GB841837A - Improvements in or relating to apparatus for secret television - Google Patents

Abstract

841,837. Television. RANK CINTEL Ltd. Oct. 31, 1957 [Oct. 31, 1956], No. 33234/56. Class 40(3). In a subscription television system in which signals generated by a camera 1 (Fig. 1) are supplied to the transmitter 6 via a coding device 4 which, under the control of a device 7, periodically changes the "mode" of transmission in any desired manner so that a satisfactory picture cannot be reproduced by unauthorized receivers (i.e. receivers not equipped with appropriate decoding apparatus) de-coding of the signals received by authorized receivers, so as to reproduce a satisfactory picture, is controlled by means of signals generated at each change of "mode", and at other times, and which comprises bursts of signals generated by generators 10A-10F, each operating at different frequencies, selected in predetermined combinations by means of a switching device 8 controlled by the "modechange" control device 7. The combined outputs of the generators 10A-10F are then undulated on to a sub-carrier frequency in device 11 the modulated wave from which is employed to modulate either the main vision carrier in transmitter 6 or the carrier for accompanying sound signals in the audio transmitter 15. At an authorized receiver (Fig. 2), the sub-carrier wave is derived from the audio channel 25 or from the video channel 26 and fed to a detector 27 in which the signals of 10A-10F (Fig. 1 ) are separately recovered and supplied to a like number of trigger circuits 28A-28F responsive to the frequency of respective ones of the generators 10A- 10F (Fig. 1) The combined outputs of the trigger circuits are then fed to a device 29 which responds to the combination of input signals thereto, (determined by the devices 7 and 8 Fig. 1), and supplies a control signal to a decoding device 30 to change the "mode" of the video signals supplied thereto from the video channel 26 in such manner as to compensate for the "mode" changes introduced at the transmitter and thus to allow the reproduction of a satisfactory picture by the picture tube 33. To improve the secrecy of the system the devices 7 and 8 (Fig. 1) initiate, in addition to "mode" change signals, further signals which do not change the "mode" of the transmitted signals and the device 29 (Fig. 2) is arranged not to respond to combinations of transmitted control signals which occur during such periods.