GB1168248A - Improvements in or relating to Colour Television Receivers - Google Patents

Abstract

1,168,248. Colour television. TELEFUNKEN PATENTVERWERTUNGS G.m.b.H. 8 Nov., 1966 [8 Nov., 1965], No. 49870/66. Heading H4F. The invention relates to the synchronization in a receiver of a switch, such as the switch which is used in the " PAL " and " SECAM " (Registered Trade Mark) systems, which is required to change state in alternate lines in response to " bursts " of oscillation of subcarrier frequency which are transmitted at the beginning of each line and comprise two components, one of constant phase in all lines and the other of a quadrature phase which is reversed (i.e. changed through 180 degrees) in alternate lines. The invention makes use of the characteristic frequency spectrum of such " burst " signals which are formed of a main component at sub-carrier frequency together with upper and lower side-band components spaced from the main component at intervals of one-half line frequency. In general a signal at half line frequency suitable for controlling the switch change of state may be derived from any two adjacent frequency components in the signal spectrum. In one arrangement, Figs: 2 and 3 (not shown), narrow bandwidth quartz filters select and separate the main component at sub-carrier frequency and the first upper or lower side-band component. These two components are then combined in a mixing circuit (a synchronous demodulator) to obtain the difference signal at half-line frequency. The switch may be synchronized by local line frequency pulses and established in correct phase by the half-line frequency signal. If desired, the separated component at sub-carrier frequency may be used directly as a datum carrier of constant phase for colour demodulation or indirectly to synchronize a datum carrier oscillator. In a second arrangement (not illustrated by drawings) a single filter of suitable bandwidth may select both the main and sideband components. These are passed to a non-linear circuit which generates signals which include the desired signal at half-line frequency. In a third arrangement (not illustrated by drawings) a filter selects the main component and both upper and lower first side-band components. The combination is then applied to an amplitude detector to produce a signal at half-line frequency.