Patent GB-672250-A (GB)
Title: Improvements in television transmitters
IPC: H04N7/169
Filed: 1949-04-28
Abstract: 672,250. Television. ZENITH RADIO CORPORATION. April 28, 1949 [June 5, 1948], No. 11340/49. Class 40 (iii). [Also in Group XL (c)] 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 time with reference therewith, at which times also a " key " signal is transmitted by wire line to each co-operating receiver so as to compensate for the phase displacement between the picture and synchronizing signals received thereby. In such a system, in order to prevent flicker occurring in the reproduced picture due to the periodicallychanging picture-signal content of the composite television signal, the transmitter comprises means for blanking the picture signal by periodic blanking signals occurring in a fixed time relation therewith and having sufficient duration to encompass the line-synchronizing signals whether or not the picture signal is in its relatively displaced condition. Picture signals from camera 1 are amplified at 6 and are passed via blanking circuit 7, synchronizing- signal mixer 8 and D.C. re-insertion circuit 9 to modulator 10, the output from which is radiated from aerial 11. Frame and line synchronizing pulses generated at 12 are mixed in with the picture signals at 8 and are applied to framescanning generator 15 and via unit 17 to linescanning generator 20, respectively, the outputs of which are applied to scanning coils 4 and 5 of the camera 1. Frame-synchronizing signals from unit 12 are also applied to a frequency divider 22, the output of which causes oscillator 24 to periodically generate a " key " signal, comprising a burst of oscillations initiated by one pulse and terminated by the next pulse from unit 22. This signal is sent by lines 25 to each co-operating receiver and is also applied to unit 26 to which frame-synchronizing signals are also applied from unit 12. The simultaneous occurrence of a frame-synchronizing signal and key signal in the unit 26 causes it to apply a switching signal to circuit 17 which thereupon delays the line-synchronizing signals passing through it to the scanning generator 20. The scanning-waveform generator at 20 is consequently relatively delayed and the picturesignal output from camera 1 is consequently delayed with respect to the synchronizing signals which are added thereto in mixer 8. Insertion of blanking signals. The synchronizing-signal output of unit 17 is applied to a blanking-signal generator 32 which applies blanking signals to the circuit 7 to blank the video signal passing therethrough. Fig. 2 shows at B and C the timing of the blanking signals with respect to the line-synchronizing signals as applied to mixer 8, which are shown at A, in the undelayed and delayed conditions respectively. D and E show respectively the form of the combined signal in both conditions. The blanking signal level is arranged to be close to the mean picture level to prevent the unauthorized use of the blanking signals for synchronizing. Specification 647,892 is referred to.