[mythtv-users] ac3 passthrough results in accelerated video

[Craig Treleaven]

At 9:31 AM -0700 5/29/08, scram69 wrote:
>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.

Me too.  I'm running .21-fixes (from a couple of months ago) on a Core2Duo mini.  The problem recordings will play in accelerated mode one, two or three times in a row and then play fine.  Lots of recordings play fine.  I think I only see this problem with ABC, NBC and CBS; not with the Canadian networks.  BTW, all are recorded OTA via an HDHomerun.

Also, the frontend hangs on exiting playback, sometimes.  I keep meaning to set up logging but haven't done it yet. 

Neither of these problems happened to any extent under .20-fixes.  As you might imagine, it is having a detrimental effect on the SAF (spousal approval factor).  Thanks to scram69 for pointing out the issue and the correlation with digital audio--I had no idea that was related!

On the Mac, what is a good tool identify the types of audio and video data in a file?  VLC doesn't give any details.

Craig