[vorbis-dev] Channel coupling in Vorbis

[David Carter]

On Mon, Aug 14, 2000 at 06:05:34PM -0500, David Carter wrote:
> On Mon, Aug 14, 2000 at 06:45:46PM -0400, Thomas Marshall Eubanks wrote:
> [snip] 
> > I would recommend NOT using the ambisonic for surround sound. I read the specs on this - it
> > uses a spherical harmonic expansion of the sound field. The trouble with that is
> > that it is 3-D. Unless you plan to have speakers in your ceiling, it will waste 1/3 of
> > your bandwidth and (except for the dipole terms) won't separate cleanly into
> > high and low bitrate channels (or horizontal versus vertical). Also, I believe that
> > ambisonic is "color blind", while clearly the separation of the monopole and other
> > terms is best NOT done on a  frequency independent basis.
> 
> Thomas,
> 
> Although Ambisonics allows for true 3D surround, it does not require it.  If
> you use the full four channels, you get 3D surround.  You can use three channels
> and get 2D (planar) surround, or you can use two channels and get M/S stereo.
> (The trivial case also gives mono with one channel, as you would expect.)  In
> Ambisonic B-format terms, spherical surround uses the WXYZ channels, planar
> uses WXY, M/S stereo uses WY, and mono-only uses W.  (M/S stereo is commonly
> used in FM radio transmission, as well as some MP3 joint stereo modes.)

I think I understand what you were getting at about this now -- if you have
a two-channel file that is M/S, there will be two channels in the Ogg stream,
one marked as W, the other as Y.  You are *not* required to have X and Z
channels if they aren't used.  (As I currently envision it, anyhow.)

        David


-- 
David Carter ** dcarter at sigfs.org ** dcarter at visi.com
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