Variable to fixed entropy coders: Why and how? (And their application to H.263)

Entropy coders fall into several general categories: Huffman and Huffman-like coders that parse the input into fixed length pieces and encode each with a variable length output, arithmetic coders that take an arbitrarily long string as an input and encode with a single output string, and Tunstall-like coders that parse the input into variable length strings and encode each with a fixed length output. This paper is about a Tunstall-like coder called BAC (for Block Arithmetic Coding). We argue that this class of coders is very appropriate for many situations, especially when the probabiliities vary, when channel errors may occur, or when fast operation is needed. In particular, we discuss how an H.263 encoder/decoder can be modified to replace the syntax arithmetic code with a block arithmetic code to get greater speed and better error resiliency.