[vlc-devel] AC3 3F1R downmix to stereo (simple downmixer)

James Turner zakalawe at mac.com
Tue Sep 8 11:46:13 CEST 2009

I've encountered a problem where AC3 3F1R is being down-mixed to  
stereo incorrectly - VLC is selecting the 'trivial_mixer' instead of  
the 'simple_mixer', and hence discarding the centre channel, which  
happens to contain all the dialogue for the content in question.

This is occurring with 1.0.1 on both Mac and Windows (tested under  

 From browsing the source code to the simple mixer, I think this is a  
straightforward case of the channel layout in question (left/right/ 
centre/centre-rear) not being supported by the downmix code. The  
attached patch adds support to the simple downmixer - it works great  
for me, but I'm not 100% confident on the scaling factors applied to  
the sample values, and would welcome some advice there.

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I'm not at all happy with the symbolic name I chose for the layout  
(AOUT_CHANS_4_CENTRE_REAR), feel free to pick a shorter/clearer/better  

I made no attempt to support the 'output.i_physical_channels ==  
AOUT_CHANS_4_0' case in the patch, but I believe it's also  
straightforward if someone wished to do that.

Note this work was done on the 1.0 bugfix branch from git, since I  
encountered compile errors with trunk (on Mac 10.5, XCode 3.1.3).


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