ures means that they must currently operate at fixed bit rates, though variable rate video coding be rearranged in time order before entering the coding loop. This is being studied (Morrison, 1991) for use on future networks which modified order is retained in the coded bit stream SO that the decoder could support this mode (CCITT, 1990b). Buffers in the coder and can process it directly. Restoration to the original time sequencing decoder provide short term smoothing, up to around a few hundred is carried out after the decoding loop and before display. These milliseconds. Adjustment of the longer term average rate of bit processes incur delay, depending on the number of contiguous generation must be accomplished by controlling the coder where, B-pictures and SO the delay through this MPEG algorithm can be typically, the state of fill of its buffer is fed back to control the significantly more than the CCITT and CCIR ones. For storage quantisation process. The control strategy need not be known to the applications this is unlikely to be a problem. Although 1 or 2 decoder. Consequently this and other items which do not affect contiguous B-pictures has been found to work well, the standard compatibility between coder and decoder may be left open in permits more or less, including none. standards. These then essentially contain only the syntax and se- Arrows indicate pictures referenced by motion vectors Figure 57.29 Example of combination of predictive and interpolative coding in MPEG algorithm Videophony and videoconferencing 57/29 mantics of the coded bit stream such that the decoder can recon- when the model fails. Encouraging results have been shown for the struct the pictures that the encoder intended. Thus, product differen- synthesis part which is carried out by decoders. The analysis task tiation between coders is possible and may well result in two coders which must be performed by encoders is more difficult. fed with identical input material giving different coded bit streams and subjectively different de