A prototype waveform interpolation low bit rate speech codec

Voiced speech is characterized by a high level of periodicity. In order to encode voiced speech with a good quality, the correct degree of periodicity must be preserved. The proposed coding algorithm attempts to reproduce the correct level of periodicity even at low bit rates. The method exploits the temporal redundancy of voiced segments in order to achieve high compression rates. Voiced speech is interpreted as a concatenation of slowly evolving pitch-cycle waveforms. The signal is synthesized by waveform interpolation from a downsampled sequence of pitch-cycles with a rate of one prototype waveform per frame (20–30ms). An original method of prototype parametrization and coding based on a proper mixed time-frequency representation allows a high quality prototype reconstruction. The effectiveness of such a parametrization renders it well suited to low bit rate applications, yet maintaining a good quality of the reconstructed signal. The method can be combined with existing LP-based speech coders, such as CELP, for unvoiced segments.