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. In this paper, we present a multi-level wavelet decomposition mechanism, using low-delay FIR filters, applied to Waveform Interpolation coding. The technique overcomes the substantial delay problems of [2] and identifies a preferred technique for the quantisation of the decomposed surfaces. 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 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.