[vorbis] what quality ?

[Troy Telford]

>
>
>could you explain a bit more lossy channel coupling?
>
>as i know that channel coupling means that the encoder uses the
>similarities between the stereo channels too.....
>
>bye,
>gabor
>
I'll give a couple of (feeble) interpretations, but they are mostly 
interpretations of what I remember reading at 
http://www.xiph.org/ogg/vorbis/doc/stereo.html

This is by necessity a very technical document.  It describes the stereo 
models Vorbis uses, as well as techniques used in programming Vorbis 
(There are all kinds of optimizations that make use of the limits of 
human hearing.)

But, for the less-mathematically inclined, here's my own (flawed) 
interpretation:

Every wave has (among other things) three distinguishing features: 
 Phase, Frequency, and Amplitude.

Just about everybody knows what the last two are.  Phase can be thought 
of as the wave being moved left-right for one cycle.  An example is that 
the cosine and sine function really are the same function, but at a 90 
degree phase shift.

Dual Stereo:  Each channel is entirely seperate, and is stored seperately.

Lossless Stereo:  Basically couples the two channels in a way that 
decodes identically to the dual stereo mode.  This is because a large 
amount of most sounds are heard by both ears, with only small 
differences in signal phase or time.  So you save the data once, with 
the difference between the two, and you get a perfect copy of both.

Phase Stereo:  Makes use of the fact that the human ear is nearly deaf 
to signal phase above 4kHz, meaning that the ear can't tell very well 
which direction it's coming from, so we can compress the data here. 
 While it's possible to detect the difference in special cases (when 
you're searching for it), if you're not looking for it, you won't find 
it.  (Of course, audophiles /do/ look for it)

Point stereo:  Eliminates the out-of-phase signal entirely.  Can be 
detected when using headphones (perfect stereo separation) so it's 
generally used where it's not noticible.

Mixed stereo:  Guess?

As I said, this is a rough idea of what is happening, but it's still 
/way/ too much information for 99+% of the human population anyway, so I 
think it may suffice.

(Any corrections would be appreciated if only for my own enrichment)

<p><p><p>--- >8 ----
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