[mythtv-users] ac3 passthrough problems

Scott list-mythtv at bluecamel.eml.cc
Thu Sep 21 01:30:11 UTC 2006

On Sep 19, 2006, at 11:24 PM, Nick wrote:
> If you specify ALSA:default as your MythTV passthrough sound device
> and your default ALSA device is not your SPDIF, passthrough will not
> work. You either need to specify your SPDIF directly in MythTV (like
> ALSA:iec958 or ALSA:spdif) or you need to follow through the HOWTO
> fully and adjust your asound.conf file to set your default ALSA device
> to your SPDIF.

Sure, make it sound easy. :) Here are the settings I have for MythTV - 
 > Settings -> General -> Audio

Audio output device: ALSA:digital
Passthrough output device: Default
Enable AC3 to SPDIF passthrough: [checked]
Enable DTS to SPDIF passthrough: [not checked]
Aggressive Sound card Buffering: [not checked]
Use internal volume controls: [checked]
Mixer Device: digital
Mixer Controls: PCM
Master Mixer Volume: 70
PCM Mixer Volume: 70
Independent Muting of Left and Right Audio Channels: [not checked]

> If you do configure your asound.conf (or .asoundrc if using per user
> settings) using the HOWTO you need to make sure you specify the
> digital device as device 1 on card 0.

See my attached .asoundrc which is based on the HOWTO. The default is  
set to dmix-digital. The ALSA:digital option is just a nice way to  
say iec958 or spdif. Using this .asoundrc file, I expect that  
anything sent to an unspecified ALSA control will end getting  
properly mixed to the correct rate and sent out my SPDIF and into my  
A/V. This is the way it seems to be working at any rate. In these  
cases I _expect_ to seem my receiver decode the input as PCM 48kHz.

In my MythTV config I have configured Audio to use the ALSA:digital  
control for all output. I expect this should tell MythTV to send all  
output out my SPDIF "as is". In the case of AC3 and DTS my A/V  
receiver should be picking them up and decoding the formats. This is  
not happening correctly.

With the config shown above, I get a nasty stuttering sound that  
might sound like a helicopter whirling it's blades :) If I do not  
check the "enable AC3/DTS passthrough" options then I get normal  
sounding audio but not DD or DTS format. My receiver says I'm instead  
receiving a PCM 48kHz signal from the SPDIF. Using mplayer on the  
same test clip shows DD 5.1 with the -afm hwac3 pass through option.

Also attached is an audio.debug file. I see MythTV does detect and  
select the 5.1 sound track but I'm not so sure it's actually passing  
it to ALSA correctly. Is there a way I can see even more debug info  
so I know exactly what the ALSA driver is receiving? I'm hoping to  
see something like I can with mplayer -v.

Bonus question: Since I have an external A/V receiver for decoding  
and volume control, shouldn't I be not checking the "Use internal  
volume controls" option?

The help is much appreciated.


-------------- next part --------------
A non-text attachment was scrubbed...
Name: aplay_devs.txt.gz
Type: application/x-gzip
Size: 164 bytes
Desc: not available
Url : http://mythtv.org/pipermail/mythtv-users/attachments/20060920/4b1db8fe/attachment.bin 
-------------- next part --------------
A non-text attachment was scrubbed...
Name: aplay_pcms.txt.gz
Type: application/x-gzip
Size: 708 bytes
Desc: not available
Url : http://mythtv.org/pipermail/mythtv-users/attachments/20060920/4b1db8fe/attachment-0001.bin 
-------------- next part --------------
A non-text attachment was scrubbed...
Name: asoundrc.gz
Type: application/x-gzip
Size: 1991 bytes
Desc: not available
Url : http://mythtv.org/pipermail/mythtv-users/attachments/20060920/4b1db8fe/attachment-0002.bin 
-------------- next part --------------
A non-text attachment was scrubbed...
Name: audio.debug.gz
Type: application/x-gzip
Size: 2500 bytes
Desc: not available
Url : http://mythtv.org/pipermail/mythtv-users/attachments/20060920/4b1db8fe/attachment-0003.bin 

More information about the mythtv-users mailing list