The MPEG video coding algorithm will be used in a large variety of video recording applications.A key constraint for video coding algorithms for consumer (tape) recorder applications is the bit stream editability requirement; i.e. it must be possible to replace N consecutive frames by N new consecutive frames on the storage media, using at most the same number of bits. In this paper this constraint is satisfied by the use of a forward rate control mechanism, such that each Group of Pictures (GoP) will be encoded using a fixed number of bits (within a tolerance margin to be minimized). The problem of performing a forward state allocation (quantizer step allocation) is limited to the picture level by performing a pre-allocation, assigning a fraction of the available bits to each of the frames of a GoP. The state allocation per picture amounts to the correct selection of the quantization step size for all slices. This is done by forming parametric models for both the rate (R) and the distortion (D), such that for a particular slice, the R-D curve can be predicted. Using the R-D curves of every slice of the picture, the state allocation can be performed. With the described algorithm the GoP rate error will be within 4% in the stationary mode; if a nonstationary mode that includes a re-allocation based on feedback information is added, the error will be within 1%.

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