temporal redundancy is reduced by DPCM coding of the transform Horizontal spatial frequency coefficients from one frame to the next. Variable length encoding then provides further compression. Figure 57.26 Example of sub-band split 57.4.9 Motion compensation DPCM inter-frame coding can be augmented by motion compensa- tion. The moving areas generate the majority of bits in an inter- 57.4.7 Vector Quantisation frame coder. However, if the motion is of a nature that it can be described by a few parameters then more efficient coding can be Vector quantisation (VQ) is the assignment of one code word to a obtained by suitably modifying the previous picture before using it group of two or more picture elements. Compression is achieved as the prediction. For objects undergoing purely translational mo- Digital video coding 57/27 PCM video in Compressed The incorporation of motion compensation in a decoder has only data out Quantiser a small effect on its complexity; the main change is that the store in the loop ceases to be a purely serial delay but needs a degree of random access capability. The impact on an encoder is very large, the basic process being one of pattern matching. For the CCITT Inverse coder, which is the least demanding of the three in this respect, an quantiser exhaustive search for the optimum vector requires over 5 X 109 additions or subtractions of 8 bit numbers per second. Fortunately, because the decoder needs to know only the vector value and not how it was determined, the search method need not be standardised. This permits simplified search methods to be implemented in en- coders, though with some loss of performance. 1 frame delay 57.4.10 CCITT Recommendation H.261 Figure 57.27 Hybrid DCT/DPCM coder This Recommendation is for a video codec for audio visual services tion, a horizontal displacement and a vertical displacement are at multiples of 64 kbit/s up to 2 Mbit/s (CCITT, 1990a). The primary sufficient. The two components give rise to the term 'motion vec- application is videotelephone and 