Fast spherical code decoding algorithms for the residual codebook in CELP coders

A new method for fast decoding when using algebraic codes for the fixed codebook in CELP speech coders is presented. This method is based on the trellis structure of a block code, and allows fast optimal search of the residual codebook even with a combined scalar gain, unlike previous search methods. The method is flexible, allowing for long block lengths and the use of any code including nonlinear ones. Currently the performance is not as high as with standard algebraic coding methods, but further refinements may make this a viable method.