[vorbis-dev] Re: Speex: Open-source, patent-free speech coding

[DSPguru]

Hi Jean-Marc,

> > these days we think about MultiLingual vorbis defenitions, and the best
> > compression we could acheive were if we could save the common track on
> > two channels, and then save the delta between the common track to the
> > specifiec language (english speech/italian speech/etc'..) on a low
> > bandwidth track. a vocoder track for this task could be very
> > interesting. don't you think ?
> 
> Probably, what kind of bandwidth (sampling rate) and bit-rate are you
> thinking about?

ampling-rate should be high (48khz), but bandwidth should be less than 
16khz (after "extracting" speech-only from the lingual track).

about bitrate, let me describe something :
up until vorbis came, people used to encode their soundtrack of movies 
at 128kbps to 192kbps MP3. now, with Ogg, we can encode the "common"
track at around 100kbps vorbis, and encode each speech track at less
than 30kbps with speex. this gives us about 180kbps for a movie with
three soundtracks (english/italian/francis, for instance).
that could make a small revolution :).

<p>> Well CELP (mostly CELP variants) is still the most widely used technique
> for speech coding. If you look at the latest standards (like G.729 and
> AMR wideband), most use ACELP (which we cannot use because of patents,
> but that's another thing). Note that CELP has nothing to do with LPC
> vocoders (aside from the fact that is uses LPC analysis). There are
> other techniques, like WI (waveform interpolation) and sinusoidal coding
> (not sure about MBE), but most of them are for low bit-rate coding (in
> the 4 kbps range). Our current goals focus more on high quality that low
> bit-rate.

you're right, VoIP applications uses variations of CELP. AMBE (& MELP)
vocoders are mostly used in military applications (Digital voice over HF). but in some way, MBE complements Speex, the same way Speex 
complements vorbis :).
anyway, it was only a thought..

just to mention, that it been some years since i studied vocoders
principles, but i still remember (correct me if i'm wrong here :>),
that CELP bitstream mostly (i believe that in LD-CELP it's even ONLY)
includes codes to exciters codebook, but both the encoder and decoder
includes LPC analysis.

you can find some info about MBE over at :
http://www.dvsinc.com/papers/mbe.htm

these days, some people are testing the difference between tracks within mutlilingual movies.
results would be published here :
http://forum.doom9.org/forumdisplay.php?s=&forumid=11

> 
> That being said, the Speex is an open-source project and the code is
> designed so that it's really easy to change any part of the codec (LSP
> quantization, pitch, fixed codebooks, ...). So you can play with it if
> you like and if you find something that works well, let us know.
> 
>         Jean-Marc

10x,
Dg. http://DSPguru.doom9.net

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