A Closed-loop Multimode Variable Bit Rate Characteristic Waveform Interpolation Coder

A variable bit rate characteristic waveform interpolation (VBR-CWI) speech codec with about 1.86kbps average bit rate which combines closed-loop multimode techniques is presented in this paper. Each kind of characteristic waveform (CW) surface is regarded as only rapidly evolving waveforms (REWs), only slowly evolving waveforms (SEWs) or mixed REWs plus SEWs in different cases of CWs evolving performance. A cost criterion based on weighted signal-to-noise (WSNR) value in the spectral domain is used to make the mode selection. Experiments show that the proposed closed-loop multimode VBR-CWI coder has reduced the average bit rate markedly and improved the synthesis speech quality to some extent compared to the original fixed bit rate coder. Further research can be done in order to have a more accurate perceptual objective quality measurement instead of WSNR and there is also need to pay attention to computational complexity of closed-loop method in real-time applications.