Patent GB-733400-A (GB)
Title: Improvements in or relating to subscription television system
Inventor: ELLETT ALEXANDER
IPC: H04N7/16
Filed: 1953-09-11
Abstract: 733,400. Television. ZENITH RADIO CORPORATION. Sept. 11, 1953, No. 25162/53. Class 40 (3). In a " coded " television system coding/decoding is controlled by the selective application of one of a plurality of coding/decoding signals. Fig. 1 shows one embodiment in which coding of a composite television signal is effected by periodically introducing a delay device 44 into the synchronizing signal path from generator 18 to the line sweep system 45, the operation of device 44 being controlled by a circuit 26, the actuation of which is initiated and terminated by the field synchronizing pulses (fed from generator) which follow the beginning and ending of coding signal pulses supplied to circuit 26 from a switch 25. The latter, which selects one of two input signals derived from rotating magnetic records 27, 28 on which distinct prescribed coding pulses are recorded, is operated between its two positions by means of a control circuit 24 which in response to a " burst " of key signal supplied by a generator 23 (controlled by field synchronizing pulses randomly divided in frequency divider 21) and delayed field synchronizing pulses from delay line 49 impresses a series of pulses on switch 25 corresponding to the " bursts " of key signal but each having leading and trailing edges corresponding to the delayed field synchronizing pulses following the beginning and end of each " burst." In this manner the coding device 44 is actuated in accordance with the code signals on record 27 during certain intervals and in accordance with those on record 28 during intervening intervals, the change from one record control to another being effected at random intervals in accordance with the operation of random frequency divider 21. To control the decoding of the television signal coded in this manner the key-signal " bursts " are transmitted as a modulation component of the sound channel of the system (or, alternatively, as an independent radiation or over a line circuit) and at a subscriber receiver (Fig. 2) provided with rotating magnetic code records 69, 70 identical with records 27, 28 (Fig. 1), the received key signal, which is derived from the sound detector 66 via filter 78 (in the case in which the key signal is transmitted on the sound channel), is employed in conjunction with delayed field synchronizing pulses from delay line 79 (similar to 49, Fig. 1) and a circuit 77 (similar to 24, Fig. 1) to control the operation of a switch 75 (similar to 25, Fig. 1) which selectively supplies the code signals from records 69, 70 to a circuit 76 (similar to 26, Fig. 1), the signals derived in which, comprising the field synchronizing pulses derived from unit 60 which follow the beginning and ending of the code signals from switch 75, control the operation of the decoding device 61 in time coincidence with the actuation of the cooling device 44 (Fig. 1) at the transmitter to compensate for the delay introduced thereby. In a second embodiment (Figs. 3 and 4, not shown) the switching between the two code records is controlled in accordance with key signals recorded on a third record pro- Aided at both the transmitter and receiver so that the transmission of a key signal is unnecessary and in a modification of this embodiment (which may also be employed in the arrangement described with reference to Fig. 1 by dispensing with the key signal generator) the two code signals and the key signal are recorded on a single magnetic disc as signal " bursts " of different frequency the control pulses being derived by means of frequency selective amplifiers (Fig. 5, not shown). In a further modification (Fig. 6, not shown) which may be employed directly in the arrangement shown in Fig. 1, and with modifications in the embodiment of Fig. 3 (not shown), the two code signals are recorded as different frequency " bursts " on a single magnetic record and selectively applied to the circuit controlling the cooling device (e.g. 26, Fig. 1) by means of frequency selective amplifiers which are keyed into operation by pulses from the control circuit 24 (Fig. 1).