[mythtv-users] ac3 passthrough results in accelerated video

Bill Williamson bill at bbqninja.com
Wed Jun 4 09:52:33 UTC 2008

On Fri, May 30, 2008 at 2:15 AM, Michael T. Dean <mtdean at thirdcontact.com>

> On 05/29/2008 11:30 AM, scram69 wrote:
> > Recently upgraded to 0.21 so I could finally get ac3 passthrough
> > working on my osx frontend.  When it works, it's great - however...
> >
> > Just about every other time I start to watch a recording, the
> > playback, both video and sound, is accelerated, as if I were watching
> > using 2x-3x timestretch or fast forward.  (I bring up the time stretch
> > menu, and it indicates 1.0x)  If I uncheck ac3 passthrough in the
> > Setup>General menu, the problem goes away.
> >
> > Unfortunately, the frontend log output is _exactly_ the same when I
> > see this accelerated playback as when I do not.
> >
> > There have been a number of posts about ac3 passthrough in 0.21, but
> > in searching, I haven't come across this issue.  Has anyone else
> > experienced this?
> Does it always happen on any given recording or are you saying that on a
> particular recording, you'll sometimes see it play incorrectly and
> sometimes see it play correctly?
> If it always happens on a given recording, it's just a matter of your
> broadcaster choosing to use 32kHz AC-3, which the vast majority of
> computer-based sound cards cannot handle (they play it as 48kHz, thus
> the "timestrech"--which is actually more of a "chipmunking" as there
> shouldn't be pitch correction).  Myth just keeps the video in sync with
> the audio as passthrough means that adjusting the audio stream is
> impossible.
> Oh, and if you enable timestretch--even to 1.05x or something--it will
> "make it work" by disabling passthrough.  TTBOMK, even if you then go
> back to 1.0x, it will still "work" (as Myth won't re-enable passthrough
> unless you exit playback and restart playback).
> Mike

-Ubuntu 8.04 + Mythbuntu 8.04  + weekly fixes repository
-X64 build
-"Great Outdoors" show on TenHD in canberra
-AC3/DTS passthrough set to ON or OFF (I'm using AC3 on an nvidia chipset

I get this bug.  ONLY this show as far as I can tell.  If I set timestretch
to 1.1 then back to 1.0 it plays fine, as advised.

Note though, this happens with AC3 and DTS passthrough OFF as well as on
(sound is always set to stereo ).

What can I do to help debug and help get this resolved?  I'm happy to upload
a sample video or logs or is the other info in this thread enough?
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