Patent GB-1026372-A (GB)
Title: Communication receiver
IPC: H04N7/16
Filed: 1963-01-29
Abstract: 1,026,372. Television; coded receivers. ZENITH RADIO CORPORATION. Jan. 29, 1963 [Jan. 30, 1962 (3)], No. 3568/63. Headings H3Q and H4F. [Also in Division H2] In a television system in which the signal is transmitted in coded form so as to be usable only in a receiver equipped with a permutable decoding apparatus which is adjusted to agree with the transmitter coding apparatus, a correlation signal is periodically transmitted which represents the correct setting of the decoding apparatus and the receiver includes a correlation testing means which compares the output of the decoding apparatus with the correlation signal and produces a control effect whereby the television signal may be utilized only providing correct correlation is found in each of a predetermined number of consecutive time intervals. The invention is disclosed applied to a system substantially similar to that described in Specification 742,140, where the code schedule is determined at the transmitter by means of a generator which develops bursts of frequencies in a random order. The bursts are applied through a multiple switch to control signal coding and are transmitted to the receiver during vertical blanking to control decoding. The receiver includes a multiple switch unit which corresponds to that of the transmitter and which has to be adjusted to an advertised condition for satisfactory decoding of the received signals. The coding and decoding signals are square wave signals with transitions occurring at times in accordance with the code schedule and are employed to introduce delays between the video and synchronizing components. In accordance with the invention one of the frequencies is separated from the code and is arranged to be transmitted during each vertical blanking period at an instant when the coding signal has a predetermined polarity. At the receiver a channel separates this one frequency and compares its instantaneous position with the polarity of the decoded signal. If the decoding device is not correctly adjusted the one frequency and the decoded signal coincides only occasionally. At the first coincidence a cam is started into rotation, but is reset to its start position if correlation is not found at a subsequent test. Unless coincidence occurs for a period of about 10 seconds (i.e. in each of 600 fields) the cam is prevented from completing its cycle. Upon completing its cycle the filament supply to one or more valves is completed to allow the picture to be reproduced. Whenever the cam is in its start position a buzzer is actuated and a warning lamp is extinguished to warn the viewer of the incorrect adjustment. In accordance with a further aspect of the invention the code to be set up at the receiver on a cyclometer register is advertized as a combination of three numbers followed by a letter, the letter denoting the charge for the programme. The cyclometer is mechanically coupled to a punched tape which sets the decoding switch and to a typing mechanism which prints a corresponding combination of numbers and letters to indicate the programme viewed and the price thereof when the correlation as previously described is achieved. In addition, the punched tape has a sine wave recorded along one edge, the appropriate part of the sine wave being erased when correlation has been achieved.