Low delay multi-level decomposition and quantisation techniques for WI coding

For efficient coding of speech, it is desirable to separate the slowly and rapidly evolving spectral components to take advantage of their different perceptual qualities. We present a multi-level wavelet decomposition mechanism, using low delay FIR filters, applied to waveform interpolation (WI) coding. The technique overcomes the substantial delay problems encountered by Chong, Burnett, Chicharo and Thomson (see Proc. Int Conf. Acoustics, Speech, Sig. Processing, Seattle, USA, vol. 1., p.513-16, 1998) and identifies a preferred technique for the quantisation of the decomposed surfaces. The phase is shown to be particularly sensitive to the compounding of quantisation errors within the tree-structured transform. The proposed solution involves the use of variable dimension vector quantisation (VDVQ) on separately decomposed magnitude/phase surfaces. This approach provides for coarse or no phase quantisation while maintaining high speech quality. The techniques discussed may also be applied to other transforms and to the quantisation of surfaces in the standard waveform interpolation coder