ITU-t g.729 extension at 6.4 kbps

This paper describes the 6.4 kbit/s CS-ACELP coder being standardized as annex D to ITU-T G.729. The coder is based on the same building blocks as the 8 kbit/s G.729 to facilitate low complexity extensions to G.729 in terms of additional memory usage. It is fully switchable with the 8 kbit/s coder and provides additional flexibility to existing and emerging G.729 applications. The fixed codebook is a 2-pulse algebraic codebook. The adaptive codebook quantization has been changed and a new conjugate structure gain codebook is used. In order to compensate for the sparser algebraic codebook, an adaptive post-processing technique is used to enhance the quality for unvoiced speech and background noise sounds. Subjective tests have indicated that the coder has a performance close to that of G.729, and equivalent to that of G.723.1 at 6.3 kbit/s for speech.