Digital processing of n.t.s.c. color television signals

Abstract

Digital processing of a 525 line N.T.S.C. color television signal is achieved by digitizing (40) with a sub-Nyquist sampling frequency f.sub.s which has a mean value 2 f.sub.sc .+-.1/2 f.sub.F, or more generally m f.sub.L .+-.(p+1/2) f.sub.F (where f.sub.sc, f.sub.L and f.sub.F are the subcarrier, line scan, and field scan frequencies, and m and p are integers). Conveniently the sampling frequency is exactly 2 f.sub.sc during each field and suffers a 180.degree. phase shift between fields. After processing, the signal is converted back to analogue form (50) and, to remove alias components, is comb filtered (30) over the frequency range f.sub.s -f.sub.v to f.sub.v (where f.sub.v is the maximum video frequency in the original signal) by averaging across a 262 line delay (24'), or more generally a delay of a field minus half a line. Preferably a second comb filter (10) is included before the digitizer (40); the two filters (10, 30) are then so arranged that the overall delay suffered over the whole frequency range is substantially the same. Digital comb filters can be used in which case the filters and converters are reversed in position and the initial sampling is at twice the required frequency f.sub.s. Improved diagonal luminance and vertical chrominance resolution is obtained with stationary pictures, and alias components are reduced.