POLYNOMIAL-TIME, SEQUENTIAL, ADAPTIVE SYSTEM AND METHOD FOR LOSSY DATA COMPRESSION

A system and method are provided for lossy compression of finite alphabet source sequences subject to an average-per-letter distortion constraint. The source sequence is sequentially parsed into phrases and each source phrase is mapped to a distorted phrase such that average per-letter distortion between the two phrases does not exceed the desired distortion. The present system adaptively maintains a codebook as the collection of all one-letter extensions of previously emitted distorted phrases. The present system uses approximate string matching and carries out a sequential procedure by iterating the following steps: (i) given the current codebook find the longest source phrase that can be transmitted at a given distortion, (ii) from all codewords that match the source phrase carefully choose that which is most likely to be useful in the future. For every new source phrase, the present system judiciously selects one of the many approximately matching codewords to balance between the code rate for the current phrase versus the code rate from resulting codebooks for the future source phrases. The present system outputs a distorted sequence that can be naturally losslessly compressed using the Lempel-Ziv algorithm or any variation thereof. Such judicious codeword selection is intended to iteratively improve the codebook quality. The entire present sequence can be implemented in quadratic-time in the length of the source sequence. The present system is sequential and adaptive.