Improvements to the analysis-by-synthesis loop in CELP codecs

In this paper extensions to the analysis-by-synthesis (AbS) loop used in code excited linear predictive (CELP) speech codecs are considered. Methods for updating the short-term synthesis filter once the excitation parameters have been determined are examined. We show that significant improvements can be achieved by updating the synthesis filter, similar to those obtained using the well known methods of interpolation and bandwidth expansion. However our proposed method of update avoids the increase in the delay of a codec that is usually associated with interpolation. Furthermore the traditional sequential method of determining the adaptive and fixed codebook parameters is examined and compared to an exhaustive search of both codebooks. Three sub-optimum techniques are proposed for improving the performance of the codebook search while maintaining a reasonable level of complexity. The most complex of these increases the codec complexity by only about 40% but provides 80% of the maximum possible 1.1 dB segmental SNR improvement associated with an exhaustive codebook search.