One very effective technique for the bit-rate reduction of digital PAL signals is sub-Nyquist sampling. This technique requires the use of comb filters which attenuate unwanted alias components produced by the sub-sampling process and also reconstruct the colour signal to standard form at the decoder. Variations are possible in the design of comb filter; in particular, the configuration can be based on line, field or picture delays.

This Report outlines the theory of comb filter operation and describes the results of subjective tests which compare the performance of the different possible sub-Nyquist systems. Certain design features are also described which are required in order that sub-Nyquist codecs may be cascaded without introducing further filtering impairment after the first such codec.