[vorbis] Pre RC4 changes

[Peter Schuller]

> "Ogg Vorbis files use lossy channel coupling, meaning that redundancies
> between the left and right channels are combined to save space."
> 
> If you can, please explain that. (give examples if you can).
> How this differs from having the original stereo image? (quality??)

I think that partiuclar part is a bit mesleading. My understanding is
that mp3 uses *lossless* channel coupling (joint-stereo), while Ogg
Vorbis supports both *lossless* and *lossy*.

Lossless means that there is no information loss with respect to stereo
separation. That is, although the sound is otherwise compressed as
normal, the stereo separation in and of it self will be  perfectly
represented.

Lossy means that, just as mp3, vorbis and other vormats performs lossy
compression of the sound itself (meaning data is removed and quality
reduced in order to save space - the trick is removing the right data),
vorbis is also capable of representing stereo separation in a lossy way.
That is, it can achieve the percepton of perfect (or near-perfect)
stereo separation while using less data to describe it than would be
required with lossless separation.

In practice, I have never heard any stereo artifacts. But I believe
there was some .wav discussed previously that releally stressed sound
codecs, in which there were channel coupling related artifacts. Someone
else should be able to tell you more about this.


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