Optimum Run Length Codes

To realize the full redundancy reducing potential of run length coding over a collection of picture segments with varying run length statistics, adaptive coding techniques have been proposed. This paper compares results for a previously proposed
A
-code, for which the block length was varied adaptively, with those obtained using a fixed block length
B
-code first proposed here. Both codes use variable length codewords integrally related to the code block length. For all pictures analyzed, the fixed block length
B
-code performed nearly as well as the adaptive
A
-code. For both, the bit rates were close to the entropy bound. The reasons for these results are discussed. It is Shown that, due to the prevalence of exceptionally long runs, the
A
-code, which is nearly optimal for exponentially distributed run lengths, performs poorly for actual pictures unless the block length is varied adaptively. The strategy used to implement the A-code adaptively is described. The optimal block length of the B-code,on the other hand, is shown to be largely independent of the picture statistics and need therefore not be varied adaptively. The hardware implementation of both coding techniques, the influence of channel disturbances on image quality, and the problems of error correction and line synchronization are discussed.