[vorbis-dev] Channel coupling in Vorbis

[David Carter]

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.)

(This also allows for easy compatability with lesser setups than the track
was made for -- if you want to play a spherical track on stereo speakers,
simply ignore the X and Z channels.)

Since M/S stereo (the technique planned for channel coupling) is exactly the
same as 2-channel Ambisonic B-format sound, there seems to be little reason
to make a seperate spec for M/S stereo.  I'm not sure whether Monty is
intending for M/S tracks to be tagged as 2-channel Ambisonic or not, but that
doesn't matter a whole lot.  As a surround format, Ambisonics seems to have
considerable advantages, especially on the patent front.  (The relevant
patents seem to have expired several years ago.)

I'm not sure what you mean by "color blind" -- could you elaborate on that as
well as why the WXYZ seperation should be frequency-dependant?  If you would
also elaborate on your other reasons (if any) for disliking Ambisonics, I'd
like to discuss them further.

        David

Layout of Ambisonic channels:

---  Front of room ---
  ^
  |  <--Y-->   Z (up/down)
X |
  v            W (omnidirectional)

---  Back of room  ---


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