[mythtv-users] ac3 passthrough problems

Michael T. Dean mtdean at thirdcontact.com
Thu Sep 21 21:00:24 UTC 2006

On 09/21/06 15:36, Scott wrote:

>  On Sep 21, 2006, at 2:17 AM, Thomas Börkel 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.
>  Not necessarily. It's perfectly possible to send analog format
>  signals out the spdif and for the receiver to decode them.

It is not the responsibility of the underlying driver to tell the 
receiver to switch modes when the application changes audio 
formats--it's the application's responsibility.  Some receivers are 
better at "auto-detecting" formats than others.  Some sound cards are 
better at creating specification-compliant data streams which enhance 
the receiver's ability to auto-detect.

Myth doesn't yet do well at informing the receiver of the audio stream 
type (because all previous attempts to add functionality to do so have 
broken more systems than they've fixed--and not just because of bad 


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