[vorbis] square polar mapping - backgrounds

[Stoffke]

> I assume you've seen http://xiph.org/ogg/vorbis/doc/stereo.html ? 

Yes I've read the specs, but it's not quite easy for me to understand .

> It describes the transformation fairly well (for the 2d case, anyway).

Does that mean there're  more than 2 dimensions possible in SPM ?

<p>Let me recapitulate the things that I understood from the specs:

The encoder  separates the spatial audio information (how exactly is not said in
the specs  ) into a "point" and a "diffuse" component. These two components are
than coded into a vector. The magnitude of the vector represents a "point image".
A point image is a sound that can clearly be located somewhere  in the stereo field. 

What if there are more than one "point images", for example two intruments, one clearly on the left and the other clearly on the right side ? 

The angle of the vector represents a "difuse image" - that seems to fill a  larger area with
a diffuse sound, like reverberation.

The L/R channels can be restored by a simple algorithm, based on subtraction and addition
of the magnitude and angle values.

Bits are saved by allowing only a few discrete angle values 
But i think the whole SPM  is discrete, right? It was said in the specs, that there are 121 possible vectors. Or was that just an example ?

With the quantisation of the vector to only a few possible angle values, some "diffuse image" information are thrown away - at least the higher frequencies above 4 kHz.
But this is little noticeable to the human ear.

Hope I got it all  right.

Regards 

Stoffke

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