Patent GB-1337397-A (GB)
Title: System for recording and or reproducing four channel signals on a record disc
IPC: H04S3/00
Filed: 1970-11-25
Abstract: 1337397 Four channel systems NIPPON VICTOR KK 25 Nov 1970 [28 Nov 1969 15 March 1970 24 March 1970 5 July 1970] 56119/70 Heading H4R A quadraphonic disc recording system (Fig. 2) applies four separate channel signals to matrix circuits 16, 17 for composing a first sum signal and a first difference signal from the first and second channel signals and a second sum signal and a second difference signal from the third and fourth channel signals. A single oscillator 24 generates a carrier wave. A modulator 22 modulates the carrier wave with the first difference signal and a modulator 23 modulates the carrier wave with the second difference signal. The lower frequency limit of the C.W. in the outputs of the modulators 22, 23 is higher than the upper frequency limit of the first and second sum signals. A mixer 25 mixes and multiplexes the first sum signal with the output of modulator 22. A mixer 26 mixes and multiplexes the second sum signal with the output of modulator 23. The outputs of the mixers 25 and 26 are recorded in a single groove of a record disc 30. In the recording studio (Fig. 1, not shown), four microphones are spatially disposed and signals from them are recorded by a tape recorder 15. In the recording system (Fig. 2) equalizers 18, 19, 20, 21 have a frequency response which increases 6dB/octave, which increase may be in a frequency range higher than a threshold frequency (e.g. in the range 100Hz to 2,000Hz), there being a flat response below the threshold frequency (Figs. 13, 14, not shown). In cutting the disc 30 the tape recorder 15 and disc turntable are operated at a lower speed than normal so that the cutter 29 will engrave signals which can be played back to embrace the frequency range 0 to 45 KHz. Thus, during the engraving, each sum signal has the frequency range a (Fig. 3a) and each frequency modulated difference signal has the frequency range b. When played back, each sum signal has the frequency range A and each frequency modulated difference signal has the frequency range B. The frequency modulated carrier, on playback, has a frequency 30KHz. The limit of frequency deviation of the difference signals is higher than the upper frequency limit of the sum signals. As an alternative the signals from the four microphones may be applied directly to the matrix circuits 16, 17 (Fig. 2) and the output from the amplifiers 27, 28 applied to a tape recorder. The recorded tape may then be played back at slow speed to feed the cutter 29 at a correspondingly slow speed. This is, however, less satisfactory because white noise is produced on normal reproduction. For providing the frequency modulated difference signal a crystal oscillator 120 (Fig. 18) is used. Its output is applied to a frequency divider 121, a generator 122 deriving a sawtooth signal. The output of generator 122 is supplied as a carrier to serrasoid modulators 127, 128 capable of effecting modulation without changing the phase of the carrier and including an equalizer and a phase modulator. Difference signals are fed to input terminals 123 and 124 and are applied to modulators 127, 128 after amplification. The modulator outputs are frequency multiplied by multipliers 129, 130 and applied to mixer detectors 131, 132. These provide an output difference signal in which variation in the output of the master oscillator 120 is substantially cancelled. In the reproducing apparatus the frequency range of the pick-up 51 (Fig. 5) extends from 20Hz to 45KHz. The multiplexed signals are passed to low pass filters 53, 55 and band pass filters 54, 56. Demodulators 59, 63 recover the difference signals. Equalizers 57, 60, 61, 64 (see Figs. 15, 16, not shown) complement the equalizers 18, 19, 20, 21 (Fig. 2). The individual signals ch1, ch2, ch3 and ch4 are recovered in matrix circuits 58, 62 and, after amplification, are fed to four loudspeakers spatially related to the position of the four recording microphones.