1. A bit allocation method for digital video, comprising the steps of: providing a plurality L of video programs at an encoder, each program having successive groups of pictures (GOPs); each GOP having an associated number of pictures; providing a super GOP comprising at least one GOP from each of said L video programs, and having a length of N pictures; calculating a first target number of bits, T, for encoding said super GOP according to the number of pictures in said super GOP, L.times.N, and an available capacity of a channel over which the video programs are transmitted; wherein each super GOP comprises a plurality N of super frames, each super frame having L pictures at a common temporal reference point; defining a respective complexity measure for each picture type in each l.sup.th program; calculating a second target number of bits for encoding each n.sup.th super frame of pictures, where n=1,. .. , N, according to said first target number of bits, T, and the complexity measure of each l.sup.th picture in the associated n.sup.th super frame, where l=1,. .. , L; calculating a third target number of bits for encoding each l.sup.th picture in the associated n.sup.th super frame according to said second target number of bits and the associated complexity measure, and a sum of the complexity measures for each picture in the associated n.sup.th super frame; and providing respective weighting factors, w, for the different video programs according to a relative priority thereof; wherein: said third target number of bits for encoding each l.sup.th picture in the associated n.sup.th super frame is calculated according to the respective weighting factor of the associated l.sup.th video program.