Patent GB-1395477-A (GB)
Title: Sound signal-converting apparatus for use in a four-channel stereophonic reproduction system
IPC: H04S3/02
Filed: 1973-04-30
Abstract: 1395477 Quadraphonic decoder SANSUI ELECTRIC CO Ltd 30 April 1973 [2 May 1972] 20534/73 Heading H4R Apparatus for reproducing four-channel signals on four output channels by combining two input channel signals has a matrix circuit for each of the input signals and is arranged to combine the two input channel signals in ratios which are dependent on their frequencies. Preferably, low frequency two channel signals are combined in a larger ratio than medium frequency signals, and high frequency two channel signals are combined in a smaller ratio than the medium frequency signals. At medium frequencies separation between front and back channels and separation between left and right channels are substantially equal. At low frequencies, separation between left and right channels is lowered whereas separation between front and back channels is enhanced. At high frequencies separation between front and back channels is lowered whereas separation between left and right channels is enhanced. A first embodiment (Fig. 1), has input terminals 11, 12 supplied with signals L and R, four matrix circuits M1, M2, M3, M4 and four output terminals 36, 38, 40, 42. Matrix circuits M1, M2, M3, M4 are the same but inverters 23, 29 are connected to matrix circuits M3, M4. Matrix circuit M1, comprises capacitors 15, 16 a resistor 17 and its output is taken from the junction of two identical resistors 13, 14. For medium frequency signals capacitor 15 has a large impedance, while capacitor 16 serially connected to resistor 17 has a small impedance so that the impedance of circuit M1 is determined substantially by the parallel resistance of the resistors 13, 17. For low frequency signals, capacitors 15, 16 exhibit a large impedance so that the impedance of circuit M1 is determined substantially by resistor 13. For high frequency signals, capacitor 15 has a far smaller impedance than resistors 13, 17 so that the impedance of circuit M1 is determined by the impedance of capacitor 15 and this is negligibly small compared with the value of resistor 14. Phase shifters 35, 37, 39, 41 are provided. An alternative circuit (Fig. 4), has sum and a difference matrix circuits 51, 52 and level varying means 53, 54 (e.g. a filter or capacitor), which increasingly attenuate the levels of signals as they fall in frequency. Inverters 56, 59, mixers 55, 57, 58, 60 and phase shifters 61, 62, 63, 64 are provided. Two channel signals are mixed in different ratios according to their frequencies as in Fig. 1. As shown in Fig. 5, a matrix circuit for producing sum signal L + R includes resistors 71, 72 of the same value in series between input terminals L, R. The sum signal L + R is delivered from the junction of resistors 71, 72. A matrix circuit 52-1 provides a difference signal L - R and consists of two identical resistors 73, 74 and a phase shifter 75, the difference signal being delivered from the junction of resistors 73, 74. An analogous matrix circuit 52-2 produces a difference signal R - L at the junction of resistors 76, 77. Four mixers 55, 57, 58, 60 are provided and each includes a capacitor, such as 53-1 in mixer 55 which provides the frequency dependent characteristic.