A significant improvement in the efficiency of excitation coding with CELP at low bit rates is achieved by a new paradigm for encoding the fixed excitation. In the proposed scheme, the non-zero fixed-codebook excitation elements are substantially localized in a set of windows with positions adaptive to the pitch peaks. Highly efficient coding is thus achieved by allocating most of the available excitation bits to capture the essential excitation events. The paradigm is validated by computer simulation of a variable-rate speech codec. The performance of the codec is evaluated by informal subjective tests and compared with TIA standard variable rate speech codecs. The results indicate that the proposed scheme can be used to reproduce speech at average bit rates from 2.3 to 3.4 kbps (i.e., in a two-way communication scenario) with very high quality and intelligibility.
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