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.

Description

This is a continuation of application Ser. No. 92,803, filed Nov. 25, 1970, now U.S. Pat. No. 3,686,471.

This invention relates to a system for recording four channel signals on and/or reproducing them from a record disc and, more particularly, to a system for recording four channel signals on a single groove of a record disc and/or reproducing these signals from this single groove.

In a conventional stereophonic record disc, two channels signals are recorded. In this stereo record, signals of the left and right channels are respectively recorded on two walls of a groove in the disc. This groove is called a 45--45 stereo system groove because both walls thereof are inclined at an angle of 45° with respect to a perpendicular line which passes through the deepest point of the groove. In this conventional two channels stereophonic system, there are two sound sources forming one sound plane, which enables this system to have a sound source reproductivity twice as large as that of a monaural system in which there is only one sound source and no sound plane whatsoever.

However, there has recently been an increasing demand for reproducing andit will beπι θ∼α in a standard level of the modulating wave in a frequency band ranging over several hundreds to several thousands Hz, which is relatively high in auditory sensitivity. This purpose of the invention is achieved by supplying carriers having the same frequency and phase to the frequency modulators 22 and 23 by means of the same local oscillator 24.

In FIG. 2, the equalizers 19 and 21 have a frequency response characteristic, as shown in FIG. 13 or FIG. 14.

In the characteristic shown in FIG. 13, the output level rises at a slope of 6 dB per octave, as the frequency increases. Accordingly, the frequency modulated waves which are obtained from the frequency modulators 22 and 24 through the equalizers 19 and 21 are substantially phase modulated waves. The phase deviation angle of the phase modulated wave shows a constant value, which is independent from variations of the frequency if the level of the modulating wave is constant.

Thus, the phase deviation angle θ of the angle modulated wave (which is obtained at the outputs of the frequency modulators 22 and 24) is set at a predetermined radian (for example, approximately 0.3 to 3 rad.), when the level of the modulating wave applied to the frequency modulators 22 and 24 is a standard level. Then, the crosstalk which may occur will not finally produce a disturbing noise in the reproduced signal.

In the characteristic shown in FIG. 14 as another example of characteristic of the equalizers 19 and 21, the characteristic is flat within a low frequency range. Sounds which are usually recorded on a record disc scarecely contain a sound of very low frequency component. Even if there is such very low frequency component, its energy is very small. Further, the phase deviation angle of the angle modulated wave, modulated by a modulating wave having a frequency corresponding to the ultra low sound frequency range, may be large. The result is a generation of the disturbing noise in the reproduced sound. The frequency component of the disturbing noise is within a range where an auditory detecting ability is relatively low. Therefore, the signal-to-noise ratio in a low frequency range is improved by making the characteristic flat below a suitable frequency, which is shown by point T in FIG. 14.

As a frequency at the point T, a suitable frequency within the range of 100 to 2000 Hz is selected. An experiment shows that a good result is obtained when the frequency around 800 Hz is selected.

In the reproducing system shown in FIG. 5, the equalizers 60 and 64 have a frequency response characteristic shown in FIG. 15 or FIG. 16. If the equalizers 19 and 20 have the characteristic shown in FIG. 13, the equalizers 60 and 64 have the characteristic as shown in FIG. 15, which is the inverse of the characteristic shown in FIG. 13. If the equalizers 19 and 21 have the characteristic shown in FIG. 14, the equalizers 60 and 64 have the characteristic as shown in FIG. 16 which is the inverse of the characteristic shown in FIG. 14.

As described hereinabove, in the system according to this invention, the phase deviation angle of the angle modulated wave (obtained by angle modulation by a modulating wave at a reference level) is set a predetermined radian. This deviation angle is set irrespective of the modulating wave frequency, particularly in a high and intermediate frequency ranges where auditory sensitivity is high. Accordingly, no disturbing noise is produced in the reproduced signal when the composite signal containing a crosstalk component is demodulated.

In case the level of the input modulating wave exceeds the standard level, a harmonic component of higher order is developed in the reproduced signal obtained by demodulating the composite signal of the angle modulated wave and the crosstalk component. The harmonic component, of higher order is in the reproduced signal obtained by demodulation, is reduced to an accoustically undetectable level by the equalizers 60 and 64, which have characteristics for attenuating signals in high frequency range. Accordingly, even in case the level of input modulating wave exceeds the standard level, no disturbing noise is produced in the reproduced signal.

The relative speed of a pickup stylus, with respect to a recored disc, in several hundred mm/sec. The size of dust particles which may fall in the groove of the record disc is several tens μ. Hence, most frequency components of noise signals generated by such dust particles are distributed within a high frequency range. Accordingly, the signal-to-noise ratio is improved by reducing the frequency characteristic of the modulated wave in a high frequency range during reproduction of the signal.

The system according to this invention is further considered from the aspect of transmission energy. A 100 percent-modulation in an amplitude modulation system corresponds to the case in which the phase deviation angle of the angle modulated wave is set at 1, radian in the system according to this invention. Accordingly, the signal-to-noise ratio is improved by about 20 dB in a high frequency range, as and about 40 dB in a low frequency range as compared with the amplitude modulation system.

In the aforementioned embodiment of the recording system, the combination of the frequency modulators 22 and 24 and the equalizers 19 and 21 is employed as a modulating means. However, the modulating means is not limited to this, but a modulator may be employed having a characteristic providing an angle modulated wave having a constant phase deviation angle relative to an input modulating wave. The deviation is at a certain level within a range above several hundred Hz.

Nextly, a specific embodiment of a frequency modulating means consisting of the frequency modulators 22 and 23 and the local oscillator 24 which are adopted in the recording system shown in FIG. 2 will be described.

FIG. 17 shows an example of a frequency modulating means which is readily conceivable from the conventional prior art. Difference signals (Ch1-Ch2) and (Ch3-Ch4) are respectively supplied from input terminals 100 and 101 to mixers 108 and 109 through reactance tubes 102 and 103, oscillating circuits 104 and 105, and buffer amplifiers 106 and 107. At the mixers 108 and 109, the difference signals are mixed with a signal from an oscillator 110. Outputs from the mixers 108 and 109 are taken out of output terminals 113 and 114 through filters 111 and 112.

The easily conceivable frequency modulating means of the aforementioned construction has the separate oscillating circuits 104 and 105 in the two channels. It is difficult to synchronize these two oscillating circuits 104 and 105. If, for example, there is no input at the terminal 100 and there is an input at the terminal 101, the frequency of the oscillating circuit 105 deviates in accordance with the input at the terminal 101. At the same time, the frequency of the oscillating circuit 104 is also deviated by the oscillating circuit 105 notwithstanding the absence of input at the terminal 100, which produces an undesirable result. If is, therefore, very difficult to control and synchronize this system so as to obtain carriers having the same frequency and phase from the terminals 113 and 114.

In the system according to this invention, it is necessary to have a modulator which is capable of obtaining signals of two channels having the same carrier frequency and phase, in order to prevent distortion and disturbing noise due to crosstalk in each channel signal. Accordingly, the aforementioned modulating means which is easily conceivable from the prior art is of no practical use in the system according to this invention.

Then, in the system according to this invention, the frequency modulating means shown in FIG. 18 is employed as a preferable embodiment. A master oscillator 120 includes a crystal oscillator with a stable output. Output from the oscillator 120 is supplied on one hand, to a frequency divider 121 at which the frequency is divided into a frequency of q/p times. A saw tooth generator 122 generates a saw tooth wave responsive to the output of the frequency divider 121. The output of the saw tooth generator 122 is supplied as a carrier to frequency modulators 127 and 128. The frequency modulators 127 and 128 respectively consist of an equalizer having a characteristic that increases in a low frequency range and a phase modulator. The difference signals (Ch1-Ch2) and (Ch3-Ch4) as are applied to input terminals 123 and 124, which are respectively supplied to the modulators 127 and 128 after being amplified by amplifiers 125 and 126.

The modulators 127 and 128 are serrasoid modulators for effecting modulation without changing the phase of the carrier. It is to be noted here that, with the modulation, a carrier and a sideband which compose a frequency modulated wave are separately considered. The sideband is changed by a modulating wave and the carrier is changed only in its level. When the modulation index is large, the phase of the carrier changes by 180°. Here, the level of the carrier has changed to negative and the phase of the carrier remains unchanged.

The output frequency modulated signals, from the modulators 127 and 128, are multiplied in the frequency thereof by frequency multipliers 129 and 130. This increases the modulation degree. The resulting signal is supplied to mixing detectors 131 and 132. At the same time, the output frequency from the master oscillator 120 is supplied to the mixing detectors 131 and 132. Obtained from the mixing detectors 131 and 132 is an output signal which has been beat down to a frequency which is the difference between the output frequency from the oscillator 120 and the frequency of output signals from the frequency multipliers 129 and 130. This output signal is taken out of output terminals 135 and 136 through filters 133 and 134.

According to the frequency modulator means of the aforementioned construction, the signals which actuate the modulators 127 and 128 and the signals which are supplied to the mixing detectors 131 and 132 are all maintained at the same frequency and in the same phase. Accordingly, it is possible to transmit the angle modulated wave without producing disturbing noises due to crosstalk, even in transmission systems in which crosstalk occurs between the L and R channels.

The modulation angle obtained from the serrasoid modulators 127 and 128 is on the order of ±1.5 radian. It is theoretically impossible to obtain a modulation angle of more than ±3 radian. Accordingly, a greater modulation angle is obtained by increasing the modulation degree by means of the frequency multipliers 129 and 130.

If it is desired to obtain carriers of the output frequency modulated signals from the terminals 135 and 136 at a low frequency, the signals must be beat down after being multiplied by the frequency multipliers 129 and 130. This will be understood from the fact that the frequency of the saw tooth wave at the serrasoid modulators must be selected at a frequency at least five times as high as a maximum modulating frequency in order to avoid the beats which will otherwise occur between the saw tooth wave and the modulating wave if the saw tooth is selected at a low frequency.

If an oscillator has an unstable output frequency and its output has already been modulated by noises such as hum, instead of the stable master oscillator 120 which supply the signal to the mixing detectors 131 and 132, the signal-to-noise ratio will extremely be deteriorated. In this case, the signal-to-noise ratio will be deteriorated in proportion to the multiplication of the frequency multiplier. If, for example, the frequency is multiplied 81-times, the signal-to-noise ratio will be deteriorated by a factor of 81.

In the system according to this invention, the output of the master oscillator 120 is multiplied after being frequency divided and frequency modulated. This multiplied output is beat down with the output from the master oscillator 120. As a result, the variation in the output of the master oscillator 120 is mostly cancelled by being beat down. Accordingly, even if the output of the master oscillator 120 is modulated in AC, there is not much variation in the beat down output and hence no deterioration in the signal-to-noise ratio.

This invention is not limited to the embodiments hereinabove described but many modifications and variations may be made without departing from the spirit and scope of the invention.

Claims

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 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, 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 and the reproduced second difference signal 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 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, 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 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.

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 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 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, each of said modulator means being a frequency modulator, 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 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, 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.