Patent GB-595730-A (GB)
Title: Multiplex intelligence transmission system
IPC: H04N7/084
Filed: 1945-03-23
Abstract: 595,730. Multiplex radio and television systems. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. March 23, 1945, No. 7420. Convention date, March 24, 1944. [Classes 40 (iii) and 40 (v)] A television and sound multiplex system comprises means for producing video and control signals of varying amplitude, means for producing sound signals of varying frequency, and means for successively interspersing the signals one with the other with predetermined equally timed durations of signal. Fig. 1 shows the arrangement at the-transmitter, in which signals from a video source 10 together with synchronizing signals from a generator 11 are fed by lead 16 to a mixer unit 18 where they are mixed with sound signals from unit 26 and then fed to an amplitude modulation transmitter 12. The sound signals are produced by a microphone 23 which modulates the frequency of an oscillator 24, the output of which is fed to a keyer valve 26 which is opened in the interval following the horizontal synchronizing pulses to pass the sound signal to mixer 18. The keyer 26 is operated by pulses from a keying pulse generator 28 which is controlled by the synchronizing pulse generator 11. The generator 28 also controls a unit 29 which supplies pulses to unit 11 for slotting the vertical synchronizing signals to allow insertion of the sound signals at the appropriate times. At the receiver, Fig. 2, the output from the I.F. amplifier 31 and second detector 32 is fed on the one hand to video amplifier 36 and cathode-ray tube 43, and on the other hand to a synchronizing signal separator 38 and deflection generator 41 to reproduce a visual image in the usual manner. The I.F. signal is also fed to a selector 51 which is controlled from the deflection generator 41 and a multivibrator 53 to open the selector for an interval after each horizontal synchronizing signal to allow the sound signal to pass to a limiter 61 and frequency discriminator 62. The output from 62 is in the form of amplitude modulated pulses which are fed to a low pass filter 63 and loud-speaker 64. The multivibrator 53 also feeds a blanking unit 59 for blanking the cathode-ray tube except during the video signals. Alternatively, the selector 51 may be fed with the signal after detection at 32 and amplification at 36 (Fig. 3, not shown). Fig. 4 shows the sound signal circuit of Fig. 1 in more detail. The signal from the microphone 23 is fed by line 69 to the grid of a reactance valve 82 forming part of the frequency-determining circuit of an oscillator 76 to give a frequency modulated signal which is fed to an oscillator-mixer 121 to give a desired frequency. After amplification at 126, the sound signal is fed to grid 129 of a keyer valve 131 which is opened periodically by pulses from a multivibrator 138 controlled over line 146 by the horizontal synchronizing pulses, and which feeds through elements 158, 159 and valve 156, to give time delay and width control of the pulses. At the same time a multivibrator 148 and valve 151 serve for slotting the vertical synchronizing pulse to allow insertion of the sound signal. The sound signal output from valve 131 receives at 166 the video signal from amplifier 163, and the combined signal is fed through amplifier 161 to the transmitter, not shown. U.S.A. Specifications 2,132,654, 2,132,655, 2,192,121, 2,256,530, 2,300,501 and 2,313,906 are referred to.