Patent GB-1391003-A (GB)
Title: Recording system for a multichannel record disc
IPC: H04S3/00
Filed: 1972-06-08
Abstract: 1391003 Quadraphonic systems NIPPON VICTOR KK 8 June 1972 [12 June 1971] 26856/72 Heading H4R In a four channel disc recording system using sum signals and angle modulated difference signals of form (Ch1+Ch2)+A(Ch1-Ch2) and (Ch3 + Ch4) + A(Ch3 - Ch4) recorded on the left and right walls of a disc groove respectively, the angle modulated difference signals A(Ch1- Ch2) and A(Ch3 - Ch4) are phase changed relative to the signals (Ch1+Ch2 and (Ch3+ Ch4) respectively during the recording process to cancel out undesired phase deviations which occur during tracking of the record disc by a reproducing stylus of predetermined radius. These undesired phase deviations occur because the stylus tip does not engage the groove wall at the ideal point. The amount of phase shift change required depends upon the radius of the stylus tip, the disc speed and the radial distance of the stylus from the centre of the disc. The phase shift for each channel is derived from the respective sum signal (Ch1+ Ch2) and (Ch3 + Ch4). Preferably, the angle modulated difference signals are frequency modulated for frequencies below 800 Hz and above 6 kHz and are phase modulated for signals in the range 800 Hz to 6 kHz. The sum signal frequency band is 30 Hz to 15 kHz while the angle modulated difference signal band is 20 kHz to 50 kHz. A system (Fig. 2), has four inputs 21, 22, 21a, 22a. Suffix a relates to circuit parts for signals Ch3 and Ch4 and these circuit parts are identical to those for signals Chl and Ch2 and are not further described. Inputs 21, 22 receive signals Ch1 and Ch2 respectively which are matrixed to form sum and difference signals. The sum signal is applied to a combined 30 to 15,000 Hz pass filter and delay 24, the delay time matching that of the difference signal in a modulation or demodulation. The difference signal is applied to an equalizer 26 having a -6 dB octave response in the frequency ranges below 800 Hz and above 6 kHz and a flat frequency response in the range 800 Hz to 6 kHz. It comprises a network of capacitors and resistors connected in a negative feedback circuit of an amplifier (Fig. 5, not shown). The equalized difference signals are angle modulated in phase modulator 27, which shares a single carrier oscillator (not shown), with phase modulator 27a. The angle modulated difference signals are mixed with the filtered, delayed sum signals in mixer 25 to form the multiplex signal. This is fed to an RIAA standard characteristic equalizer 28, amplified and applied to the driving coil of a disc cutter 30. The required phase shift is obtained by taking a portion of the output sum signal from matrix 23 and feeding it to an RIAA equalizer 31 having a velocity amplitude versus frequency response characteristic (Fig. 7, not shown). The output from equalizer 31 is applied to an attenuator 32 whose value is altered in dependance on the radial distance of the cutter from the disc centre, and thus upon the linear velocity of the stylus relative to the groove. The output signal from attenuator 32 is proportional to tan #, where tan #=y/x (x is the linear velocity of the stylus relative to the groove and y is the velocity amplitude, i.e. the velocity normal to the direction of such linear velocity). Function tan # is converted to sin # in a network 33 comprising resistors and diodes. A variable resistor 34 is set in correspondence with the radius r of the stylus to be used in playing back the disc. The output from resistor 34 is representative of r.sin # and it can be shown that the phase shift required to compensate for the undesired phase deviations is also r.sin #. Thus the output of resistor 34 is applied to the phase modulator 27 to provide desired compensation. A circuit 35 changes the signal phase by either 0 or 180 degrees to conform with the standard that the groove walls move upward (downward), when the left (right), channel is of positive polarity. Circuitry 31, 32, 33, 34, 35 is described in detail (Fig. 14, not shown).