Work out what kind of problem it is
People say subtitles are “out of sync” when the text and the speech do not match in time. Before fixing anything, find out how they are out of step. Play the beginning, the middle and the end, and note how far off the text is each time. The pattern tells you most of what you need.
- A constant offset: The text is the same amount early or late everywhere.
- Sync drift: The error is small or zero at the start and grows steadily.
- A jump: The text is fine until a certain point, then is suddenly off by a fixed amount.
- Random or irregular errors: Some cues are fine, others are not, with no clear pattern.
Each pattern has typical causes and typical remedies.
Constant offset
A constant offset is the easiest to fix. It often arises when the video has a few extra seconds at the start, such as a logo, a different intro or a slate, or when the subtitle file was timed against a version without them. The remedy is to shift all cues earlier or later by the same amount. Most subtitle editors and many players have a shift or delay control for exactly this purpose.
To find the amount, compare the first line of speech with its cue. If the cue appears two seconds before the voice, shift the whole file later by two seconds. Check the last line afterwards. If it is also right, the offset was constant. If it is not, you are dealing with drift.
Drift
Drift means the error changes as the video plays. The most common cause is a mismatch in speed or frame rate between the video the subtitles were made for and the video you are playing. A film timed at one rate and played at another is the standard example, and a hypothetical illustration is a 100-minute film at 24 frames per second played at 25, which would run 96 minutes. The cues would then be steadily later than the speech.
Another cause is a version difference, in which the second video has been trimmed in many small places, such as ad breaks removed throughout. A file made for the longer version will lag more and more behind, with sudden steps wherever material was removed.
The remedy for a speed mismatch is to scale the timecodes. A common approach, offered by many tools, is to give two reference points: the time of one early cue and the correct time of one late cue. The tool then works out a shift and a stretch that fit both. It is wise to check a few cues in the middle afterwards, since a result that matches at two points can still miss elsewhere if the underlying cause is not a simple speed difference.
Jumps
If the text is accurate until a point and then suddenly off, something in the video was added or cut at that place. Examples include an inserted scene, a removed commercial break, or a different edit of a chapter. The remedy is to treat the file in sections. Shift the part after the jump by the amount of the change. If there are several jumps, repeat the process for each. This can be slow, and where the difference between versions is large it may be quicker to retime the file against the new version. Teams that reuse subtitles when a video is re-edited often keep a record of what changed so they can adjust the right sections.
Irregular errors
When errors are scattered, the cause may be a file that was timed by a method that produced inconsistent results, a conversion that rounded times poorly, or a file combining parts from different sources. Occasionally the problem lies with the player: a device that drops frames or buffers heavily can make text seem uneven. Test the same file and video in another player before changing the file. If the problem is only in one player, the file may be fine.
Text encoding or format problems can also make a player skip or misplace cues. A file that is damaged or in the wrong format can behave strangely, so check that it opens and displays properly in a plain text editor.
A cautious fixing routine
A sensible order of work is:
- Keep a copy of the original file so you can start again.
- Check the first, middle and last spoken lines against their cues.
- Decide whether the error is constant, growing, stepped or irregular.
- Apply the matching fix: a shift, a scale, or a section-by-section adjustment.
- Re-check the same three points, plus a few more.
- Save the corrected file with a clear name or version number, which is where version control helps.
When fixing is not the right answer
If a file needs many repairs, consider whether it was made for the right video. Looking for a version of the subtitles that matches your video is often easier than rescuing a mismatched one. For professional work, the better route is to return to the master and produce a fresh, correctly timed file. Retiming is a patch, and the quality of the result depends on how well the patch fits.
Also remember that even a perfectly timed file may not suit your viewers if the cues are too short or too fast. That is a different problem from drift and needs a different solution, usually editing the cues themselves.