[mythtv-commits] Ticket #12396: Mythvideo subtitle timing problem

MythTV noreply at mythtv.org
Mon Apr 20 02:56:46 UTC 2015


#12396: Mythvideo subtitle timing problem
-------------------------------------+---------------------------
 Reporter:  ismota@…                 |          Owner:  jyavenard
     Type:  Bug Report - General     |         Status:  new
 Priority:  minor                    |      Milestone:  0.27.5
Component:  MythTV - Video Playback  |        Version:  0.27.4
 Severity:  medium                   |     Resolution:
 Keywords:  subtitle timing          |  Ticket locked:  0
-------------------------------------+---------------------------

Comment (by stichnot):

 Replying to [comment:4 jyavenard]:
 > Replying to [comment:2 stichnot]:
 > > Lack of complaints likely due to:
 > > 1. MythTV doesn't record .avi files (i.e. you have to go to extra work
 to import them), so most people don't play them.
 > > 2. Most people don't care about captions/subtitles.
 > >
 >
 > this has happens to me many times... subtitles are good at first and
 then they start diverging to the point they are unusable. I've never
 managed to reproduce the problem correctly, and often I just blamed the
 subtitles and moved on.

 That sounds like subtitle drift, which is separate from the OP's issue, I
 think.  Drift can come from a mismatch between the video and the subtitle
 file, or even from inconsistency between the container's reported
 framerate and actual content (#11415 comes to mind).

 >
 > > IIRC, what's happening is that without a seektable, MythTV must ask
 the decoder (ffmpeg) to seek to the desired frame, but ffmpeg only seeks
 to the nearest keyframe, while MythTV believes it is at the exact frame.
 If keyframes in the .avi file are far apart, subtitles are likely to be
 out of sync.  But if a seektable is present, MythTV seeks to a keyframe,
 then frame-by-frame advances to the desired frame.
 >
 > That seems like a bug to me... Yes ffmpeg will seek to the next
 keyframe, but you still get the right time and adjust the subtitle for
 that time.
 > Also, why not do like with the seek table present a move frame by frame
 then?

 I thought I had debugged in the past the issue of seeking in a particular
 container type and seeing this kind of imprecision from ffmpeg, but I
 could be misremembering.  In any case, the OP reporting that it snaps to
 +/-3sec off, plus no seektable difference, suggests that it's something
 else.

--
Ticket URL: <https://code.mythtv.org/trac/ticket/12396#comment:6>
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