Waveform Interpolation

Waveform interpolation (WI) has proved to be an efficient procedure for high quality coding of speech at low bit rates. In this method, the speech signal is described by a sequence of characteristic waveforms, which are interpolated during reconstruction. Originally, the characteristic waveform was identified with a pitch cycle, and WI was applied to voiced speech segments only. A number of implementations, which use CELP for unvoiced signal segments, showed that the procedure can provide high performance. Recently, the method was extended to include unvoiced speech and background noise. To this purpose the characteristic waveform is decomposed into a slowly evolving waveform (representing the periodic component of the signal), and a rapidly evolving waveform (representing the other components of the signal). The rapidly evolving waveform requires high time resolution and only low quantization accuracy, while the slowly evolving waveform requires less time resolution and a more precise description. With this decomposition, switching between different coding models is avoided, and a robust coding method results.