Patent GB-669582-A (GB)
Title: Improvements in coded electrical signal transmitters
IPC: H04N7/167
Filed: 1949-05-17
Abstract: 669,582. Television transmitters. ZENITH RADIO CORPORATION. May 17, 1949 [July 21, 1948], No. 13120/49. Class 40 (iii). The invention relates to a television system wherein, in order to prevent useful reception of the transmitted signals by other than authorized persons, the picture signals, which occur in alternation with line synchronizing signals, are periodically displaced in tune in respect therewith, at which times a " key " signal is also transmitted by wire line to each cooperating receiver so as to compensate for the phase displacement between the picture and synchronizing signals received thereby. In such a system the transmitter comprises means for generating an identifying signal, this signal and the picture signal being transmitted as modulations of the same or separate radiofrequency waves. At a co-operating receiver, as described in Specification 669,583, reception of the identifying signal renders decoding means responsive to the " key " signal so that the receiver performs a decoding function only when receiving a coded signal, since other signals will not be .accompanied by the identifying signal. The transmitter comprises a camera unit 1 the output from which is amplified at 6, mixed with synchronizing and pedestal signals at 7, passed through D.C. re-insertion device 8 and modulated on to a radio-frequency carrier on modulator 9. Frame and line synchronizing signals are generated at 11 and are thence applied to mixer 7. Frame synchronizing scanning generator 14, the output of which is applied to the appropriate deflecting coil of the camera unit. Line synchronizing signals are applied through a delay line device 16 to the scanning generator 15 the output of which is applied to the other coil of the camera unit. Frame synchronizing pulses are also applied to frequency divider 17 the output of which is applied to key-signal generator 18 which generates a burst of key signal whenever a signal is applied thereto from circuit 17. The key-signal generator is connected to telephone lines 19a whereby the key-signal may be transmitted to co-operating receivers and is also connected to unit 20 to which is also applied the frame synchronizing signal from unit 11. Circuit 20 so controls the delay-line device 16 that, commencing during a frame flyback time, when key signal is being generated by the generator 18, the line synchronizing signals are delayed thereby displacing the video signal generated in camera unit 1 with respect to the synchronizing signals applied to mixer 7 The condition in which the synchronizing signals are not delayed is restored during the frame fly-back time immediately following the end of the key signal. The concomitant sound transmission is radiated from modulator and transmitter 24 and the identifying signal, consisting for example of a continuous 25 kc/s. signal, is generated in unit 26 and modulated on to the signal radiated from unit 24. This identifying signal may alternatively be modulated on to the carrier-wave radiated from transmitter 9. Specification 647,892 also is referred to.