[mythtv] [mythtv-commits] Ticket #5552: AC3 passthrough causes accelerated playback on OSX 0.21 frontend

Craig Treleaven ctreleaven at cogeco.ca
Sat Jul 19 18:35:13 UTC 2008

At 1:52 AM -0400 7/19/08, Michael T. Dean wrote:
>On 07/18/2008 10:32 PM, Craig Treleaven wrote:
>> At 12:37 AM -0400 7/18/08, Michael T. Dean wrote:
>>> On 07/17/2008 12:45 AM, MythTV wrote:
>>>> #5552: AC3 passthrough causes accelerated playback on OSX 0.21 frontend
>>>> When AC3 passthrough is selected on the OSX frontend, occasionally the
>>>> video an audio begin playing at an accelerated rate.  This usually happens
>>>> when the show is not bookmarked - i.e. playback starts from the beginning
>>>> of the file.  It never happens if AC3 passthrough is un-checked.
>>>> I've attached a log file.  The accelerated playback session begins at the
>>>> top of the file and ends at 21:29:24.  The normal playback session begins
>>>> immediately afterward at 21:29:25.  The most obvious difference between
>>>> the two is the appearance of a slew of " NVP: Video is x.xxxx frames
>>>> behind audio (too slow), dropping frame to catch up." messages once
>>>> playback begins in the first case.
>>> Are you sure the broadcaster isn't switching to 32kHz AC-3 at that location and your sound card is continuing to read data at 48kHz?  Most all computer-based sound cards work only with 48kHz AC-3.
>>> http://www.gossamer-threads.com/lists/mythtv/users/275748#275748
>> I don't think this is the problem since the exact same portion of the recording will sometimes play fine and other times play in accelerated mode.
>It would do exactly that if something disabled passthrough (or caused
>Myth to re-encode to 48kHz AC-3--which I think it can now do for

Absolutely no settings are changed between attempts to play the same file.  Often it will play in accelerated mode three, four times in a row and then finally play normally.  Obviously if a file plays fine the first time I generally don't exit out and start playing it again.  ;-)

What happens is, I will attempt to playback a recording.  If it starts playing accelerated, I'll quit back to the recordings screen, and start it playing again (either from the bookmark or from the beginning of the file).  A number of times, I've let it play back (accelerated) so that the bookmark is in the main program rather than in any commercials beforehand.  (This on the theory that the audio parameters might be different in the commercials as opposed to the main program.)  This seems to make no reliable difference as to whether the recording will play back correctly or not.  Sometimes the frontend becomes unresponsive while trying to exit playback from accelerated mode.  If I kill it and go back to trying to play the file, it makes no reliable difference in whether the file will playback OK or not.

The recordings come from both digital OTA and digital cable via an HDHomerun.  Experience the problem with several different ATSC broadcast stations (ABC, NBC, CBS, Fox in Buffalo; City, CBC in Toronto).  Seems to happen somewhat less with SD digital cable recordings.

VLC always seems to be able to pay the files OK if I resort to that option.

For example, just now, I tried a few recordings.  One, (Silent Movie) I had watched for about 20 minutes last week.  (I don't recall any trouble getting it to play that time.)  I tried to start watching from the bookmark and it played in accelerated mode.  It did the same thing on two more attempts.  Finally, on the fourth attempt, it played normally.  Played a minute or two and exited.  On restarting playback at the bookmark, it played accelerated.  No problems playing the same recording on my remote frontend (.21-fixes under OS X, analog audio) a couple of times.

I wonder if the problem is caused by some uninitialized memory object that sometimes contains garbage.  If there is something I should be logging, I'd be glad to give it a try.


(Sorry for the length.)

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