The coded data stream of today's powerful source codecs still contains residual redundancy. Due to the fact that coding is usually done frame by frame, this redundancy remains both inside a frame and also in a time correlation of successive frames. The redundancy should be used as a priori information at the receiver to improve the decoding result. Often the majority of the source parameters are almost uncorrelated, but never the less a small number of correlated parameters can be observed. Especially the most significant bits of these parameters are most correlated. We assume a periodic order of the a priori bits. Using the distance spectrum and Viterbi's (1979) upper bound formula, we show that it is possible to determine the optimal (with respect to bit error rate) order of the a priori bits and to calculate an estimate of the resulting bit error rate for perfect a priori information.