal responsive to the first and second channel signals and also a second sum signal and a second difference signal responsive to the third and fourth channel signals of the four separate channel signals, single oscillator means for generating a carrier wave having a predetermined frequency, first modulator means for angle modulating said carrier wave with the first difference signal, second modulator means for angle modulating said carrier wave with the second difference signal, each of said modulator means comprising means whereby the phase angle deviation of said carrier wave is between 0.3 and 3 radians for frequencies which are higher than a predetermined frequency when the level of each of said difference signals applied respectively to said two modulator means is a standard level, the lower frequency limit of the carrier wave in the output of each of said modulator means being higher than the upper frequency limit of the first and second sum signals, first mixer means for mixing and multiplexing the first sum signal with the output signal of the first modulator means, second mixer means for mixing and multiplexing the second sum signal with the output signal of the second modulator means, and means for simultaneously recording the output signal of said first mixer means on one wall of a single groove of the record disc and recording the output signal of said second mixer means on the other wall of the groove. 
     
     
       16. A system for recording four channel signals in a single groove cut on a record disc, said system comprising signal source means for supplying four separate channel signals, matrix circuit means for composing a first sum signal and a first difference signal responsive to the first and second channel signals and also a second sum signal and a second difference signal responsive to the third and fourth channel signals of the four separate channel signals, single oscillator means for generating a carrier wave having a predetermined frequency, first modulator means for angle modulating said carrier wave with the first difference signal, second modulator means for angle modulating said carrier wave with the second difference signal, each of said modulator means comprising means whereby the phase angle deviation of said carrier wave is between 0.3 and 3 radians for frequencies which are higher than a predetermined frequency when the level of each of said difference signals applied respectively to said two modulator means is a standard level, said predetermined frequency lying in the difference signals and being a frequency selected within a range extending from 100 to 2,000 Hz, the lower frequency limit of the carrier wave in the output of each of said modulator means being higher than the upper frequency limit of the first and second sum signals, first mixer means for mixing and multiplexing the first sum signal with the output signal of the first modulator means, second mixer means for mixing and multiplexing the second sum signal with the output signal of the second modulator means, and means for simultaneously recording the output signal of said first mixer means on one wall of a single groove of the record disc and recording the output signal of said second mixer means on the other wall of the groove. 
     
     
       17. A system for recording four channel signals in a single groove cut on a record disc, said system comprising signal source means for supplying four separate channel signals, matrix circuit means for composing a first sum signal and a first difference signal responsive to the first and second channel signals and also a second sum signal and a second difference signal responsive to the third and fourth channel signals of the four separate channel signals, single oscillator means for generating a carrier wave having a predetermined frequency, first modulator means for angle modulating said carrier wave with the first difference signal, second modulator means for angle modulating said carrier wave with the second difference signal, each of said modulator means comprising means whereby the phase angle deviation of said carrier wave is between 0.3 and 3 radians for frequencies which are higher than a predetermined frequency when the level of each of said difference signals applied respectively to said two modulator means is a standard level, said predetermined frequency being a frequency in the difference signals and in the vicinity of 800 Hz, the lower frequency limit of the carrier wave in the output of each of said modulator means being higher than the upper frequency limit of the first and second sum signals, first mixer means for mixing and multiplexing the first sum signal with the output signal of the first modulator means, second mixer means for mixing and multiplexing the second sum signal with the output signal of the second modulator means, and means for simultaneously recording the output signal of said first mixer means on one wall of a single groove of the record disc and recording the output signal of said second mixer means on the other wall of the groove. 
     
     
       18. A system for recording four channel signals in a single groove cut on a record disc, said system comprising signal source means for supplying four separate channel signals, matrix circuit means for composing a first sum signal and a first difference signal responsive to the first and second channel signals and also a second sum signal and a second difference signal responsive to the third and fourth channel signals of the four separate channel signals, single oscillator means for generating a carrier wave having a predetermined frequency, first modulator means for angle modulating said carrier wave with the first difference signal, second modulator means for angle modulating said carrier wave with the second difference signal, each of said modulator means comprising means whereby the phase angle deviation of said carrier wave is between 0.3 and 3 radians for the first and second difference signals at a standard level, the lower frequency limit of the carrier wave in the output of each of said modulator means being higher than the upper frequency limit of the first and second sum signals, first mixer means for mixing and multiplexing the first sum signal with the output signal of the first modulator means, second mixer means for mixing and multiplexing the second sum signal with the output signal of the second modulator means, and means for simultaneously recording the output signal of said first mixer means on one wall of a single groove of the record disc and recording the output signal of said second mixer means on the other wall of the groove. 
     
     
       19. A system for recording four channel signals in a single groove cut on a record disc, said system comprising signal source means for supplying four separate channel signals, matrix circuit means for composing a first sum signal and a first difference signal responsive to the first and second channel signals and also a second sum signal and a second difference signal responsive to the third and fourth channel signals of the four separate channel signals, single oscillator means for generating a carrier wave having a predetermined frequency, first modulator means for angle modulating said carrier wave with the first difference signal, second modulator means for angle modulating said carrier wave with the second difference signal, each of said modulation means comprising means whereby the phase angle deviation of said carrier wave is between 0.3 and 3 radians for the first and second difference signals at a standard level and for frequencies higher than a predetermined frequency, the lower frequency limit of the carrier wave in the output of each of said modulator means being higher than the upper frequency limit of the first and second sum signals, first mixer means for mixing and multiplexing the first sum signal with the output signal of the first modulator means, second mixer means for mixing and multiplexing the second sum signal with the output signal of the second modulator means, and means for simultaneously recording the output signal of said first mixer means on one wall of a single groove of the record disc and recording the output signal of said second mixer means on the other wall of the groove. 
     
     
       20. A system for recording four channel signals on a record disc, said system comprising signal source means for supplying four separate channel signals, matrix circuit means for composing a first sum signal and a first difference signal responsive to first and second of said four channel signals and a second sum signal and a second difference signal responsive to third and fourth of said four channel signals, first equalizer means for receiving the first difference signal, second equalizer means for receiving the second difference signal, said first and second equalizer means having a characteristic frequency responsive including at least a slope portion of 6 dB/octave, single oscillator means for generating a carrier wave having a prescribed frequency, first modulator means for angle modulating said carrier wave responsive to the output signal of the first equalizer means, second modulator means for angle modulating said carrier wave responsive to the output signal of the second equalizer means, the lower frequency limit of said modulated carrier wave being higher than the upper frequency limit of said first and second sum signals, first mixer means for mixing and multiplexing the first sum signal with the output signal of the first modulator means, second mixer means for mixing and multiplexing the second sum signal with the output signal of the second modulator means, and means for simultaneously recording the output signal of said first mixer means on one wall of a single groove of the record disc and the output signal of said second mixer means on the other wall of the groove. 
     
     
       21. A system for reproducing the four channel signals recorded by the system of claim 20, said reproducing system comprising first separator means for separating the first sum signal and the angle modulated signal which is modulated with the first difference signal, said first sum signal and the associated angle modulated signal being electrical signals generated in response to the signals recorded on the one wall of the groove of the record disc, second separator means for separating the second sum signal and the angle modulated signal which is modulated with the second difference signal, said second sum signal and the associated angle modulated signal being electrical signals generated in response to the signals recorded on the other wall of the groove of the record disc, first demodulator means for demodulating the angle modulated signal separated by the first separator means and thus reproducing the first difference signal, second demodulator means for demodulating the angle modulated signal separated by the second separator means and thus reproducing the second difference signal, third equalizer means for receiving the demodulated first difference signal, fourth equalizer means receiving the demodulated second difference signal, said third and fourth equalizer means having a characteristic frequency response including at least a slope portion of 6 dB/octave, said slope portion of the characteristic of the third and fourth equalizer means having a reverse slope with respect to the slope of the slope portion of the characteristic of the first and the second equalizer means in the recording system, and second matrix circuit means for separately reproducing the original first and second channel signals responsive to the separated first sum signal and the output signal of the third equalizer means and also the original third and fourth channel signals responsive to the separated second sum signal and the output signal of the fourth equalizer means. 
     
     
       22. A system for recording four channel signals on a record disc, said system comprising signal source means for supplying four separate channel signals, matrix circuit means for composing a first sum signal and a first difference signal responsive to first and second of said four channel signals and a second sum signal and a second difference signal responsive to third and fourth of said four channel signals, first equalizer means for receiving the first difference signal, second equalizer means for receiving the second difference signal, each of said equalizer means having a characteristic frequency response including at least a flat portion for frequencies lower than a predetermined frequency and a slope portion of 6 dB/octave for frequencies higher than said predetermined frequency, single oscillator means for generating a carrier wave having a prescribed frequency, first modulator means for angle modulating said carrier wave responsive to the output signal of the first equalizer means, second modulator means for angle .[.demodulating.]. .Iadd.modulating .Iaddend.said carrier wave responsive to the output signal of the second equalizer means, the lower frequency limit of said modulated carrier wave being higher than the upper frequency limit of said first and second sum signals, first mixer means for mixing and multiplexing the first sum signal with the output signal of the first modulator means, second mixer means for mixing and multiplexing the second sum signal with the output signal of the second modulator means, and means for simultaneously recording the output signal of said first mixer means on one wall of a single groove of the record disc and the output signal of said second mixer means on the other wall of the groove. 
     
     
       23. A system for reproducing the four channel signals recorded by the system of claim 22, said reproducing system comprising first separator means for separating the first sum signal and the associated angle modulated signal which is modulated with the first difference signal, said first sum signal and the associated angle modulated signal being electrical signals generated in response to the signals recorded on the one wall of the groove of the record disc, second separator means for separating the second sum signal and the associated angle modulated signal which is modulated with the second difference signal, said second sum signal and the angle modulated signal being electrical signals generated in response to the signals recorded on the other wall of the groove of the record disc, first demodulator means for demodulating the angle modulated signal separated by the first separator means and thus reproducing the first difference signal, second demodulator means for demodulating the angle modulated signal separated by the second separator means and thus reproducing the second difference signal, third equalizer means for receiving the demodulated first difference signal, fourth equalizer means for receiving the demodulated second difference signal, said third and fourth equalizer means having a characteristic frequency response including at least a flat portion for frequencies lower than the predetermined frequency and a slope portion of 6 dB/octave for frequencies higher than said predetermined frequency, said slope portion of the characteristic of the third and fourth equalizer means having a reverse slope to the slope of the slope portion of the characteristic of the first and the second equalizer means in the recording system, and second matrix circuit means for separately reproducing the original first and second channel signals responsive to the separated first sum signal and the output signal of the third equalizer means and also the original third and fourth channel signals responsive to the separated second sum signal and the output signal of the fourth equalizer means. 
     
     
       24. A system for recording four channel signals on a record disc, said system comprising signal source means for supplying four separate channel signals, matrix circuit means for composing a first sum signal and a first difference signal responsive to first and second of said .[.fourth.]. .Iadd.four .Iaddend.channel signals and a second sum signal and a second difference signal responsive to third and fourth of said four channel signals, first equalizer means for receiving the first difference signal, second equalizer means for receiving the second difference signal, each of said equalizer means having a characteristic frequency response including at least a flat portion for frequencies lower than a predetermined frequency and an increasing slope portion of 6 dB/octave which increases in proportion to an increase of frequency higher than said predetermined frequency, .[.and each of said modulator means being a frequency modulator,.]. single oscillitor means for generating a carrier wave having a predescribed frequency, first modulator means for angle modulating said carrier wave responsive to the output signal of the first equalizer means, second modulator means for angle modulating said carrier wave responsive to the output signal of the second equalizer means, .Iadd.each of said modulator means being a frequency modulator, .Iaddend.the lower frequency limit of said modulated carrier wave being higher than the upper frequency limit of said first and second sum signals, first mixer means for mixing and multiplexing the first sum signal with the output signal of the first modulator means, second mixer for mixing and multiplexing the second sum signal with the output signal of the second modulator means, and means for simultaneously recording the output signal of said first mixer means on one wall of a single groove of the record disc and the output signal of said second mixer means on the other wall of the groove. 
     
     
       25. A system for reproducing four channel signals which are recorded on a record disc in such a manner that a first sum signal is multiplexed with a first difference signal angle modulated on a carrier wave responsive to a first and a second channel signal of the four channel signals, said multiplexed signal being recorded on one wall of a single groove of the record disc and a second sum signal is multiplexed with a second difference signal angle modulated on a carrier wave responsive to a third and a fourth channel signal of the four channel signals, said second named multiplexed signal being recorded on the other wall of the groove, the phase angle deviation of the carrier wave in each of the first and second angle modulated difference signals being between 0.3 and 3 radians, the lower frequency limit of the carrier wave of the first and second angle modulated difference signals being higher than the upper frequency limit of the first and second sum signals, said system comprising first separator means for separating the first named multiplexed signal including the first difference signal angle modulated on the carrier wave picked up from the one wall of the groove of the record disc into the first sum signal and the first angle modulated difference signal, second separator means for separating the second named multiplexed signal including the second difference signal angle modulated on the carrier wave picked up from the other wall of the groove of the record disc into the second sum signal and the second angle modulated difference signal, first demodulator means for demodulating the 0.3 to 3 radian phase angle deviations of the carrier wave bearing the first angle modulated difference signal separated by the first separator means and thus reproducing the first difference signal, second demodulator means for demodulating the 0.3 to 3 radian phase angle deviations of the carrier wave bearing the second angle modulated difference signal separated by the second separator means and thus reproducing the second difference signal, first matrix circuit means responsive to the separated first sum signal and the demodulated first difference signal for separately reproducing the original first and second channel signals, and second matrix circuit means responsive to the separated second sum signal and the demodulated second difference signal for separately reproducing the original third and fourth channel signals. 
     
     
       26. An apparatus for reproducing four channel signals which are recorded on a record disc in such a manner that a first sum signal is multiplexed with a first difference signal angle modulated on a carrier wave responsive to a first and a second channel signal of the four channel signals, said multiplex signal being recorded on one wall of a single groove of the record disc and a second sum signal is multiplexed with a second difference signal angle modulated on a carrier wave responsive to a third and a fourth channel signal of the four channel signals, said second named multiplexed signal being recorded on the other wall of the groove, the phase angle deviation of the carrier wave in each of the first and second angle modulated difference signals being between 0.3 and 3 radians .Iadd.for at least those freqencies which are higher than a predetermined frequency when the level of each of said difference signals applied respectively to said two modulator means is a standard level.Iaddend., the lower frequency limit of the carrier wave of the first and second angle modulated difference signals being higher than the upper frequency limit of the first and second sum signals, said apparatus comprising first separator means for separating the first named multiplexed signal including the first difference signal angle modulated on the carrier wave picked up from the one wall of the groove of the record disc into the first sum signal and the first angle modulated difference signal, second separator means for separating the second named multiplexed signal including the second difference signal angle modulated on the carrier wave picked up from the other wall of the groove of the record disc into the second sum signal and the second angle modulated difference signal, first demodulator means for demodulating the 0.3 to 3 radian phase angle deviations of the carrier wave bearing the first angle modulated difference signal separated by the first separator means and thus reproducing the first difference signal, second demodulator means for demodulating the 0.3 to 3 radian phase angle deviations of the carrier wave bearing the second angle modulated difference signal separated by the second separator means and thus reproducing the second difference signal, first matrix circuit means responsive to the separated first sum signal and the demodulated first difference signal for separately reproducing the original first and second channel signals, and second matrix circuit means responsive to the separated second sum signal and the demodulated second difference signal for separately reproducing the original third and fourth channel signals.