[mythtv-users] ac3 passthrough problems

Thomas Börkel thomas at boerkel.de
Thu Sep 21 06:17:33 UTC 2006


Scott wrote:

> 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.

Apparently, Myth sends raw AC3 data to Alsa and Alsa does not switch 
your receiver to DD.

I have wrestled with this for several days and now it works. Try this:

First throw away your .asoundrc and restart mythfrontend.

Set this in Myth:
Normal device: ALSA:default
Passthrough device: ALSA:spdif
AC3 passthrough: Checked

With these settings, you should:
- get DS sound for MP2 and AC3 2.0
- get DD sound for AC3 5.1

Now, for mixing 44 KHz audio to 48 KHz, you need some special .asoundrc. 
Using the .asoundrc from the HowTo, you should be able to set normal 
device to ALSA:dmix-digital and passthrough device to ALSA:digital-hw. 
BUT, you have to set the right device number in the digital-hw section 
of the .asoundrc, otherwise your receiver won't switch to DD. Try "aplay 
-l" to see the devices. For me (SB Live 5.1), it was device 2.

Also, for AC3 5.1 working, you have to switch OFF "Use video as 
timebase" in the frontend/tv/playback settings.


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