[vorbis] bitrate peeling

[Christian.Buchner]

> do any other audio formats support peeling? cuz afaik you just reencode mp3z
> 'n' such to get lower bitrates also.

Depends on the coding they use.

As far as I know MP3 stores subband by subband in Huffman coding.
That means you can cut away whole subbands to cut off the high
frequencies for example. But that's about all you can do without
resorting to re-quantization.

To be really peelable you have to code the data progressively.
And that is a form of art. ;)

Bitplane coding for example can be used achieve that. You would
then encode bitplane by bitplane, for example using arithmetic
coding. And for peeling data off, you would simply drop the least
significant bitplanes. That is (somewhat simplified) what JPEG
2000 does.

I do not know enough about the entropy coding used in Vorbis
to comment on it. Maybe someone can shed some light on it with
a few sentences.

Christian

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