[mythtv-users] AC3 passthrough setup problems

Jonathan Rogers jonner at teegra.net
Fri Nov 9 04:54:52 UTC 2007

On Nov 8, 2007 8:37 PM, Bob <mythtv at cox.net> wrote:

> James Fidell wrote:
> > I have everything working fine with normal stereo sound, so I thought
> > I'd finally get around to trying to get AC3 passthrough working on my
> > main front-end.
> >
> > I'm following the guidelines in the wiki at
> >
> >   http://mythtv.org/wiki/index.php/DigitalSoundHowTo
> >
> >
> http://mythtv.org/wiki/index.php/Configuring_Digital_Sound_with_AC3_and_SPDIF
> >
> > but when testing with mplayer (using an ISO rip of a DVD) mplayer
> > spews a52 CRC errors, plays choppy sound and can't display the video
> > properly either.
> >
> > I've tried changing the mythtv config as suggested and playing the same
> > iso back with the internal player but get no sound at all.  In "normal"
> > stereo mode, playback is fine on both video and audio.
> >
> > I think first I need to confirm that I can play back properly through
> > the SPDIF connector outside mythtv.  Is there another easy way to do
> > that?
> Try speaker-test:
> http://alsa.opensrc.org/index.php/Speaker-test
> There are a lot of options, more than the web page indicates.  I was
> never able to get speaker test to make pink noise through 6 speakers,
> but I do have surround sound working.

Unfortunately, speaker-test doesn't have anything to do with S/PDIF or
IEC958 passthrough. It can produce test sounds for two channels through a
digital output just like for analog stereo output, but James Fidell said he
already had regular stereo (I assume PCM) working.

I found that configuring passthrough was much simpler than the Wiki
describes. I believe all I had to do was to add "-afm hwac3" for mplayer and
select "ALSA:iec958:{ AES0 0x02 }" in the "Passthrough output device:"
listbox in the MythTV configuration. I have no idea what "{ AES0 0x02 }"
means, but it seems to be correct for AC3 passthrough.

Jonathan Rogers
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