[vorbis] Ogg Voxpop

[Keith Wright]

I have been thinking about what is needed to make language
teaching/learning tools.  (Like talking flash cards.)
The main thing needed is a low bit-rate encoding of
human voice.  At first I thought I could take one of
the gevernment standard vocoders and embed it as an
Ogg stream.

But:
 (1) there is not a standard vocoder, there is are half a dozen, at least.
 (2) they are fixed bit rate, we really don't want to waste bits
     while the teacher waits student to respond
 (3) they include error correction (not needed here because
     we assume the underlying storage and transport mechanism
     takes care of that
 (4) one might need several different quality/bit rate options
     For example, both hours of Spanish radio broadcast packed
     into as small a file as possible, and a high audio quality
     demonstration of the difference in pronounciation of a "D"
     in English, Spanish, Chinese, and German, using as much space
     as needed to make the difference sound clear.

My current thought is to filter the input down to a bandwidth
of 7KHz or 4KHz (traditional values for high and low quality
speech), decimate the samples so that the sound is sampled
at, say, 44/3=14.6KHz or 44/5=8.8KHz, then run it through
the standard Vorbis encoder.  Vorbis then sees an ordinary
20KHz bandwidth stream that sounds like a tape recording
running at 3 to 5 times normal speed and encodes it as
usual.

I checked the mailing list archives, and found an old thread
about low bit-rate encoding that quickly degenerated into
a highly bogus discussion of the proper way to decimate
the sample sequence.  We don't need to do that again, so
assume the filtering and decimation is done properly,
is there any reason this scheme could not work?  Are
there hooks in the Vorbis stream format to tell the decoder
that this as been done so that it will know to play back
slower than normal?  Do I have to write all this myself,
or is it already in there if I just know the parameter to set?


-- 
     -- Keith Wright  <kwright at free-comp-shop.com>

Programmer in Chief, Free Computer Shop <http://www.free-comp-shop.com>
         ---  Food, Shelter, Source code.  ---

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