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Re: [Discuss-gnuradio] Proposal Draft: Next Generation Digital VoiceCod

From: Jeff Brower
Subject: Re: [Discuss-gnuradio] Proposal Draft: Next Generation Digital VoiceCodecs and Vocoders for Amateur Radio
Date: Wed, 14 May 2008 22:40:42 -0500


> > We've been working on a new lowlatency codec for speech and music,
> > CELT, and are about to public a paper on it.  (celt-codec.org).
> > While CELT wouldn't be useful for narrowbanded voice some of the
> > components of CELT would be very useful in an AMBE killer.
> > (Particularly CWRS, the algebraic vector quantizer).  CELT itself
> > could be used for a 10-20khz wide mode (40-60kbit/sec with decent SNR)
> > to replace FM with something with much greater quality if there was
> > interest...   But a lower bandwidth mode will be far more interesting.
> >
> Would this be useful to solve the garble problem when there's a
> firefighter with a loud chainsaw in the background? That just kills the
> codecs they use on APCO 25 right now.

If the codec can handle music and other audio signals besides speech, then the 
is probably yes.  That would mean the codec uses "perceptual" techniques, which 
general, rather than specific technique based on a human vocal tract model.  The
vocal tract model is what gets codecs like AMBE and MELPe into trouble when they
encounter non-speech sounds.  MP3 uses perceptual techniques.

But like Gregory said, lower bandwidth operation is key.  Without that codecs 
will AMBE persist.  MELPe goes as low as 600 bps.  Essentially they accomplish 
by making a wide range of assumptions based on speech-like signals.


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