Subtitles question

[vanderbog]

Hi guys,
I have a question about srt files and couldn't find the answer:
- since the srt has a timeline for each line of text, why is the framerate important ?

I have a movie recorded with my PVR in trp format. I extract the CC from it in srt format. I play the video with BSplayer and load this subtitle and it works perfectly sync. If I download another subtitle and sync the first and the last line.....it doesn't work. It's in sync at the very beginning and at the vey end and in between it's all messed.
Yesterday, something magic happened I downloaded a srt file in 29.97fps and align the very first line to start at the right time and it works perfect all the way to th end. Why ? Is it because the framerate ?
How do you "read" the framerate of an srt ?
For the first movie I didn't have the subtitle in 29.97 and tried to sync it with SubtitleCreator. Then I converted the subtitle framerate to 29.97 and didn't work either.
So it's a bit of a fog here Can anyone clear it a bit ? Thanks a lot.

[manono]

Hi-

since the srt has a timeline for each line of text, why is the framerate important ?

It's not, not for the SRT file. It may be important for the video to which you try and attach it. If, for example, the SRT file is from an NTSC sourced movie (23.976/29.97fps) and the video to which you add it is PAL (25fps), then they won't match up without resynching one or the other. It's usually easier to synch the subtitle to the video. If you try and synch the video with the sub, you have to adjust the length of both the video and the audio. The video part is easy. The audio part isn't.

If I download another subtitle and sync the first and the last line.....it doesn't work.

An educated guess is that you were trying to synch it with a different version of the movie, one with different scenes included or left out, like one was the theatrical version and the other the director's cut, something like that.

Is it because the framerate ?

Sort of. The SRT file was from an NTSC version of the movie, and the movie to which you matched was also at NTSC framerate.

How do you "read" the framerate of an srt ?

You don't. If the SRT file doesn't say somewhere what format DVD it's from (PAL or NTSC), it can be real hard to figure which version of the movie it's from. The sites that have subs are supposed to tell you which format of the movie it's from.

For the first movie I didn't have the subtitle in 29.97 and tried to sync it with SubtitleCreator. Then I converted the subtitle framerate to 29.97 and didn't work either.

If it's from an NTSC movie, you probably want to synch it to 23.976fps, and not speed it up to 29.97fps. Movies are 24fps (changed to 23.976 for NTSC). But NTSC DVDs are required to output 29.97fps and they usually use 3:2 pulldown to achieve that. The "base" framerate is still 23.976fps. Both have the same length. If converting from PAL to NTSC you generally slow it by about 4%, and not speed it up to make it play much faster.

[vanderbog]

Thanks a lot Manono, for your detailed answer.
II guess this is the only explanation: the movies have some cuts sometimes, because some are working perfectly and it's very easy to syncronize them, but some are just a nightmare.
I was playing just now with Nim's Island. The CC I extracted from the recorded file is impecable, then I try to sync another one and it's just almost good, it starts ok and then graduatly it looses sync but only about 2sec in the end.

[manono]

Gradually loses synch? Starts OK but 2 seconds off at the end? Then that should be easily fixable by stretching the subs the right amount. Sounds like maybe that's a 29.97 .vs 30.00fps issue (or 24.975 .vs 25fps).

I was thinking later on. It's pretty easy to find if you have PAL subs for an NTSC movie or vice-versa. PAL 25fps is about 4% faster than an NTSC 23.976fps. In a hundred minute movie (NTSC at 23.976fps) the PAL version will last about 96 minutes. You can find the time of the last sub, and then in the AVI find the time of the last dialog. Comparing them should give you the information you need to fix the problem by stretching the subs. Also, comparing the time of the first line of dialog will tell you if maybe one has some extraneous stuff at the beginning (a logo, for example, that one version might have that the other doesn't), which will mean adding a delay in addition to stretching the subs.

[vanderbog]

Thanks again.
I was able to sync other few movies subtitles with the sync option from Subtitle Creator. It works very good.
I don't know what happened with the first two movies. Maybe my PVR had a glitch and missed a couple of seconds or something like that.
This thing with the fps in subtitles...I don't get it. I mean, the movie is let's say 2hrs , the subtitle in srt format is 2hrs with timeframe for each line, so as long as you start in the same point it should work if the movie doesn't have any cuts. The PAL vs. NTSC is just a matter of displaying the movie, but the movie doesn't get shorter in NTSC because the fps is bigger.
So, if the srt subtitle is nothing more than a text file with numbered lines and on-off timeframes for each line, why are we talking about fps ? This is the thing I really don't understand.

[manono]

The PAL vs. NTSC is just a matter of displaying the movie, but the movie doesn't get shorter in NTSC because the fps is bigger.

I don't quite get you, but an NTSC movie gets longer because the FPS is lower (23.976 .vs 25fps). Which is why, for synching purposes, it's important to know from which source the subs are, and from which source your video is.

So, if the srt subtitle is nothing more than a text file with numbered lines and on-off timeframes for each line, why are we talking about fps ?

I did say in my very first reply that FPS for an SRT file isn't important. Some sub formats (as I learned the other day from AlanHK) don't have timed dialog, but have it by frame count. Then FPS is very important as they use the 29.97fps framecount for NTSC, and the 25fps count for PAL. But it is useful to know the FPS of the video (DVD usually) from which those SRT subs were taken.