Non-linear techniques for pitch and waveform enhancement in PWI coders

Two non-linear interpolation techniques are introduced for enhancing speech reproduction in prototype waveform interpolation (PWI) and similar encoders. A temporal differential rate (TDR) vector is used to characterise the non-uniform evolution of pitch cycle temporal structure during interpolation. Experimental results show a clear improvement in the accuracy of decoded pitch cycle lengths and in the reproduction of periodicity in general. It is also shown that waveform reproduction can be significantly improved by vector quantising sets of optimal combination coefficients (OCC) aimed at maximising the similarity between interpolated and target signal segments. Both time domain waveform similarity and frequency domain spectral envelope similarity derived OCC are tested. Subjective assessment suggests a general preference for non-linear interpolation methods and the scheme using frequency domain derived OCC with perceptual weighting provided the best subjective preference