[vorbis-dev] Vorbis as a Quicktime codec

[Kevin Marks]

At 12:50 PM +0200 9/19/01, Vitalijus Slavinskas wrote:
>KM> This is going to be tricky, as QT audio compressors assume 2 things
>KM> that Vorbis violates: a fixed compression ratio and uniform frame
>KM> sizes.
>
>maybe i'm mistaken but i think that purevoice qt codec is also 
>variable bitrate

It is, but not in the same way. PureVoice has a fixed frame size (160 
samples) and encodes this to a variable size chunk (1-35 bytes in 
full mode, 1-17 bytes in half mode). Within QT movies, it pads these 
out to the maximum size in each case (otherwise the table to index 
the sizes would be bigger than the data).
When streaming, the packetizer discards the padding and interleaves 
the chunks in the stream. The reassembler expands it back to the 
fixed ratio before handing it to QT.

QT supports VBR MP3 decoding - I've posted links to how this works 
before, and I believe Segher is using this technique in his 
importer/decoder.

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