GB1391289A - Four channel stereophonic system - Google Patents

Abstract

1391289 Stereophonic broadcasting systems GENERAL ELECTRIC CO 15 Sept 1972 [21 Sept 1971] 42843/72 Heading H4L The conventional two-channel stereophonic radio system is modified to carry four stereophonically related signals. As described, the four signals are designated L F , L R , R F , R R and the transmitter includes matrix means which produce outputs L F + L R + R F + R R , (L F + L R ) - (R F + R R ), (L F - L R ) - (R F - R R ) and (L F - L R ) + (R F - R R ). The first and second of these signals form the sum and difference signals as in a conventional stereo system, the difference signal amplitude modulating a 38 kHz subcarrier which is then suppressed. A second 38 kHz subcarrier in quadrature with the first subcarrier is generated and is suppressed carrier amplitude modulated with the third signal (L F - L R )-(R F - R R ). The fourth matrix output amplitude modulates a 76 kHz subcarrier, which may be depressed or suppressed, the maximum degree of modulation being arranged by limiting to be between 30 and 90% of that otherwise obtainable, 60% being suggested as a suitable value. A filter removes most of the upper sideband. The resulting signals are combined together with a 19 kHz pilot signal, Fig. 1, delay means being included to equalize the envelope delay of the four signals, and the composite signal thus obtained frequency-modulates the main carrier wave. If the 76 kHz subcarrier is transmitted it may be used at the receiver to provide an indication of quadrophonic reception and to switch the appropriate circuitry, the 19 kHz pilot signal similarly indicating stereophonic reception. The receiver includes means responsive to the pilot signal and including a phase-locked loop for regenerating the first, second and third subcarrier waves for detecting the four matrix output signals, and de-matrix means responsive to these outputs to reproduce the four discrete audio-frequency input signals. Detailed circuitry relating to the transmitter and the receiver is described, the receiver being designed to use inductorless integrated circuitry.