A 6.4-KBIT S VARIABLE-BIT-RATE EXTENSION TO THE G.729 (CS-ACELP) SPEECH CODER/

This paper proposes a 6.4-kbit/s extension to G.729 (conjugate structu re algebraic code excited linear prediction: CS-ACELP). Each G.729 mod ule was investigated to determine which bits could be removed without hurting the speech quality, then two coders that have different bit al locations were designed. They have two different algebraic codebooks ( a 10-bit algebraic codebook that has two pulses and an 11-bit algebrai c codebook that has two or three pulses). This paper also proposes a c onditional orthogonalized search for a fixed codebook to improve the s peech quality. The conditional orthogonalized search chooses one of tw o search methods (orthogonalized or non-orthogonalized) based on the o ptimum pitch gain. The quality of the two coders was evaluated using o bjective measurements (SNR and segmental SNR) and subjective ones (mea n opinion score: MOS and a pair-comparison test). The selected coder w as evaluated under practical conditions. Subjective test results have indicated that the quality of the proposed coder (10-ms frame length) is equivalent to that of the 6.3-kbit/s G.723.1 coder, which has a 30- ms frame length.