Improved excitation for phonetically-segmented VXC speech coding below 4 kb/s

Techniques for combating two types of distortion that degrade the quality of vector excitation coded (VXC) speech are presented. One degradation, the presence of noiselike components between the intended harmonics in voiced speech segments, is reduced by adaptive comb filtering, controlled by a smoothed pitch estimate. The other degradation arises with front vowel sounds whose second and third formants tend to be attenuated in VXC coders. This is improved by adding high-frequency emphasis to the perceptual weighting when computing the distortion between original and reconstructed speech. It is shown that the first technique can benefit any VXC coder, whereas while the second is applicable specifically when phonetic segmentation is used as a front end to VXC coders