 same parity as the field being coded. Motion com- CCIR 601 source quality cannot be maintained at the target bit rates pensation is performed with half pel resolution within the range 15 and downsampling is usually employed before the coding kernel. pels horizontally and 7.5 lines vertically. The inter-field mode uses Typical sampling rates are 288 non-interlaced lines of 352 lumin- the immediately previous field and without motion compensation. ance pels repeated 25 times per second and 240/352/29.97, these Because of interlace, the previous field has a vertical spatial offset. being simply related to 625 and 525 line television respectively. This is compensated for by taking the average of two pels in the To support random access, some pictures are coded entirely in previous field, one directly above and one directly below, as the intra mode, i.e. without reference to any information outside that prediction. individual picture. These are termed I-pictures. Two forms of mo- Weighting matrices are applied to the transform coefficients in the tion compensated prediction are employed as shown in Figure quantisation process. The matrix for the two colour difference 57.29. P-pictures are coded with respect to a previous picture but signals is different from the luminance one. not necessarily the immediately previous one. The intervening B- pictures use the previous picture or the next picture or an average of 57.4.13 Buffering and coder control the two as a prediction, the selection being possible for each 16 pel by 16 block. The latter mode uses two motion vectors for the block. Video coders utilising compression techniques inherently produce The motion compensation resolution is half pel and half line, but the coded data at a non-uniform rate because the effectiveness of these permitted range is many times larger than CCITT H.261 or CCIR methods is dependent upon the spatial and temporal activity of the Rec.723. input pictures and these are not constant. Transmission channels The use of prediction from future pict