Speex dev - Aug 2004 - Psycho Acoustic models i Speech Coding

Hi,

Does anyone have an idea about the possibility to apply psychoacoustic
models as the ones in mp3 or AAC to a CELP coder? Thanks!

/Pontus

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(This is almost out of topic but anyway...)

It is surprising how little research effort have been put into psy-acou
models for CELP. The basic problem lies in that it is not easy to alter
the LP model without distroying the minimum-phase property (ie. the
stability of the predictor). That leaves us with psy-acou modelling of
the noise-part only. However, my own research is in constrained
LP-models that addresses just this problem. Unfortunately, applications
are still at least a couple of years away.

<p>Tom

<p>Christian Buchner wrote:
> 
> > Does anyone have an idea about the possibility to apply psychoacoustic
> > models as the ones in mp3 or AAC to a CELP coder? Thanks!
> 
> > /Pontus
> 
> This is (sort of) done in the decoder with the optional perceptual
> filtering. Speex tries to shape the noise so that it sounds more
> pleasant, I believe.
> 
> To really use perceptual coding, one would require fine granular
> control over quantization noise in various spectral bands. Speex
> is not a subband coder and has no such control. But it already does
> a better job than MP3 and other codecs at these bitrates without
> using a PSY model.
> 
> Besides those PSY models are mostly FFT based which would greatly
> increase the encoder complity. We don't really want that. Do we ;)
> 
> Christian
> 
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-- 
Tom Bäckström
Helsinki University of Technology, 
Laboratory of Acoustics and Audio Signal Processing
email: Tom.Backstrom@hut.fi
http://www.acoustics.hut.fi/~tbackstr/
phone: +358-9-451 5843 (office), +358-50-341 3841 (mobile)
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> Does anyone have an idea about the possibility to apply psychoacoustic
> models as the ones in mp3 or AAC to a CELP coder? Thanks!
> /Pontus
This is (sort of) done in the decoder with the optional perceptual
filtering. Speex tries to shape the noise so that it sounds more
pleasant, I believe.

To really use perceptual coding, one would require fine granular
control over quantization noise in various spectral bands. Speex
is not a subband coder and has no such control. But it already does
a better job than MP3 and other codecs at these bitrates without
using a PSY model.

Besides those PSY models are mostly FFT based which would greatly
increase the encoder complity. We don't really want that. Do we ;)

Christian

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