Phase adjustment in waveform interpolation

This paper describes a method of improving the quality of the Waveform Interpolation (WI) speech coder by adjustment of the phase information. In WI, a slowly-evolving waveform (SEW) and a rapidly-evolving waveform (REW) represent the periodic and the non-periodic part of the signal. The phase of the synthesized signal is determined by the SEW and REW, and thus the correct quantization of these parameters are important to producing natural speech quality. A method is described, whereby the phase of the synthesized signal is adjusted by modifying the quantized REW spectrum as a function of the fundamental frequency. This essentially attempts to correct the discrepancies in phase that arise due to variation in pitch and also accounts for the difference in noise sensitivity between female and male speech. The overall effect would be the same if multiple codebooks (depending on pitch) were used to code the REW spectrum. Experimental results confirm that the new method results in significantly improved performance.