Patent GB-1402320-A (GB)
Title: Decoder for use in 4-2-4 matrix playback system
IPC: H04S3/02
Filed: 1972-10-18
Abstract: 1402320 Quadraphonic decoders SANSUI ELECTRIC CO Ltd 18 Oct 1972 [25 Oct 1971 30 Oct 1971 4 Nov 1971 10 Nov 1971 29 Dec 1971 23 March 1972 17 April 1972] 47930/72 Heading H4R A decoder for use in a 4-2-4 quadraphonic playback system has at least one control unit responsive to the instantaneous amplitude relationships of the directional audio input signals in the first and second channel signals. A first variable matrix means, controlled by one of the control signals, produces two audio output signals corresponding to two directional audio input signals. A second variable matrix means, controlled by the other control signal produces two audio output signals corresponding to two remaining directional audio input signals. One embodiment (Fig. 4) has encoded L and R signals applied at terminals 23, 24. A first variable matrix circuit includes resistors 35, 36, 37 and a second variable matrix-circuit includes resistors 39, 40, 41 and an inverter 42. Resistors 36 and 40 comprise photosensitive CdS elements illuminated by incandescent lamps 44, 45 respectively. The control unit 32 (see Fig. 5) is a phase discriminator and has limiters 50, 53 to transform signals L,R into rectangular wave signals. Output signals of opposite polarities produced by the limiter 53 are amplified by amplifiers 56 and 58 respectivelyand their outputs are applied to switching circuits 60 and 61 thereby causing these switching circuits to be ON and OFF alternately. The output from limiter 50 is coupled to the common input of switching circuits 60, 61. Their output terminals are grounded through capacitors 62, 63. Slidable arms of potentiometers 64, 65 supply control outputs EC1, EC2. The control circuit 32 operates to switch signal L by alternatively rendering ON and OFF switching circuits 60, 61 in response to signal R, thereby discriminating the phase difference between signals R and L. Returning to Fig. 4, where the phase difference between signals L and R is about zero, a loud sound is present at the front and a small sound at the rear. The first control output EC1 from control unit 32 is large, whereas the second control output is small. As a result, lamp control circuits 46, 47 operate to pass a large current through lamp 45 but a small current through lamp 44. Consequently resistor 40 has a small resistance value whereas element 36 has a large resistance value. Accordingly the level of signal L in decoder output FL<SP>1</SP> is increased but the level of signal R contributing to cross-talk is decreased. The level of signal R in decoder output FR<SP>1</SP> is increased whereas the level of signal L contributing to cross-talk is decreased. On the other hand the level of signal R in output RR<SP>1</SP> is decreased while the level of the signal L contributing to cross-talk is increased. This means a separation between rear channels. However, as the level of front channel signals is increased and the level of rear channel signals is decreased the separation between front and rear channels is improved. Where the phase difference between signals L and R is about 180 degrees the separation between the front channels is improved and that between rear channels is improved. Where the phase difference between signals L and R is about 90 degrees (i.e. where sounds at the same level are present at front and rear) the control outputs EC1 and EC2 have the same level and the circuit (Fig. 4) operates as a fixed matrix circuit. In an alternative circuit (Fig. 8) sum and difference signals (L + R) and (L - R) are applied to logarithmic amplifiers 70 and 72 respectively. A boot-strap circuit 74 applies signals proportional to log (L + R) of opposite phases across a rectifier 78. Similarly a bootstrap circuit 76 applies signals proportional to log (L-R) of opposite phases across a rectifier circuit 79. The output of rectifier 78, proportional to log |L+R|, and the output of rectifier circuit 79, proportional to log |L-R|, are applied to transistors 80, 81 constituting a differential amplifier 82. A first control output EC1 corresponds to log A second control output EC2 corresponds to log Another embodiment (Fig. 9) has a first matrix circuit 90 for the front channels comprising a differential amplifier 91 including two transistors 92, 93. Signal L is coupled to the base of transistor 92 while signal R is coupled to the base of transistor 93 through an emitter 94. The collector of transistor 92 provides the first output while the collector of transistor 93 provides the second output. A first control circuit 99 includes a F.E.T. 100 connected to a control input terminal so that it acts as a variable resistor. The control circuit 99 operates to vary the A.C. impedance of the emitter circuits of transistors 92, 93 in accordance with the magnitude of the control input so as to control the gain of the differential amplifier 91. A similar matrix circuit 105 for the rear channels is provided but has no inverter. A further embodiment has, for the front channels, matrix circuits 120, 121 (Fig. 10) for producing signals (L+R)and (L-R). (The signal (L-R) is applied to a third matrix circuit 123 via a variable gain amplifier 122 to be added to the outputs from matrix circuit 120. Circuit 123 functions to produce an output expressed by (1+f)L and (1-f)R, and an output -(1-f)L, -(1+f)R which is coupled to terminal 28-2 through an inverter 124. The rear channel signals are obtained by matrix circuits 125, 126, 128 and a variable gain amplifier 127. The gains of variable gain amplifiers 122, 127 are varied in opposite directions by control outputs EC1 and EC2 for control unit 32. Phase chargers 29, 30 are provided. A detailed circuit is given (Fig. 11, not shown). With the circuit described, reproduced high frequency sound may be wrongly positioned at the front or rear by the over-riding influence of low frequency sounds, or vice versa. This may be eliminated by using variable gain amplifiers 122, 127 having a low gain for frequencies below 200 Hz, a high gain for signals higher than 5 kHz and gain control in the frequency band of 200 Hz to 5 kHz. Filter circuits to achieve this are described (Figs. 11, 12, not shown). Two elaborations of the decoder of Fig. 10 are described (Fig. 13, 25, not shown) for independent control of front-left and -right channels and the rear-left and -right channels. This includes various phase shifters, matrix circuits and either phase discriminators (Fig. 13) or comparators (Fig. 25). In a further elaboration (not shown) the two channel signals are divided into a plurality of frequency bands and control signals are produced for the respective frequency bands.