[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 
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


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