BBC R&D

Abstract

A number of PAL and NTSC decoding methods, several of which involve comb filters based on line delays, are compared, primarily for use in standards conversion. To simplify comparisons, the circuits are all converted into a general form consisting of a chrominance-pass filter, a subtractor used to obtain luminance and a chrominance demodulator. The theoretical performance of each circuit is summarised as a set of vertical frequency characteristics which show luminance and chrominance resolution, cross-luminance and cross-colour and, where present, other cross-talk and aliasing effects. In addition, practical impressions of the performance of each method have been obtained from a fully digital decoder using line-locked sampling and switched filters.

The optimum PAL decoding arrangements for direct viewing, for subsequent recoding to PAL and for standards conversion are all found to be different. For direct viewing, a two-line luminance circuit with nulls to reject subcarrier components, combined with a two-line version of the conventional delay line chrominance circuit produces the most acceptable pictures. For applications in which decoded signals are to be encoded as PAL subsequently, the complementary Weston circuit using two line delays is the most suitable of those tested. However, the degree of chrominance-luminance separation obtained is unlikely to be adequate for the circuit to be used as an input to component signal processing equipment in studios. In a standards converter, the vertical-temporal filtering action of the interpolator can be designed to have a beneficial effect, suppressing cross-effects and other impairments left by the decoder. Because of this, the combination of a two-line luminance comb filter and simple chrominance demodulation retains good spatial resolution and, after the interpolator, achieves a low level of cross-effects. A similar arrangement is also found to be satisfactory for NTSC signals.