Split a Subtitle File

One subtitle track, one video that arrived in two files. This comes up with long recordings split for upload, with a two part film distributed as two downloads, and with a lecture cut into sessions after it was captioned in one pass. Every cue after the cut has to move, and the two things that make it awkward are both invisible until someone watches the result.

You can split SRT, WebVTT, ASS and SBV here — the format is read from the content rather than from the extension, so a mislabelled file is cut correctly anyway. What you cannot do is split subtitles by cue number and expect it to line up with anything: a video is cut at a time, so a subtitle file has to be cut at the same time.

The second half has to be retimed, or it is an hour late. A subtitle file for part two of a video starts its own clock at zero, because the video does. Cut a track at 01:02:30 and hand the remainder over unchanged and every line of it sits an hour and two minutes past the end of a file that is only forty minutes long — so nothing appears at all, and nothing reports an error either. Subtracting the cut point from every timestamp is what makes the second file usable, and it is on by default.

The switch to turn it off is there for the other case: cutting a track up for review or for a QC pass, where the timings have to keep matching the original so a note about cue 412 still means cue 412.

Some cue is almost always on screen at the moment you cut. There is no answer that keeps it whole in both files. It could be duplicated into both, truncated at the boundary, or assigned to one — and of the three, assigning it is the only one that does not change what the file says. It goes with the half it starts in, on the grounds that a line belongs to the scene it began in, and the count of how many did that is shown rather than left to be discovered.

Numbering restarts. Both halves are renumbered from one, because SubRip requires a sequential index and a file starting at 413 is a file half the players in circulation will renumber for you anyway.

Press Load a sample. Five cues, and the cut is set to 00:00:10. The status line reads part one 3 and part two 2.

Cue 3 is the interesting one: it runs from 00:00:09,000 to 00:00:11,000, so it is on screen when the cut lands. The note under the controls says one cue is in that position, and cue 3 is in part one — where it starts.

Look at part two. Its first cue reads 00:00:04,000 --> 00:00:07,000, and in the original that cue was at 00:00:14,000. Ten seconds have been taken off every timestamp, which is exactly the ten seconds of video that part two no longer contains.

Now turn retime it to start from zero off. The same cue reads 00:00:14,000 again. Nothing else changes — the split point is the same and the same cues are in the same halves. That is the difference between a file for a video and a file for a reviewer.

Splitting is the reverse of merging, and the two use the same number: the offset you would merge back with is the time you cut at. If the halves then need to line up with a differently encoded video, shift each one after splitting rather than before, so the two shifts stay independent.