[Vorbis-dev] 5.1 surround channel coupling

[qduaty]

2007/1/26, Arek Korbik <arkadini at gmail.com>:
> How about the 'Ogg Vorbis stereo-specific channel coupling discussion'
> ( http://xiph.org/vorbis/doc/stereo.html )? Came across it just yesterday.
> In my understanding it suggests the approach is extensible and doesn't
> necessarily require 2^n (2*n even?) channels or separate couplings?

Maybe I don't understand it well, but I didn't ever see any solution
for surround in that discussion, and I have read it several times.
Just two channel coupling and a promise that the model is extensible.

After googling for "site:lists.xiph.org surround ambisonic 5.1", I
started to encode some movie soundtracks as 3-channel, B-Format
Ambisonics, with such assumptions:

1. Movies have just several sources of sound (namely, speakers) so
even if B-format is encoded with plain FMH equations, the only
artifact noticeable will be that dialogs are in front of the screen
(equations assume all speakers have the same distance from the
microphone). This is not an issue - the goal is to check whether
Vorbis can encode it all at 128 kb/s, with reasonable quality.

2. As there are no known directx filters for Ambisonic-encoded movie
sound tracks, the decoding has to be done the same way as encoding,
i.e. without latencies, nor filtering.

3. My encoding setup consists of 5 channels; LFE is simply mixed down
into W. Four channels are put in a rectangle, where X axis (in
ambisonic terms) is a bit longer than Y. This is to cancel the
mentioned dialogs-related artifact. Center channel stays in its place
between L and R.

4. As there is also no known audio standard that took 3 channels of
sound (except Ambisonics) it's even safe to assume these encodings
will be decodable in future, by assuming they are Ambisonic WXY
B-Format.

MPlayer + oggenc + OGMuxer (or mkvtoolnix, or ffmpeg2theora) can
handle it all. Actually, I just started listening...