[mythtv-users] ac3 passthrough results in accelerated video

Michael T. Dean mtdean at thirdcontact.com
Thu May 29 16:15:44 UTC 2008

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 

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).


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