Convert subtitles to a different frame rate
Pick the frame rate the subtitles were made for and the one your video plays at. Every line is rescaled exactly, so the drift is gone from the first line to the last.
Why subtitles drift by 4%
Films are shot at 24 frames a second; Blu-ray and web copies usually play at 23.976. European television and PAL DVDs play the same film at 25 — about 4% faster, so a two-hour film is almost five minutes shorter. Subtitles timed to that version run ahead of a Blu-ray or web copy, and the gap grows by about four seconds every hundred.
Converting rescales every time by the exact ratio — 23.976 is really 24000/1001, and the rounding matters over two hours. The usual case is 25 → 23.976; the menu lists the others, with the release each one comes from.
Not sure which rates?
If you do not know where the subtitles came from, sync by two lines instead: it works out the correction from the video itself and tells you when it is exactly a frame-rate change. If the lines are wrong by the same amount everywhere, it is a delay, not the frame rate.
Files timed in frames
MicroDVD .sub files store frame numbers, not times, and many do not say which frame rate they mean. Such a file is read at 23.976 and the page says so; if the text is wrong from the very first line, re-read it at another rate from the same form — that is a different fix from converting.
Questions
- My subtitles end five minutes before the film.
- That is a 25 fps file on a 23.976 video: convert 25 → 23.976. If they end five minutes after it, go the other way.
- Does the frame rate change the file format?
- No. An SRT stays an SRT; only the times inside change. “Save the fixed file” keeps the format it came in.