Patent GB-1377138-A (GB)
Title: Four channel arrangements
IPC: H04H20/89
Filed: 1972-04-14
Abstract: 1377138 Multiplex radio signalling MATSUSHITA ELECTRIC INDUSTRIAL CO Ltd 14 April 1972 [15 April 1971 (2)] 17455/72 Heading H4L Quadraphonic signals are transmitted by the use of an additional 38 KHz subcarrier in quadrature with the normal 38 KHz subcarrier of a conventional stereophonic broadcasting system. The system is compatible with monophonic and stereophonic radio receivers and may be added to an existing stereophonic broadcasting transmitter. Transmitter.-Input signals L1, L2, R1, R2, Fig. 1, are supplied via amplifiers 5 to 8 respectively, which include pre-emphasis circuits, to switching circuits 9 to 12. A 19 KHz pilot signal is generated at 20 and converted to a 38 KHz subcarrier as in a conventional stereo system at 19 and a further 38 KHz subcarrier in quadrature at 18. The subcarrier from 18 controls the switching circuits 9, 10 to pass signal L1 on one half-cycle and L2 on the other half-cycle to a matrixing means 13 which adds the two signals, the switching circuit 11, 12 being likewise controlled to pass signal R1, R2 to a matrixing circuit 14. The outputs of circuits 13, 14 are supplied to switching circuits 15, 16 respectively, which are controlled by the subcarrier from 19 to pass the output from 13 on one half-cycle and the output from 14 on the other to a combining circuit 17. The 19 KHz pilot signal from 20 is added to the output of 17 at 21. A further pilot signal of 57 KHz is generated at 25 and via switch 26 may be combined with the composite signal at 21. This pilot signal may be used at a four channel receiver to indicate that a four channel signal is being received and to provide automatic switching to the correct mode. The output from 21 is supplied via a low pass filter 22, suppressing undesired harmonics, to a conventional F.M. transmitter 23. Various frequency response characteristics for the filter 22 are described with differing crosstalk and signal to noise values in the resulting composite signal. Receiver.-Conventional receiving means 41, Fig. 4, produce a frequency discriminated signal which is supplied via an equalizing means 42 to switching circuits 43, 44. The 19 KHz pilot signal is selected at 57 and controls the generation of a 38 KHz switching signal at 58 as in a conventional stereophonic receiver and a further quadrature phase 38 KHz switching signal at 59. A switch 61 is closed and a lamp 62 operated when the 57 KHz pilot signal is detected at 60. Switches 43, 44 are controlled by the switching signal from 58 so that the input from 42 is supplied to a switching circuit 64 on one halfcycle of the switching signal and to switching circuit 65 on the other half-cycle. Switching circuit 64 is controlled by the switching signal from 59 so that signal L1 is supplied via amplifier and de-emphasis circuit 49 to loud-speaker 53 on one half-cycle of the switching signal and L2 is supplied via amplifier and de-emphasis circuit 50 to loud-speaker 54 on the other. Switching circuit 65 is controlled likewise by the switching signal from 59 to supply signals R1, R2 to loud-speakers 55, 56 respectively. Instead of using the equalizing circuit 42 a portion of the composite signal from 41 may be applied to each of the outputs of the switching circuit 66 in appropriate polarity to reduce crosstalk.