Patent GB-1328141-A (GB)
Title: Audio systems
IPC: H04H20/88
Filed: 1970-08-28
Abstract: 1328141 Quadrasonic systems P SCHEIBER 28 Aug 1970 [28 Aug 1969 29 Dec 1969] 13891/70 Heading H4R A quadrasonic system employs only two transmission or recording channels, signals for four speakers being encoded on the channels. As shown in plan view four microphones at a recording or broadcasting station are arranged to receive sounds from the four quadrants round a point. The outputs of left and right facing microphones 12L and 12R are applied directly to the channels A and B respectively and the sum and difference signals respectively of the outputs of the front and rear facing microphones are applied with an attenuation factor #2/2 to the channels A and B respectively. At the reproduction station the signals on the A and B channels are applied respectively to loudspeakers 14L and 14R positioned on the left and right hand sides of a listener position 16. The sum of the two channel signals is attenuated by the factor #2/2 and applied to a front loudspeaker 14F and the difference between the two channel signals is attenuated by the factor #2/2 and applied to a rear speaker 14X. The net effect is that a signal picked up by any microphone is reproduced at the correspondingly positioned loudspeaker and is reproduced at a lower equal level at the two adjacent loudspeakers. To the listener the adjacent loudspeakers do not effect the sound direction which appears to come exclusively from the correspondingly positioned loudspeaker. An encoding network employing two differential amplifiers, the outputs of which are connected to the channels and the inputs of which are selectively coupled to the microphone outputs via attenuating resistors is described (Fig. 3, not shown). A decoding network (Fig. 4, not shown) in which the two channels are coupled directly to first and second outputs and to primaries of corresponding transformers, each having two secondaries, the first secondaries being coupled in series aiding fashion to provide a third output and the second secondaries being coupled in series opposition to provide a fourth output is also described. Automatic gain control.-In order to provide better separation between the channels automatic gain control circuits may control the signals to each oppositely positioned pair of loudspeakers in accordance with the signals associated with the other pair of loudspeakers. Various gain controlling arrangements are described each working on the assumption that when one channel transmits a signal while the other does not, then the signal originated from the microphone 12L or 12R directly connected to the channel on which the signal is received and the loudspeakers associated with the other microphones should be silient, while if both channels transmit equal signals, the signals originated from microphone 12F or microphone 12X both equally coupled to both channels and the loudspeakers associated with the microphones 12L and 12R should be silent. Accordingly in one such arrangement the envelopes of the signals on the two channels are detected and the logarithms of the amplitude of these envelopes are produced in analog fashion. The two analog logarithmic values are applied as the two inputs of a differential amplifier which gives an output corresponding to the logarithm of the ratio. When the signals on the two channels are equal this output is zero and the gain of the amplifiers for the loudspeakers 14L and 14R is adjusted to zero while the gain of the amplifiers for loudspeakers 14F and 14X is adjusted to a maximum while when there is no signal on one channel the output is theoretically infinite and the gain for loudspeakers 14F and 14X is adjusted to zero while that for loudspeakers 14L and 14R is adjusted to a maximum. In another embodiment (Figs. 7 and 8, not shown) the ratio of the power from one oppositely positioned pair of microphones to the power of all four microphones is determined and a 20 Hz. tone is transmitted on both channels at amplitudes in accordance with the ratio. At the reproducing station the gain in the corresponding loudspeaker input amplifier is varied in accordance with this ratio. Further automatic gain controlled amplifiers may be provided between the loud speaker input amplifiers commonly controlled to compensate for variations in total power of the input amplifiers (Fig. 9, not shown). In a further embodiment (Figs. 10 to 14, not shown) an analog calculating circuit formed from numerous operation amplifiers which may be discrete coponents or may utilize integrated circuits of monolithic or hybrid construction varies the gain in the loudspeaker amplifiers in accordance with complex logarithmic formulae. The gain control element in each amplifier is photo-responsive resistor PR 201 illuminated by a semi-conductor diode light source D201 controlled by a signal "V CONTROL" supplied by the analog calculating circuit. In a yet further embodiment each loudspeaker has an independent gain controlling element which increases the gain when a high level signal is present. The element may be a "T" network comprising a doped silicon conductor element in one of the loudspeaker supply conductors and an incandescent lamp connected across the supply conductors.