A robust and fast DP-CELP (double-pulse CELP) vocoder at the bit rate of 4 kb/s

It has been reported that in a CELP vocoder the perceptual quality of synthesized speech can be improved by replacing the traditional stochastic codebook with a mixed codebook containing periodic SP (single-pulse) and noise-like sequences. At the bit rate of 4 kb/s, the frame and subframe sizes are relatively long to reduce the bit rates for coding the gains and the synthesis filter parameters. In this case, the coding of noise-like sequences requires many more bits than the coding of periodic SP sequences to obtain high-quality speech, thus, a periodic DP (double-pulse) excitation can be introduced while the bit rate required to code the excitation is not significantly increased. In the 4 kb/s DP-CELP vocoder presented in the paper, a mixed codebook containing periodic SP and binary noise-like sequences is first searched, avoiding V/UV decision and exhaustive searching of all possible candidates. If the optimal sequence is of periodic SP, a second periodic SP sequence is combined with it to generate a periodic DP excitation. In the decoder, a proposed short-term postfilter is added to increase the clarity of synthesized speech