Improving LSF quantization performance with sorting

All coders require that the used linear prediction (LP) filter is stable. All current line spectral frequency (LSF) vector quantization routines may cause that the resulting quantized LSF vector is out of order, and thus unstable. In those cases the vector is stabilized after the quantization by sorting the LSF vector. This, however, means that the found vector may not be optimal, since this sorting was not taken into account in the quantization loop. The proposed codebook search routine orders the LSF vector inside the quantizer loop, and the optimal code vector is always found. The proposed method improves LSF quantization performance in terms of spectral distortion (SD) while maintaining the original bit allocation. The method can be used both with predictive and non-predictive split (partitioned) vector or multistage vector quantizers. The proposed method has more effect on performance when higher order LPC models or matrix quantizers are used, because in those cases LSFs are closer to each other and invalid ordering is more likely to happen. The proposed method is used in Texas Instruments, Nokia, and Ericsson ITU-4k candidate speech coder [A. McCree et al., May 2002].