[mythtv] DVD playback issues, patch

Davin McCall davmac at davmac.org
Fri Nov 13 12:35:36 UTC 2009

Hi all,

Regarding the DVD playback issues I mentioned earlier. This is 
(possibly) related to the following tickets:

Although I originally found that revision 22120 was causing the issue, I 
no longer believe this is the case: that revision just altered the 
timing which presented other problems. Basically the code in the AVSync 
method in NuppelVideoPlayer.cpp (libs/mythtv/NuppelVideoPlayer.cpp) is 
highly suspect and the code in vsync.cpp (same directory) is also 
dubious. I posted some questions about this before but received no reply 
so I'm assuming that the errors I saw really were errors. In any case, 
altering the code as per the patch below fixes the problem for me.

Incidentally one of the problematic revisions seems to be 21665 by gigem:

This seems to max out the av sync adjustment to the refresh rate, which 
frankly isn't enough as sometimes the per-frame sync drift is higher 
than that. I've undone that change in the patch and also set the vsync 
delay so that it always tries to exactly re-sync audio and video (well, 
it uses avsync_avg which is a weighted average of the sync drift). I 
can't think of any reason why it wasn't like that in the first place but 
maybe someone can enlighten me. (Perhaps something to do with the fact 
that vsync was accumulating the delay rather than using it as a one-off, 
see my earlier post).

The other change in the patch takes all the smoothing logic out of 
vsync.cpp and leaves it to do what it surely was meant to do: 
synchronize to vertical refresh, nothing more, nothing less. In 
particular it no longer accrues a large negative trigger delay (which 
causes video to playback at a stupidly high speed for an extended period).

I don't know how things work here so if I should be sending this 
somewhere else please let me know.


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