[mythtv] Time-stretch for DVB-T captions?

Stephen Backway stev391 at exemail.com.au
Sun Sep 27 00:19:14 UTC 2009

On Sat, 2009-09-26 at 13:35 +0200, Håkon Alstadheim wrote:
> Hi, I just added this : "Allow time-stretch of subtitles, at least 
> proportional to playback timestretch, preferrably independent. (tested 
> using dvb-t in Norway, may be a bug, but subtitles stay on-screen for 
> short time even when playback is slowed down) Submitter: hakon, Sat, 26 
> Sep 2009 11:17:22 +0000" to the wiki wish-list.
> There may be a bug in when/how long subtitles are displayed for dvb-t in 
> Norway, but here is a feature request independent of that. Subtitles 
> (aka captions) are put up on screen just as the last word is spoken. 
> This is probably "correct" in some sense of the word, but it feels a bit 
> frustrating having to wait until a whole sentence is spoken before 
> knowing what is being said. Anyway, whether or not this is "correct" for 
> some value of "corect", there seems to be a fixed, quite short time 
> before it disappears again. I have trouble following sometimes, not to 
> mention my 4th grader. Trying to reduce the speed (hit "A" and left 
> arrow), slows down playback, but the captions disappear in the same, 
> frustratingly short, time :-/.
> I'd try to hack a patch but the code is way to dense for me to grasp 
> (yet). Any feed-back on the soundness of my idea and how hard it would 
> be to implement is appreciated. Thanks, Håkon.

The DVB subtitle problem is a known FFmpeg issue.  It also affects
BluRay subtitles (however a quick work-around is in FFmpeg for BluRay

The problem is caused by an incorrect start time (to display caption)
then a almost correct end time.  To overcome this look in the dvb
subtitles code (in libavcodec) and replace the end time with a large
timeout, instead of the correct end time.  However this is a nasty hack
and should really be fixed properly upstream in FFmpeg.

I think it has something to do with the FFmpeg mpegts demuxer.


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