All-pass excitation phase modelling for low bit-rate speech coding

This paper is concerned with the phase spectra of voiced speech segments digitised at low bit-rates using sinusoidal interpolative coding techniques. Such phase spectra are either disregarded at the decoder or regenerated from the magnitude spectra according to assumptions about the human speech production mechanism. The inaccuracy of the commonly used minimum phase assumption is demonstrated and a means of correcting the phase thus obtained using a second order all-pass filter is presented. The application of the all-pass filter to three speech coding techniques, i.e. STC, PWI and IMBE is demonstrated.