Low Bit-Rate System for Digital Coding of the Television Picture
Peter Rainger, P.A. Ratliff · SMPTE Journal · 1979
System design of bit-rate reduction equipment for digital television involves striking a compromise between hardware cost and the cost of transmission or storage of the digital signal. This paper is concerned with “cascadable” systems, in which there is a small but definable loss of quality when the signal is first coded though there is no increase in degradation as it passes through further stages of digital coding. Bit-rate economy is sought by reducing the sampling frequency and the number of bits per sample; degradation due to analog to digital conversion is ignored and — for the sake of simplicity — the system input and output are assumed to be linear pulse-code-modulated signals of 8 bits per sample. Experimentation with various alternative systems has shown that for various reasons differential pulse code modulation (DPCM) is to be preferred, although in some cases its advantage is small. The discussion is in terms of the 625-line PAL signal, but many of the points made are applicable to other television standards. The effectiveness of DPCM coding is considered for the cases of 1-D, 2-D, and 3-D prediction, and the cascadability of such a system is considered in detail. Sub-Nyquist sampling is examined as an important bit-rate reduction system, and the necessary requirements of cascadability are studied in an attempt to identify the principles involved so that the technique may be extended. Experience gained with a system sampling at twice color subcarrier and using a 2-D system of filtering was employed to develop the theory of a 3-D system.