es applied to previously coded information. For composite colour signals sam- pled at setting the delay element to N samples reduces the (57.4) occurrence of large outputs from the subtractor. Although the sub- carrier is a high frequency it must be coded accurately as it is carrying low frequency colour information. DPCM coding with 5 to 6 bits per pel is considered to offer the (57.5) otherwise 1 subjective quality of 8 bit PCM. The disadvantage is the increased susceptibility to transmission errors which affect all subsequent (57.6) samples until a reset can be performed, for example at the end of a otherwise scan line or bottom of the picture. One way to mitigate against, but not eliminate, this is to introduce a leak factor in the feedback loop Blocks are commonly 8 by 8 or 16 by 16 though the horizontal and by multiplying the prediction by slightly less than unity. The error vertical dimensions are not required to be equal, nor to be a power then diminishes over a number of pels which depends on the leak of 2. Small blocks can fail to take maximum advantage of correla- factor. Leak factors further from unity give faster recovery but at the tions from low spatial frequencies and may also incur a higher expense of coding efficiency. proportion of overhead data to address and describe them. Large CCIR Recommendation G.721 describes a DPCM codec for use transform blocks have a higher implementation difficulty. The at 140 Mbit/s (CCIR, 1990b). The input video is in 4:2:2 component transformation does not of itself produce data compression as the form according to Recommendation 601. The predictor is two number of output values, termed coefficients, is the same as the dimensional using the average of the pel immediately above in the input. However, the process tends to concentrate the energy into a same field and the pel immediately to the left on the same line. few coefficients, usually those representing lower spatial frequen- cies, and the others can be omitted or coarsely quantised. The (0,0) 57.4.4 Variable 