Sync a subtitle file against a reference video or subtitle file, entirely on your machine. Nothing is uploaded.
A browser tab cannot silently overwrite a file on your disk. On Firefox and Safari, "Overwrite" here downloads the corrected file under the original name, and the backup, if kept, downloads as .bak, both landing in your downloads folder. On Chrome, Edge and other Chromium browsers, each file opens a normal Save dialog instead, and choosing the original file there replaces it directly.
Reads .srt, .vtt, .ass or .ssa and writes it out as any of the other three. Timing is left as it is.
Reads the subtitle tracks out of a video file. Text tracks (SubRip, ASS, mov_text, WebVTT) come out as text. PGS and VobSub tracks are images, not text, and come out as a raw .sup file, same as the native engine.
Moves every line by a fixed amount, in milliseconds. No audio, no video, no engine, so it is instant. For a sync that came out close but not quite right.
Writes a copy in OpenDyslexic, larger and with wider letter spacing, matching the preset in the Jellyfin plugin's Readable Subtitles. The font is embedded in the file itself, so it does not depend on the font being installed wherever the file is played. Scripts other than Latin keep the larger text and heavier outline, but get a font that has their letters in it instead, since OpenDyslexic does not cover Arabic, Hebrew, Thai or CJK.
Subtitles: .srt, .ass, .ssa, .vtt, .sub, .sup, .idx, .smi, .ttml, .dfxp, .sbv. Reference video or audio: anything FFmpeg can decode, including .mkv, .mp4, .avi, .mov, .webm, .mp3, .wav and .m4a. The Convert and Dyslexia-friendly tools under Advanced are simpler tools written separately from the sync engine and only handle .srt, .ass, .ssa and .vtt.
| Setting | What happens |
|---|---|
| Write a new file | The original is left alone. Movie.en.srt gets Movie.en.synced.srt beside it. |
| Write a new file, keep a backup | Same, and a copy of the original is kept as .bak as well. |
| Overwrite, keep a backup | The subtitle is replaced and the old one is kept as .bak. Default. |
| Overwrite, no backup | The subtitle is replaced and nothing is kept. |
| Test | lapse | alass | ffsubsync |
|---|---|---|---|
| Normal (shift or drift) | 36 / 39 | 31 / 39 | 33 / 39 |
| Split, turned on | 32 / 39 | 27 / 39 | 17 / 39 |
| Median sync time, cached | 0.66s | 11.3s | 17.1s |
39 feature films picked to be hard to sync: long runtimes, quiet stretches, heavy score under the dialogue, films with more than one cut. Numbers are from an older lapse release; the project's own notes say accuracy and speed have both improved since. Full methodology and every per-film result is on GitHub.
These numbers are for the same engine this page compiles to WebAssembly and runs, not a separate implementation.
Checking the compiled engine's own version...
lapse can also be used through AutoSubSync, a third-party desktop application that supports it alongside other sync engines.
A product of Schwponaco Organisation.