Abstract
A simple method of applying sub-Nyquist sampling techniques to digital coding of PAL colour video signals is described. This technique employs a sampling frequency of twice the colour subcarrier frequency, the sampling instants being maintained at a specified phase relative to the reference subcarrier. For System I (625-line, PAL) video signals the resulting sampling frequency is about 8.87 MHz. Comb filtering in the upper part of the video band is used both to recover a standard PAL video signal and to remove unwanted alias components from the sampled signal.
The results of subjective tests indicated that for a single coding and decoding operation, broadcast-quality colour pictures can be obtained from an 8-bit per sample PCM system employing this sub-Nyquist sampling frequency; with a 6-bit DPCM system the impairment was graded 'just perceptible' on critical pictures. Further tests are required to examine the suitability of this sub-Nyquist technique for general use in a broadcasting network which might include several codecs (coding and decoding operations) in tandem.
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