GB1427858A - Reproducing quadrophonic sound - Google Patents

Abstract

1427858 Quadraphonic systems CBS Inc 17 April 1974 [17 April 1973] 16797/74 Heading H4R In a matrix decoding system for a quadraphonic sound reproduction system a logic circuit operates by comparing the phases of the signals supplied to the two front reproducers to provide a first phase difference signal, comparing the phases of the signals supplied to the two rear reproducers to provide a second phase difference signal, and then comparing the first and second phase difference signals to provide a control signal which is used to differentially change the gain of amplifiers feeding the front reproducers relative the gain of amplifiers feeding the rear reproducers. As shown in Fig. 1 matrix coded L T and R T signals 254 and 256 are applied to a matrix decoder 28 to provide L<SP>1</SP> f , R<SP>1</SP> f , L<SP>1</SP> b , R<SP>1</SP> b signals. The matrix decoder signals are applied via respective gain controlled amplifiers, 270, 272, 274, 276, to respective loud-speakers, preferably after mixing a small proportion of the opposite channel signal with each of the signals in the combining circuits 278, 280, 282, 284, to improve channel separation, as described in Specification 1,396,352. Control signals for the gain controlled amplifiers are derived in the logic circuit 286 in which the decoder matrix output signals, after opposite channel signals have been mixed in by combiners 288, 290, 292, 294, are amplified and hard limited to provide substantially rectangular waves having the same zero crossings as the input signals. The two limited front signals from limiters 304 and 306 are added in combiner 312 and the resulting wave full wave rectified to provide a signal whose level depends on the relative phase displacement of the L<SP>11</SP> f and R<SP>11</SP> f signals. Similarly the combiner 316 and rectifier 318 provides a signal whose level depends on the relative phase displacement of the L<SP>11</SP> b and R<SP>11</SP> b signals. The two phase displacement dependent signals are fed to comparator 336 to provide a difference signal which after amplification and shaping is fed, via a limiter 342, rectifier 348, and time constant circuit 352, to control the gain of the amplifiers feeding the front loud-speakers, and also via an inverter 344, limiter 346, rectifier 350 and time constant circuit 354 to control the gain of amplifiers 274 and 276 feeding the rear loud-speakers. The circuit is such that with a predominant front signal amplifiers 270 and 272 have a gain of 1 and amplifiers 274 and 276 a gain 0, the gain values being set by the limiting levels + E 1 and - E 2 of limiters 342 and 346 respectively; while predominantly rear signals will provide gains of 0 for 270 and 272 and 1 for 274 and 276; while with side signals or signals having no predominant directional effect the gains of all amplifiers will be À7, with the range of signal over which the gain remains at À7 being set by the slicers 356 and 358.