Digital video: Comb filtering in sub-Nyquist PAL codecs.
It is possible in theory to reduce the slight impairment introduced by line-delay comb filters in a sub-Nyquist PAL codec by using alternative forms of comb filtering. A description is given of the theoretical effects of line-delay, field-delay and picture-delay comb filters. A brief experimental investigation of the effect of a field-delay filter compared with a line-delay filter in the decoder showed that although the expected changes were visible on special test pictures no significant differences were seen on typical (but critical) television pictures. However, further work is considered desirable to extend the investigation to both pre- and post sampling filters, including picture-delay filters, and to PAL signals for which cross-colour has been reduced.
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