Switched orthogonalization of fixed-codebook search in code-excited linear-predictive speech coder: Derivation of conditions for switching

Abstract

This document presents an algorithm of switched orthogonalization of fixed-codebook (FCB) search in code-excited linear-predictive (CELP) speech coder and derivation of conditions for switching. Orthogonalization of FCB search is an early 1990's technology, and later efficient implementation was developed on some algebraic CELP (ACELP) speech coders standardized in 2000s. ITU-T Recommendation G.729.1 is such speech coder standardized in 2006. Orthogonalization of FCB search does not degrade CELP coder performance if ideal gain parameters are applied to codebook vectors. However, because of limited performance of gain quantization and overall optimization of CELP coding algorithm, the orthogonalization does not always give improved coder performance. This document presents a switched orthogonalization algorithm based on an estimated adaptive codebook (ACB) gain parameter, which is obtained through the orthogonalized FCB search. The algorithm was evaluated in G.729.1 coder. While the orthogonalization was switched off on 20% of voicing frames, segmental SNR was improved by around 0.1 dB in average.