Fixed bit-rate PWI speech coding with variable frame length

This paper presents a harmonic vocoder for digitising speech at fixed low bit-rates based on the prototype waveform interpolation (PWI) technique. A variable frame-length form of this technique is proposed. The basic idea is consistent with that of the original PWI algorithm, but improves its performance at low bit-rates and reduces its computational complexity. This variable frame-length coding technique, which uses vector quantised cepstral coefficients for representing the spectral envelope of individual pitch periods produces good perceptual quality at 2.4 kb/s.