[vorbis] Partial Format converters?

[R.J.J.H. van Son]

On Mon, 2002-08-19 at 11:55, Mark Hetherington wrote:
> It is possible to lower bitrate without decoding on Ogg Vorbis files, but the 
> tools do not currently exist.
> 
> It is not possible to directly convert from any compressed format to a 
> different compressed format without decoding->encoding. It is a logical 
> impossibility. The data is of a completely different type.
> 
> 

I have only a very limited knowledge of the different file formats, and
I will immediately accept that direct translation between file formats
is impossible.

As I suspect that some of the problems have to do with the introduction
of quantization noise (both in amplitude and frequency(?)), there might
still be something to win if we could do a half-way decoding/encoding. 

This mailing list might not be the best place to discuss this rather
technical point (if so, name a better place and I will move it there).
But if I have the idea that there is something to win over just
doing/refraining from repeated compression, I can make a point for (or
against) building translators to people willing to hire someone to
actually code it.

I mean, all compression starts (and decompression ends) with a spectral
(band-filter) representation of the sound of some kind. At this point,
there is information on the frame-rate, spectral resolution (band-filter
set), intensity coding etc. Might a "translation" of the spectrum at
this point not prevent some of the problems that are added from a full
waveform synthesis and a NEW frame/spectral analysis? That is, you skip
the conventional frame-rate and band-filter, DCT coeeficient allocation
decissions and make do with whatever (band-filter) spectrum you get
delivered from, say, ATRAC3 (Minidisc).

I understand that the Ogg Vorbis file format is quite flexible and could
accommodate most frame rates. If it could accommodate their band-filter
sets too, a lot would have been won already.

I know that this is not easy stuff, but I have seen some very complex
things accomplished. If it would be worthwhile (which is rather
uncertain), we might get corpus groups to hire some people who DO know
about these things. Think of it, these corpus people are EACH spending
millions of dollars to construct their databases. They have access to
some very good people in acoustics and programming (e.g., Max Planck
gesellschaft in Germany, ATR in Japan, LDC in Philadelphia are all
collecting speech). A "spare" programmer is rather cheap in these
circles. So I just might be able induce them to consider this idea of
building translators if they think it might work (e.g., ATRAC3 to Ogg
Vorbis would be a real marketing stunt). 

Rob


-- 
           Rob van Son
 Institute of Phonetic Sciences/ACLC
       University of Amsterdam

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