[mythtv] Accuracy of external subtitles

Jim Stichnoth stichnot at gmail.com
Tue Mar 9 07:24:44 UTC 2010


I have noticed that when using external subtitles in MythVideo
(through a .srt file), synchronization starts off fine, but after
jumping forward or backward, synchronization is usually off, and
subtitles display too early, anywhere from about a half second to 5 or
6 seconds early.  The offset stays consistent as long as there is no
jumping -- there is no drift and it looks like subtitles are turned on
and off correctly except for the shift.  This also happens when
starting playback from the bookmark.

I checked TextSubtitles::GetSubtitles() and it appears that the
subtitle timecodes are correct and that the timecode argument passed
in is being matched to the correct subtitle.  It appears that the
timecode argument does not necessarily represent the actual frame
being displayed.  I tried running "mythcommflag --rebuild --file ..."
in case it's a seektable issue, but that didn't help.  There are
comments in NuppelVideoPlayer::DisplayTextSubtitles() warning about
subtitles being out of sync, but I don't see how those warnings would
explain timecodes changing as a result of jumping around within the
same playback instance.

I've never seen anything like this with EIA-608 closed captions, and I
use them all the time.  It seems that EIA-608 captions use
GetLastShownFrame()->timecode whereas text subtitles use
GetLastShownFrame()->frameNumber.  Are these fields allowed to be
inconsistent?

Any thoughts/comments on this?

Jim


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