[mythtv-users] ac3 passthrough problems

Scott list-mythtv at bluecamel.eml.cc
Tue Sep 26 13:59:46 UTC 2006

On Sep 25, 2006, at 9:16 PM, Brian J. Murrell wrote:

> On Fri, 2006-22-09 at 01:53 -0400, Scott wrote:
>> Audio output device:
>> ALSA:iec958:CARD=0,AES0=0x6,AES1=0x82,AES2=0x0,AES3=0x2
>> Passthrough output device: Default
> Hrm.  I did
> Audio output device: ALSA:iec958
> Passthrough output device:  
> ALSA:iec958:CARD=0,AES0=0x6,AES1=0x82,AES2=0x0,AES3=0x2
> and I get digital PCM just fine for non AC3/DTS sound (i,e. analog  
> recordings) but nothing for a (say) 5.1 sound track.  ~sigh~

When you get no sound for a 5.1 sound track, continue to use the  
above settings but try running mythfrontend -v audio > audio.debug  
and then playing your 5.1 sound track source. In the audio.debug file  
look for the ALSA lines to see if MythTV had trouble with any ALSA  

> All of this is of course after trying the much more simple and
> straightforward "ALSA:iec958" or "ALSA:spdif" in the Passthrough  
> option.
> Frequently I get the "helicopter blades" sound characterized elsewhere
> in this thread.

I've come to the conclusion that "helicopter blades" is the result of  
sound being routed correctly BUT the headers are missing as Mike  
stated a while back. Using the hex codes above would "fix" this. No,  
I'm not sure how accurate this statement is but it's my current  
working theory.

> Any further ideas?

1) In MythTV locate a 5.1 source that fails to play correctly
2) Exit Myth Frontend
3) Using mplayer, run "mplayer -v -ao alsa -afm hwac3  
source_file_from_mythtv.mpg" and verify the sound works as expected

At step 3, if the sound doesn't work as expected then most likely the  
problem is ALSA is not configured correctly yet. An asoundrc file  
might help out. If the sound does work correctly then look for the  
alsa-init lines produced by mplayer verbose output. They should tell  
you exactly which device mplayer is using to get you 5.1 sound. Copy  
that information down and paste it ot the list. The info mplayer  
outputs won't be usable in the MythTV device fields as is but it can  
be a good hint of the direction to run in.

Of course, all this assumes you're doing the following hookup: sound  
card -> spdif -> A/V receiver. :)


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