Patent GB-1001441-A (GB)
Title: Television communication system
IPC: H04N7/16
Filed: 1961-07-14
Abstract: 1,001,441. Secret and subscription television systems. TELEGLOBE PAY-TV SYSTEM Inc. July 14, 1961 [July 20, 1960], No. 25610/61. Drawings to Specification. Heading H4F. In a subscription television system, secrecy is achieved at the transmitter by suppressing the horizontal synchronizing signals from the transmitted signal and to permit a receiver to reproduce correctly the signal there is provided instead a " key " signal, consisting preferably of a sinusoidal signal at horizontal synchronizing frequency. At a receiver a " coding " circuit forms the " key " signal into horizontal synchronizing pulses and combines them with the received signal so as to re-establish a normal television signal. The " key " signal may be transmitted to the receiver by way of the video or audio signal carrier, the signal being so located in the transmission band as not to interfere with the video or audio signal. Alternatively, the signal may be sent to the receiver by telephone line. In one arrangement where the " key " signal is transmitted with the audio signal using frequency modulation, the " key " signal and normal audio signal are separated at the output of the audio. detector, the " key " signal is used to reform the horizontal synchronizing signals as described above, whilst the audio signal is remodulated by frequency modulation on a locally produced carrier which is then added to the reformed video signal so as to provide a conventional composite television signal. In order that the audio signal may be available only to subscription receiver, it may be transmitted to receivers over the telephone network. A subscriber wishing to use the service communicates with the transmitting station by telephone call, dialling a code number or closure of a switch, reference also being made to the signalling of payment by coin, or use of a token or punched card. Reference is made to the use of a " key " signal which is wobbled in frequency to increase the secrecy of the system. The " key " signal source at the transmitter is not synchronized with the transmitter synchronizing pulse generator, but the circuit at the receiver by which the " key " signal is converted to horizontal synchronizing pulses includes a phase adjuster by means of which, it is stated, the correct synchronous operation may be achieved. Specification 1,001,442 is referred to. Reference has been directed by the Comptroller to Specifications 917,591 and 922,183. is achieved at the transmitter by suppressing the horizontal synchronizing pulses from the transmitted signal and to permit a receiver to reproduce correctly the signal there is provided instead a " key " signal in the form of a sinusoidal signal of predetermined frequency (preferably the horizontal scanning frequency) which is transmitted by means of " key " signal carrier. At a receiver a " coding " circuit forms the " key " signal into horizontal synchronizing pulses and combines them with the received signal so as to re-establish a normal television signal. In addition to the video and " key " signal there is also transmitted an associated audio signal. The " key " signal may modulate the video or audio carrier, or separate carrier located at a suitable position in the transmitted signal transmission band. Reference is made to the use of " key " signal which is wobbled in frequency to increase secrecy. The " key " signal may modulate its carrier by either amplitude or frequency modulation. In one receiving arrangement when the " key " signal is sent by separate carrier, the received signal after detection is separated by filters in two parts, the first comprising the video and audio signals and the second the " key " signal modulated on its carrier. This carrier signal is then converted to an intermediate frequency by a local oscillator and detected to derive the " key " signal. The " key " signal is converted to horizontal synchronizing pulses which are combined with the first part of the received signal to re-establish the normal video signal. The " key " signal source at the transmitter is not synchronized with the transmitter synchronizing pulse generator, but the circuit at the receiver by which the " key " signal is converted to horizontal synchronizing pulses includes a phase adjuster by means of which, it is stated, the correct synchronous operation may be achieved. A subscriber wishing to use the service provided by the system communicates with the transmitter by phone call or the dialling of a code number. Reference is also made to operations involving coins, tokens, punched cards, clocks, meters and recording mechanisms. Specification 1,001,441 is referred to. Reference has been directed by the Comptroller to Specification 922,183.