GB849622A - Improvements relating to colour and monochrome television transmitters and receivers - Google Patents

Abstract

849,622. Television. ELECTRIC & MUSICAL INDUSTRIES Ltd. Dec. 12, 1956 [Dec. 13, 1955], No. 35695/55. Drawings to Specification. Class 40(3). The invention relates to a television system for the selective transmission of colour or monochrome signals in which during the transmission of colour signals, which is effected with the aid of a chrominance subcarrier, the frequency difference between the main vision carrier and the chrominance subcarrier is arranged to be a simple fraction (i.e. the ratio of two integers each less than ten) of the difference between the main vision carrier and a sound carrier so that, at a receiver, a homodyning oscillation for detecting the subcarrier may be derived by reference to the latter difference frequency. In accordance with the invention, in such a system, during the transmission of monochrome signals, the frequency difference between the main vision and sound carriers is changed slightly from that which prevails during colour transmission, and in a receiver a device responsive to the difference is employed to disable the chrominance channel by reducing its gain so that noise in the channel does not disturb the monochrome picture. The homodyning oscillation is derived by a regenerative frequency divider supplied with the main vision and sound intermediate frequencies and the device may detect the change in frequency difference as reflected in the output of the divider. Alternatively, the device may be operated from the sound main or intermediate frequency carrier, or from a frequency discriminator circuit to which are applied the main vision and sound carriers. The frequency differences between the subcarrier and the main vision and sound carrier are preferably'each made equal to an odd integral multiple of half line frequency, an example being quoted for the 405 line system.