Re: [codec] possible issues to track

"Benjamin M. Schwartz" <> Fri, 26 March 2010 00:58 UTC

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Date: Thu, 25 Mar 2010 20:58:37 -0400
From: "Benjamin M. Schwartz" <>
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Subject: Re: [codec] possible issues to track
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> ----- Original Message -----
>> 3. The format should provide a way to specify N named output channels
as linear combinations of the
>> coded channels.

Michael Knappe wrote:
> Just to clarify, is #3 what's sometimes called a fold-down scheme? (DVD-Audio for instance could generate stereo mixdowns from multi-channel content on the fly)

That would be one good use of it.  It could also be used to increase
efficiency by statically decoupling surround-sound channels.  For example,
a "7.1" channel mix could be transmitted as just two stereo pairs, with an
8x4 mixing matrix to produce the 8 output channels.  This obviously
wouldn't make full use of the speakers, but it would be allowable, and
maybe useful.  Even if the number of channels in and out is the same
(square mixing matrix), decorrelating the channels could improve compression.

Another possibility is to specify a number of different mixing matrices,
so that a single stream could have defined mixes for different output systems.

The biggest problem I see with this system is agreeing on channel names.
I wonder if there's an existing multichannel mapping or mixing
specification somewhere that we could borrow.