Abstract: A single field or picture of a PAL color television signal is stored in a store and provides a signal at an input (52). The stored signal consists of 2f.sub.sc samples taken at 45.degree. to the U-axis. The signal is separated by a filter (56) and subtractor (58) into a high frequency or chrominance signal and a low frequency or luminance signal. The chrominance signal is delayed by delay means (62) such that the chrominance components of two lines which are an odd number of lines apart are made available simultaneously. These signals are combined by switches (S1, S2) and/or combining circuits (68) to provide a signal at 4f.sub.sc which is combined with the luminance signal in a combining circuit (70) to provide a 4f.sub.sc PAL output (72). The manner of the combination of the chrominance components is changed on an 8-field cycle.

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.