Approximated multi-symbol arithmetic coding method and apparatus

Abstract

A method and apparatus for compressing and decompressing data is described. The most frequent symbols (A-Group.) are encoded using an Arithmetic Code, then the remainder of the symbols (H-Group) are first encoded using Huffman's algorithm (or any Prefix code) and then combined with the Arithmetic code resulting in a hybrid Arithmetic/Huffman code. After being encoded into a Huffman code, the H-Group symbols are made into a 'super-symbol' which fits into an Arithmetic subinterval allocated to the symbols in the H-Group. The Arithmetic subintervals for the symbols in the H-Group preferably are a negative power of 2 (e.g., 1/2, 1/4, 1/16, 1/32, etc.) of the code space. Each such H-group subinterval has its own associated subset of H-group symbols comprising one Huffman code table that fits into that respective interval. Decoding in an AMSAC system first treats the code stream as Arithmetically encoded. Standard prior art Arithmetic decoding is performed until an interval assigned to the super-symbol(s) is encountered. The Arithmetic super-symbol for this interval is then processed to obtain the Prefix code by reversing the scaling and offsetting, if any, that was needed to fit the super-symbol into the assigned Arithmetic subinterval. The Prefix code is then decoded into the original symbol using standard prior art Prefix techniques.