GB1511230A - Colour television - Google Patents

Abstract

1511230 Television BRITISH BROADCASTING CORP 9 May 1975 [13 May 1975] 21118/74 Heading H4F A digital PAL signal is derived from an analogue PAL signal by sampling the analogue signal at a frequency f s which is twice the colour subcarrier frequency f sc and maintaining the sampling points at instants spaced one eighth of the subcarrier period from the zero crossings of the U components of the PAL signal. 'Alias' components in the recovered signal, caused by the use of a sample frequency at the Nyquist limit, are suppressed by comb filtering the analogue chrominance components prior to sampling and A-D converting for transmission and further comb-filtering the same frequency band f s -f v to f v (i.e. the chrominance frequency band) in the recovered D-A converted signal upon reception. The transmitter circuit of Fig. 3B generates a mean signal ¢(E 1 + E 2 ) in the frequency range f s -f v to f v which has all the odd harmonics of half the line frequency removed. These harmonics, however, create additional 'alias' components at the receiver, Fig. 4, within frequency range f s -f v to f v . These are removed by comb filtering the odd integral harmonics of half line frequency from the reconstituted analogue signal.