PSEUDO-GRAY CODING

A pseudo-gray code is an assignment of n-bit binary indexes to 2n points in a Euclidean space so that the Hamming distance between two points corresponds closely to the Euclidean distance. Pseudo-Gray coding provides a redundancy-free error protection scheme for vector quantization (VQ) of analog signals when the binary indexes are used as channel symbols on a discrete memoryless channel and the points are signal codevectors. Binary indexes are assigned to codevectors in a way that reduces the average quantization distortion introduced in the reproduced source vectors when a transmitted index is corrupted by channel noise. A globally optimal solution to this problem is generally intractable due to an inherently large computational complexity. A locally optimal solution, the Binary Switching Algorithm, is introduced, based upon the objective of minimizing a useful upper bound on the average system distortion. The algorithm yields a significant reduction in average distortion, and converges in reasonable running times. The use of pseudo-Gray coding is motivated by the increasing need for low bit-rate VQ-based encoding systems that operate on noisy channels, such as in mobile radio speech communications.