[mythtv-users] ac3 passthrough results in accelerated video

scram69 scram69 at gmail.com
Thu May 29 16:31:23 UTC 2008

On Thu, May 29, 2008 at 9:15 AM, Michael T. Dean
<mtdean at thirdcontact.com> wrote:
> 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.

Sorry, I should have been more clear in my first post.  On any
particular digital recording (does not happen to recordings from the
analog source), sometimes I will get accelerated playback, and other
times it will play just fine.  The broadcaster for the digital source
in this case is Comcast cable QAM-256.

But yes, "chipmunking" is a more accurate description of the audio as
there is no pitch correction.  And it does seem to explain why the
video would be affected by an audio setting.  Perhaps sometimes myth
mis-identifies the AC-3 frequency, and other times gets it right?
> 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).

Actually, if I enable timestretch during these accelerated playback
episodes I get a return to the pre-0.21 AC-3 passthrough behavior -
loud static popping.

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