4 kb/s multi-pulse based CELP speech coding using excitation switching

This paper proposes an MP-CELP (multi-pulse based CELP) speech coding at 4 kb/s. In MP-CELP, amplitudes or signs of multi-pulse excitation are simultaneously vector quantized (VQ). In order to improve speech quality for background noise conditions, the excitation signal is switched between voiced and unvoiced speech, and the number of pulse is greatly increased for unvoiced speech by restricting pulse locations. Further, in order to improve voiced speech quality, the optimal combination among adaptive codebook lag, pulse location, sign codevector and gain codevector is selected which minimizes distortion by employing delayed-decision search. The subjective evaluation results show that speech quality for 4 kb/s MP-CELP is close to that for ITU-T G.723.1 (6.3 kb/s) and G.729 (8 kb/s) in M-TRS clean speech condition. For background noise conditions, the introduction for the excitation switching and the pulse location restriction significantly improves the mean opinion score (MOS) value by 0.4. However, further improvement is still required, except for interference talker condition.