23.976 to 29.97 weird FPS problem

[michcio]

Hey!

I try to author a DVD with several subtitles tracks. Now in the beginning my .avi is perfectly synced with all my subtitles. After some conversion (avi -> mpeg, wav->ac3 and subtitles .srt -> .sub) and after I author the DVD and make my files then check the made files my subtitles and video gets a bit out of sync. The subtitles are a bit to late (though they were perfect while checkin avi and srt). So I thought the conversion avi->mpeg change the FPS from 23.976 ti 29.97 and maybe that's why my subtitles are a bit out of sync. So before I convert my srt to sub I change the srt's FPS from 23.976 to 29.97 with subtitleworkshop. (When I then checked the avi-srt sync my subtitles were really out of sync) After a new attempt of my dvd-authoring with the new subtitles my subs showed totaly out of sync. :S

Any ideas what I possibly could be doing wrong?

michcio

[jagabo]

Are the audio and video still in sync? Just the subs are out of sync? By how much at the end of the movie? How long is the movie? Is the loss of sync gradual throughout the movie or does it happen all of a sudden? If the sync loss is gradual it's easy to adjust them to match the video. For example, if the movie is 7200 seconds long (2 hours) and subs near the end early by 50 seconds, you just need to "stretch" the subs a factor 7200/7150. SubtitleWorkshop has that ability.

If the subs are too far out of sync at the end of the movie you can use the timing over a shorter period to make the adjustment. For example if they're 20 seconds early at 300 seconds into the video you need to stretch them by a factor of 300/280. You might then have to go back and use the full length of the movie to fine tune the timing.

[michcio]

Thanks for your fast answer!

Yes the video and autio are still in sync and just the subs are out of sync. At 5-6 seconds at the end of the movie and the movie is 40-50 minutes long. The loss of sync is gradual through the movie. It starts up just fine and gets wors for every minute. But I don't understand how they can't be synced if I make the subtitles FPS conversion from 23.975 -> 29.97 it's how the movie FPS is changed so why doesn't same change make my subtitle good?

[jagabo]

When you converted from 23.976 fps to 29.97 fps you duplicated frames or fields (depending on what software you used) so the running time didn't change (that's why the audio is still in sync). So the subtitles don't need to change either. It's not clear why they are now a little out of sync at the end though. Just adjust the sub timing as I indicated earlier.

[manono]

It's most likely a drop-frame problem. The movie's at 23.976/29.97fps and the subs were created at 24/30fps. For a 50 minute video the subs will play about 3 seconds early by the end. That can be easily adjusted. Or perhaps there's a setting in Subtitle Workshop that can prevent the problem to begin with. Or you can use a different subtitling program. I work a lot with subs but don't use Subtitle Workshop. And never have that problem.

The problem may be occuring during the conversion from SRT to SUB (IDX/SUB?). You might check and compare the timings before and after, especially those near the end of the movie.

[michcio]

The thing is I have tried to manually adjust the subs to play 1-2 seconds to early (because that was what I saw in the middle of the movie .. they where playing 1-2 seconds to late) but then they got perfectly synced in the middle but out of sync in the begining and the end.

Anyhow I think I now solved my problem. I didn't change the FPS of my subtitles at all , so it is 23.976 all the time. The only thing I changed was under my .srt -> .sup conversion in Subtitle Creator I now choose PAL instead of NTSC (which is kind of weird because while converting my .avi -> .m2v I choose "NTSC_film" as the new FPS). Then my subtitlse are still perfectly synced with my video.

Really weird :P any ideas why setting the subitltes to PAL solved my problem here? :P

[manono]

The thing is I have tried to manually adjust the subs to play 1-2 seconds to early (because that was what I saw in the middle of the movie .. they where playing 1-2 seconds to late) but then they got perfectly synced in the middle but out of sync in the begining and the end.

But that's not how you fix something like that since your asynch is progressive - it gets worse as the movie goes along. What you needed was the subtitle equivalent of stretching audio. You needed to stretch the subs by a factor of 1001/1000 or 1.001 (same as 24/23.976).

any ideas why setting the subitltes to PAL solved my problem here? :P

No, and what you say you did flies in the face of the problem as you described it. While my diagnosis fits the problem as described perfectly, converting the subs to PAL would increase the problem quite a bit. Where a 24fps .vs 23.976fps difference of about 3 seconds is probably correct, converting the subs to PAL should make the difference not 3 seconds by the end of a 50 minute video, but more than 2 minutes. Either you described the problem or the solution incorrectly.

[michcio]

Ah then I think I misunderstand the streching? Won't streching just move each line of the subtitles a bit? Or will it just convert the subs from one FPS to another like 1001/1000 will make it from 24 to 23.976 ? .. Then why should I convert from 24 to 23.976 if my subs were good synced with 23.976 and I changed my video to 29.97 wouldn't a 23.976/29.97 strech be good in this case ? (that's what I've been trying to do in both subtitlecreator and subtitleworkshop but it just made the subtitles worse)

as to the problem and solving .. maybe I did describe something weird so let me try again :P:

I got my subtitles good synced while my movie is 23.976 and then after I convert my .avi -> .m2v and the framrate changes to 29.97 my subtitles gets a bit out of sync (in the beginning they are a bit to early, in the middle they are kind of ok and then at the end they are to early again).
So my first approch to solve the problem was to convert the subs from 23.976 to 29.97 (by scretch I think ) but this only made my problem worse (my subs where now totaly out of sync)
Only way of solving my problem seems now to be if I open my .srt file (normal in FPS: 23.976) in SubtitleCreator and instead of Video Format choose PAL and not NTSC.

EDIT: Also maybe I should add that I use GUI for dvdauthoring/muxman where I choose muxman as my authoring engine. I've tired to author with dvdauthor and choose the 23.976 FPS - subtitle and it worked just fine after the authoring.

[manono]

Ah then I think I misunderstand the streching? Won't streching just move each line of the subtitles a bit?

No, you're describing a delay - moving them all forward or back by the same amount of time. Which is what you tried and you discovered what that did. Stretching is like adjusting the framerate. But changing the framerate to 29.97fps, as you also discovered, makes it much worse. That's because you stretched them by a factor of 29.97/23.976=1.25. Anyway, an NTSC DVD with pulldown to output 29.97fps doesn't change the length at all (as jagabo described earlier). It plays more frames at a higher framerate with the result that the video lasts exactly the same amount of time at both framerates.

But whatever the problem, I'm glad you got it sorted out.

[michcio]

Hehe ok then I get it .. but I didn't scretch them by 29.97/23.976 but the other way (Subtitle creator showed the factor of 0.8 and not 1.25 and in subtitleworkshop I went from 23.976 to 29.97 as well).

Ah so 29.97 doesn't change the framerate :O. Didn't know that...so in theory my 23.976 subs should work just fine with my 29.97 video?

Yeah well Im really glad I solved my problem as well. It just seems really weird that setting it till PAL solved it :S

[manono]

t...so in theory my 23.976 subs should work just fine with my 29.97 video?

Yes.