  One key aspect of the CELP algorithm is that it shapes the coding noise using
a simple, yet effective, weighting filter. In this paper, we improve the noise
shaping of CELP using a more modern psychoacoustic model. This has the
significant advantage of improving the quality of an existing codec without the
need to change the bit-stream. More specifically, we improve the Speex CELP
codec by using the psychoacoustic model used in the Vorbis audio codec. The
results show a significant increase in quality, especially at high bit-rates,
where the improvement is equivalent to a 20% reduction in bit-rate. The
technique itself is not specific to Speex and could be applied to other CELP
codecs.
