Modified symmetrical reversible variable-length code and its theoretical bounds

Reversible variable length codes (RVLCs) have been adopted in emerging video coding standards-H.263+ and MPEG-4-to enhance their error-resilience capabilities (which are important and essential) in error-prone environments. This study proposes an efficient algorithm to construct a symmetrical RVLC from a given Huffman code. In addition, theoretical bounds on the maximum codeword length for fixed-length Huffman codes, and on the optimal average codeword lengths for sources with exponential distribution are provided.

I. Introduction

Almost all image coding standards, such as the JPEG still image coding standard [1], the ITU series of H.261 and H.263 video coding standards [2], [3], the ISO series of MPEG-1 and MPEG-2 standards [4], [5], adopt variable-length codes (VLCs) as their entropy coding stage. Due to the variable code length nature of VLCs, they are very sensitive to errors occurring in noisy environments. Even a single bit error is extremely likely to induce propagation errors such that the data received after the bit error position becomes useless and results in a serious problem.