Regarding the size of the VQ Codebook: is there something to say that the size 256 optimal? Would increasing the size affect the speed of decoding?
A. For more diverse acoustic environments, having a larger codebook size would result in better models and better recognition. We have been using 256 codewords primarily for use with the SPHINX-II decoder, since for historical reasons it does not handle larger codebbok sizes. The original sphinx-II used a single byte integer to index the codewords. The largest number possible was therefore 256. The format conversion code which converts models from SPHINX-III format to SPHINX-II format accordingly requires that your models be trained with a codebook size of 256. The standard Sphinx-III decoder, however, can handle larger codebooks. Increasing the codebook size would slow down the speed of decoding since the the number of mixture-weights would be higher for each HMM state.
