[mythtv] [patch] multi channel audio patch 1

Michael T. Dean mtdean at thirdcontact.com
Fri Dec 30 13:38:38 EST 2005

Mark Spieth wrote:

> attached is the next version of the multi channel audio patch.
> I have tested it with ALSA and it works up to 6 channels with 
> timestretch with a AC3.5.1 avi I have. I have only listened to 2 
> channels connected to my sound card though so this needs more testing. 
> Ive limited it to 6 channels at the moment.
> AC3 passthrough support is not handled yet so you need to use all 3 
> jacks from your audio card to test currently. So dont use it yet (ac3 
> passthrough).
> however from my alsa reading you can get alsa to reencode te 6 channel 
> audio back to AC3 but I dont know how this is done. If this is true, I 
> may just leave it at that and not reencode in myth (easier for me :) ).
> There is a bit of code for support of ac3 reencode in audio layer 
> which I may back out if an ALSA setup is viable.

Instead of re-encoding to AC-3, ALSA will allow 6-channel PCM output via 
the digital connection.  This is actually preferrable to AC-3 
re-encoding because AC-3 is a lossy compression format (and PCM is 
uncompressed and, therefore, not lossy).

That being said, I haven't tested your patch (holiday visitors) and I 
only have 2 speakers in my setup (although I could "fake" the others 
with creative shuffling).  As soon as the visitors are gone (next week), 
I'll finally have a chance to work on my HDTV setup (complete rebuild), 
and I'll do some testing (and write up an ALSA config, if required).  
(I'll need to do this because I can't watch TV without timestretch, 
anymore, and the only way I'll do more than 2-channel output is if 
timestretch works with it--so thanks for the timestretch and for the 
multi-channel work.  :)

The only benefit I can see of fixing the AC-3 would be allowing the user 
to offload AC-3 decoding to an external A/V receiver.  However, keeping 
the AC-3 decoding on the Myth box should make A/V sync easier to manage 
(because we can pretty much assume PCM decoding will be "instantaneous", 
whereas AC-3 decoding may not be).  Therefore, I would recommend you 
save your time and don't worry about AC-3--at least, not yet.  (I'm even 
volunteering to reply to all the "How come it won't do AC-3 with 
timestretch?  I want AC-3 for better sound quality..." posts.  :)


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