Patent GB-1410381-A (GB)
Title: Angle modulated wave demodulation system
IPC: H04S3/00
Filed: 1972-10-03
Abstract: 1410381 Quadraphonic systems; frequency demodulators NIPPON VICTOR KK 3 Oct 1972 [5 Oct 1971 16 Feb 1972 23 Feb 1972 3 March 1972 6 April 1972 22 April 1972 24 May 1972] 45538/72 Headings H4R and H3A An angle modulated wave demodulation system in a quadraphonic sound system (e.g. in a four channel disc record having two multiplexed signals Ch1 + Ch2 + F(Ch1-Ch2) and Ch3 + Ch4 + F(Ch3 - Ch4) recorded on its walls as described in Specification 1,337,397) comprises a phase locked loop system which reduces or eliminates noise generated in the demodulator caused by abrupt loss of the carrier for example due to the pickup stylus moving over dust in the record groove, or due to imperfections of the record groove, or because the waveform on the groove is very fine. The level of the input angle modulated wave applied to the phase comparator of the phase locked loop system is preset. The phase locked loop system 19 (Fig. 1) comprises a phase comparator 20 supplied, via a level adjuster 18, with an angle modulated wave signal, e.g. F(Ch1-Ch2), and a voltage controlled oscillator 23 which is supplied with a part of the output of the phase comparator 20 as a control voltage to control its oscillation frequency and supplies its output to the phase comparator 20. PLL 19 has a lock range defined within the curves of Fig. 3 and the input to the phase comparator 20 is adjusted in level to be within the narrow lock range shown by the shaded areas, e.g. the lock range is 30 kHz 6 kHz when the input signal level is - 50 dB and 30 kHz 600 Hz when the input signal level is - 70 dB. If the angle modulated wave decreases or disappears, the voltage controlled oscillator 23 makes a free running oscillation at the carrier frequency of 30 kHz and reinforces or replaces the carrier wave. PLL 19 includes an amplifier 21 (which in another embodiment acts as a limiter) and, optionally, a low pass filter 22. A carrier detector 26 acts on a muting circuit 25 to eliminate any output from PLL 19 if a record without carrier information is being played. To overcome noise produced by PLL 19 if the input signal level is low, if the turntable speed deviates and if the frequency of the VCO 23 deviates the output of the PLL preferably includes a time constant circuit (Fig. 6) to stabilize the loop system. Low frequencies below audio frequency pass through the time constant circuit without much attenuation whereas at audio frequencies attenuation is greater by virtue of capacitor C1. Thus when the frequency deviation is below audio frequency the phase locked loop has a lock range defined by curves A, A<1> (Fig. 5) while at audio frequencies the lock range is defined by curves B, B<1>. The shaded portion of curve B, B<1> is used, i.e. between levels - 40 dB and - 60 dB. The same effect as in Fig. 5 is obtained by omitting the time constant circuit (Fig. 6) and using a voltage controlled oscillator 23 whose conversion efficiency is large at very low frequencies and small at higher frequencies. In other embodiments (Figs. 8, 9A, 9B, 10, not shown), which have the same block diagram as Fig. 1, the loop gain of the PLL is also varied by changing the conversion efficiency of the voltage controlled oscillator in accordance with frequency. Another embodiment (Fig. 11) has a PLL comprising a phase comparator 26 and a voltage controlled oscillator 66. A level equalizer (in practice a potentiometer) matches the pickup cartridge to the PLL. A time constant control circuit 68 operates in response to the output from level equalizer 67 to control the time constant of circuit 65. Variable time constant network 65 comprises a FET (Fig. 13, not shown) used as a variable resistor, the value of its resistance being determined by a voltage applied from circuit 68.