An adaptive equalizer for analysis-by-synthesis speech coders

An equalizer to enhance the quality of reconstructed speech from an analysis-by-synthesis speech coder, e.g., CELP coder, is described. The equalizer makes use of the set of short-term predictor parameters normally transmitted from the speech encoder to the decoder. In addition, the equalizer computes a matching set of parameters from the reconstructed speech. The function of the equalizer is to undo the computed set of characteristics from the reconstructed speech and impose the set of desired characteristics represented by the transmitted parameters. Design steps for the equalizer and its implementation both in time and frequency domain are described. Experimental results of applying the equalizer to the output of a standard coder, viz., EVRC (Enhanced Variable Rate Coder) operating at half-rate (4000 bps), are presented. Objective evaluation using an ITU recommended voice quality tool shows that the equalizer can help enhance the quality of the reconstructed speech significantly.