GB1347994A - Sound recording and reproduction - Google Patents

Abstract

1347994 Four channel stereophonic systems COLUMBIA BROADCASTING SYSTEM Inc 8 June 1971 [8 June 1970] 19488/71 Heading H4R In an apparatus for deriving from first and second composite signals four signals for reproducing on four loud-speakers, four separate channels of programme information, and in which each composite signal contains component signals representing respective ones of two of the channels together with, in each composite signal, in-phase portions of a third channel and phase-displaced portions of a fourth channel, the composite signals are appropriately combined to derive a first set of signals each containing a major portion of the signal in respective ones of the four channels of information, together with signals in the two adjacent ones of the four channels of information. The four signals are fed to the reproducing circuits via amplifiers whose gain is controlled by a circuit which determines which channel contains the predominant signal and increases the gain of the amplifier in the corresponding channel relative the gains of the amplifiers in the other channels. The four channels can be controlled independently or controlled in pairs, each pair comprising the channels feeding opposed signals. Fig. 7 shows one embodiment in which the two composite signals from amplifiers 40 and 42 are fed directly to respective gain controlled amplifiers 96 and 102 to provide signals for the left L and right R speakers while the sum and difference of the two composite signals are fed to respective gain controlled amplifiers 98 and 100 to provide signals for the front C and back D loud-speakers. The composite signals are also fed via equalizers 59 and 61, having inverse equal loudness characteristics, to gain controlled amplifiers 58 and 60 the output from which are fed to a matrix circuit to derive the four output channels. The signals in the four channels are rectified by rectifiers 68 to 74 to derive control signals for the gain controlled amplifiers 96, 98, 100, 102 in the main channels. The four control signals are also combined on -resistor 84 to provide a control signal for the gain of amplifiers 58 and 60 which tend to stabilize the combined signal on resistor 84 so that the individual signals, e l , e c , e d , e r are held to a specific rate. Each control signal has a bias 104, 106, 108, 110 added to it and the arrangement is such that a signal present in the right channel R tends to cut off the amplifier handling the left L signal and vice versa, and a signal in the C channel cuts off the amplifier handling the D signal, and vice versa. In one alternative arrangement, Fig. 9 (not shown), the four gain control signals are applied to each gain control amplifier via weighting networks, which weight control in favour of the signal level in the diagonally opposed channel, while in a second alternative system, Fig. 11 (not shown), the signal level in the left and right channels is compared with that in the front and rear channels and the gains of those pairs of channels controlled inversely.