[mythtv-users] ac3 passthrough problems

Thomas Börkel thomas at boerkel.de
Thu Sep 21 20:39:16 UTC 2006


Scott wrote:

>>> With the config shown above, I get a nasty stuttering sound that might
>> Apparently, Myth sends raw AC3 data to Alsa and Alsa does not switch
>> your receiver to DD.
> Not necessarily. It's perfectly possible to send analog format signals 
> out the spdif and for the receiver to decode them. Again, with the -afm 

Yes, I know. But I was talking about 5.1 data.

> hwac3 option, mplayer sends the raw AC3 sound track out to alsa and my 

That's good.

> Instead it uses the iec958 (aka spdif) of my sound card. This all works 
> regardless of if I'm using an asoundrc file or not.


> Audio output device: ALSA:default
> Passthrough output device: ALSA:spdif
> Enable AC3 to SPDIF passthrough: [checked]
> Enable DTS to SPDIF passthrough: [not checked]
> Aggressive Sound card Buffering: [not checked]
> Use internal volume controls: [not checked]
> The options for mixer and pcm aren't available because Use Internal 
> volume controls was not checked. With this config audio from MythTV 

I have this checked and "default" as mixer.

> makes a nasty stuttering sound that might sound similar helicopter 
> blades or such. If I uncheck "enable AC3 to SPDIF" then I get normal 
> sound but it's not DD. It comes through as PCM 48kHz which I think is 
> just an analog format.

This sound to me as if ALSA:spdif does not switch your receiver to DD.

> I tested this using "-v audio" and attached an audio.debug file. Looking 
> at the log, why does MythTV detect the audio twice? Is this expected?

This I do not know.

> The help is much appreciated. I'm really at the end of my rope on this 
> one. What is so frustrating is seeing both xine and mplayer pass the AC3 
> correctly but having MythTV fail on the same exact sources.

This is all strange. Could you try with the .asoundrc from the Wiki, 
setting digital-hw in the default section and using ALSA:default as 
passthrough device? You already tried that, IIRC, but please check, if 
the device in the .asoundrc in the digital-hw section is the correct 
one. It is NOT the same, as in the analog section for me. For analog, I 
use device 0 and for digital, I use device 2. At one point, I has the 
wrong device for digital and then I also had stuttering sound and the 
receiver did not switch to DD.


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