Patent GB-742140-A (GB)
Title: Improvements in or relating to subscription television system
IPC: H04N7/16
Filed: 1953-09-11
Abstract: 742,140. Television. ZENITH RADIO CORPORATION. Sept. 11, 1953 [Dec. 15, 1952], No. 25163/53. Class 40 (3). In a " secret " television system in which the television signal is transmitted in one or more different " coded " operating modes operation of the coding device of the transmitter and of the decoding device of the receiver is effected under the control of a coding (decoding) signal comprising a combination of signal components individually having a predetermined identifying characteristic and collectively defining a coding (decoding) signal in accordance with the. order of the components within the combustion. As shown in Fig. 1, coding of the transmitted signal is effected by the selective introduction of a delay device 12 between the video signal output of the camera tube 10 and the mixing amplifier 13 in which the vide signals are combined with the synchronizing &c. components from generator 19. Operation of device 12 (which has two operating conditions, " delay " or " no-delay ") is controlled by a switching pulse N (Fig. 3) supplied over conductors 100 from a multi-vibrator 103 (Fig. 2), which is triggered from one to another of its two stable states by pulses L (Fig. 3) supplied thereto via diodes 85, 86 and, depending on its initial condition, from one state to another or not triggered at all, by pulses K and M (Fig. 3) supplied via diodes 84 and 87 respectively. The triggering pulses K, L, M (Fig. 3) which are derived from a transposition device 70, the settings of the switches 71 ... 76 of which are made known to privileged persons (e.g. subscriber receivers), are generated in generators 41 ... 47 which supply " bursts " of signal of frequency f1 to f7 (J, Fig. 3), respectively, generators 41 ... 46 being " turned-on " in response to signals from a cathode-ray device 38 across the electrodes 40a ... 40b of which the electron beam modulated by a series of delayed line pulses (I, Fig. 3) during field retrace intervals is deflected in random manner under the control of a " noise " generator 35 and generator 47 being " turnedon" in response to delayed line pulses (F, Fig. 3) derived via delay device 25 and gate 26. The signal train (J, Fig. 3) thus generated is supplied over conductor 102 and the primary of transformer 58 to frequency selective circuits 51 ... 57, 51 ... 56 of which supply the switches 71 ... 76 of device 70 via diode rectifiers 61 ... 66 whilst circuit 57 supplies a pulse (via diode 67) coincident with pulse F (Fig. 3) which ensures that the multi-vibrator 103 is periodically re-set in a reference condition. To enable the coded transmission to be decoded of a subscriber receiver the signal train J (Fig. 3) is supplied to mixer amplifier 13 over conductors 101 and radiated with the composite signal as shown at O (Fig. 3). At a subscriber receiver (Fig. 4, not shown), the signal train J (Fig. 3) is separated from the received composite signal and employed to control, via a circuit similar to Fig. 2; the selective introduction of a de-coding device (delay line) between the receiver picture tube and the video amplifier to compensate for the coding action at the transmitter.