[Vorbis-dev] Vorbis Ambisonic coupling

[Richard Lee]

Richard Lee wrote:

> But there is a caveat.  Does Vorbis coupling preserve "phase" relations?  Any references explaining this in simple detail?

Sebastian :

>"coupling" is a rather loose term in the sense that it doesn't exactly 
specify HOW it's done. It can be understood as the opposite of coding 
channels independently. Obviously there are many ways to do coupling.

>Both a "lossless" variant and a "lossy" is possible within the Vorbis 
specification. The former is supposed to exploit redundancies between 
channels only while the latter is also a kind of irrelevancy reduction 
(like ignoring phase relations for intensity stereo).

>If you really want to know the gritty details you should take a look at 
the Vorbis specification and the Vorbis Stereo modes HTML page.

Thank you for Sebastien.  I am new to compressed audio coding so please correct any stupid assumptions I make ... 8>D

>From the Vorbis spec and Stereo mode page, my tentative recommendations as a (pseudo) Soundfield and Ambi guru are as follows.

- The Ambisonic B-format WXYZ channels should be should be lossless coupled together over most of the frequency range.  This will give the most gain as W "usually*" has everything that XYZ has.  A perfect B-format signal has scaled versions of XYZ in W.*

- We allow "4 phase coupling" at higher frequencies.  A perfect B-format set of WXYZ should only have real relation to each other*.  The deviations from perfection can take 2 forms.

a) Minimum Phase deviations of XYZ from W.  These start happening above about 6kHz with present Soundfields.

b) Random Phase deviations of XYZ from W.  These occur at High Frequencies when a real Soundfield Mike no longer has flat frequency response in all directions.  Above 10kHz with present Soundfields.

- B-format from Ambisonically panned material (as opposed to natural Soundfield recordings) will be perfectly in phase so this recommendation will not result in loss.

This needs investigating.  Presently no Ambisonic effects units use 


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