[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
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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
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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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