One key aspect of the CELP algorithm is that it shapes the coding noise using a simple, yet effective, weighting lter. In this paper, we improve the noise shaping of CELP using a more modern psychoacoustic model. This has the signi cant advantage of improving the quality of an existing codec without the need to change the bit-stream. More speci cally, we improve the Speex CELP codec by using the psychoacoustic model used in the Vorbis audio codec. The results show a signi cant 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 speci c to Speex and could be applied to other CELP codecs.
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