[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.
--
Scott
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