Arithmetic coding revisited

During its long gestation in the 1970s and early 1980s, arithmetic coding was widely regarded more as an academic curiosity than a practical coding technique. One factor that helped it gain the popularity it enjoys today was the publication in 1987 of source code for a multi symbol arithmetic coder in Communications of the ACM. Now (1995), our understanding of arithmetic coding has further matured, and it is timely to review the components of that implementation and summarise the improvements that we and other authors have developed since then. We also describe a novel method for performing the underlying calculation needed for arithmetic coding. Accompanying the paper is a "Mark II" implementation that incorporates the improvements we suggest. The areas examined include: changes to the coding procedure that reduce the number of multiplications and divisions and permit them to be done to low precision; the increased range of probability approximations and alphabet sizes that can be supported using limited precision calculation; data structures for support of arithmetic coding on large alphabets; the interface between the modelling and coding subsystems; the use of enhanced models to allow high performance compression. For each of these areas, we consider how the new implementation differs from the CACM package.