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List:       vorbis-dev
Subject:    Re: [Vorbis-dev] 5.1 surround channel coupling
From:       Ralph Giles <giles () xiph ! org>
Date:       2007-01-31 23:16:42
Message-ID: 20070131231642.GF3567 () ghostscript ! com
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On Thu, Jan 25, 2007 at 02:31:13PM +0000, Ivo Emanuel Gonçalves wrote:

> Someone on the Xiph wiki added some interesting links for Ambisonics
> stuff.  That may be of use to you as well.

So, ambisonics are just expanding the sound field in spherical 
harmonics. This is the proper mathematical generalization of mono and 
stereo, whereas the dolby N.M schemes assume a particular speaker 
placement. With the extra layer of abstraction (and a lot of speakers) 
one can achieve more accurate recording and reproduction of sound 
fields.

However, most of the ambisonic literature is based on the the original 
quadrophonic implementations (where were also patented) and this isn't 
enough channels to accurately represent the planar source arrangment 
most content is mixed for. Furthermore, since the vorbis spec has no 
room for a channel mapping table, there's no efficient way to specify 
a subset of ambisonic modes you want to encode for a better match.

So, while it's an interesting thing to work on, it's a different and 
longer-term project than implementing good support for the 5.1 surround 
mixes most people currently want to encode. The tradeoff becomes more 
attractive when you have 8-12 surround channels, not just 6, and care 
about 3D soundscapes (like in games, art installations, and so on) than 
in film.

 -r
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