ture by the coder to obtain maximum bit 57/26 Video transmission d.c. coefficients Variable length coder video in Re arrange Quantisers to 8x8 blocks Compressed data out Variable length a.c. coefficients coder Figure 57.25 JPEG encoder efficiency. An additional parameter allows scaling of all the quan- through the number of available code words being less than the tisers thus providing flexibility of quality and hence compression number of permutations of the possible values of the input pels in ratio. the group. Thus several input patterns are mapped to one output code. This again exploits the likely similarities between pels in the 57.4.6 Sub-band Coding same neighbourhood of space or time or both and the masking of distortion in highly detailed areas. Sub-band coding (SBC) has similarities to transform coding in that Although conceptually simple, the coding process is complex as the video input is split into frequency bands. Because each sub-band the number of input permutations increases very rapidly with the contains a limited range of frequencies it can be reversibly subsam- number of pels in the group. For example, to implement a VQ pled SO that the total number of pels in all the sub-pictures equals encoder operating on a 2 by 2 block of 8 bit pels by means of a direct the number in the original. The resulting sub-pictures are individ- lookup table would need 32 address bits, which is significantly in ually coded, e.g. by DPCM, each with quantisation distortion appro- excess of current integrated circuit technology. 64 kbyte ROM has priate to its visibility (Gharavi, 1988). The splitting is performed by just 16 address lines; 65536 such devices would be required. For this filters, usually two dimensional (spatial only) but sometimes three reason it is common to employ a multi-stage approach though this dimensional including the temporal axis. The technique has more may be sub-optimal. A variation is to extract the mean level and flexibility than the DCT in its partitioning of the input energy. In scale th