[mythtv] A/V Sync problem only on some PBS Hi-def shows

R. G. Newbury newbury at mandamus.org
Wed Apr 16 16:23:27 UTC 2008

DaveD wrote:
> I've posted this problem a couple of times to the users list, so far no 
> solutions.  I have bad (unwatchable) A/V sync on some of our 720p HD PBS 
> shows.  Some shows (Desert Speaks) always work, some shows (e2) never 
> work.  They are all recorded over cable (QAM), some times with a 
> PC-HD3000, some times with a Fusion 5 Gold.  Same problem in playback on 
> two different systems that have no trouble with any 1080i shows from 
> commercial stations.  One user on the list replied that he has identical 
> problems.  He also has Comcast in Seattle.  He noticed changing frame 
> rates when watching problem shows in Mplayer.  I got an answer from the 
> engineer at our PBS station who said all their shows use the same 
> compression algorithm.  I have had this problem for some time.  It seems 
> to have started in between Myth upgrades, while using a stable version 
> (0.20, I think) so I assume it's something that has changed in the way
> they are broadcast and not due to any changes in Myth, although I can't
> be certain of that.  No upgrade (0.21 trunk or 0.21 release) has 
> affected it.  All shows play fine in Mplayer and Xine.

> Any ideas?

I noticed this last night while watching an HD recording of "Emma" on 
Masterpiece Theatre. In the course of fiddling with some things earlier, 
I had unchecked "Use Video as Timebase" in Settings->Settings 

Without that set, the Audio was up to 2 seconds out of sync. With it 
checked the Audio was spot on the PBS HD programs. I did not notice it 
on recordings of "Dexter" or "Lost", both of which are broadcast as 
1080i while PBS is 720p...which may have something to do with it..


I did some checking later and interestingly, this defect was only 
noticeable on

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