Auto-sync subtitles to a reference
Have a subtitle that's out of sync but another one that's correct? Drop both in and SubAlign automatically retimes the broken file to match the good one, using only the cue timings, instantly and in your browser.
100% private. Your files are processed locally in your browser and never uploaded to a server.
1. Correctly-timed reference
Drop the in-sync file
the subtitle whose timing is already right
2. File to fix
Drop the out-of-sync file
the subtitle you want corrected
How should it align?
How automatic subtitle sync works
Some subtitle problems can be fixed without ever touching the video. If you already have one subtitle file that plays in perfect sync, say the original English track, you can use it as a reference to fix another file that is off, such as a translation downloaded from a different release. SubAlign lines up when each line appears in the two files and applies the timing correction that makes them match. It never reads the words, so the two files can be in completely different languages.
Constant shift vs. fixing drift
Start with constant shift. It finds the single offset that best aligns the broken file to the reference, which is the right fix when subtitles are early or late by the same amount from start to finish. It works even when the two files have a different number of lines.
Switch to fix drift (linear) when the subtitles start roughly right but slide further out as the video plays. That is usually a frame-rate speed difference (for example a 25 fps file against a 23.976 fps video). Linear mode anchors on the first and last lines of each file and rescales everything in between, correcting both the starting offset and the gradual drift at once.
When to use this vs. the other sync tools
Use this tool when you have a correctly-timed file to copy from. If you don't, fix the timing by hand with the manual sync tool using a timestamp you read from the audio, or rescale a frame-rate mismatch with the frame rate converter. To fine-tune individual lines after aligning, open the result in the subtitle editor.
Is anything uploaded?
No. Both files are read and aligned entirely in your browser with JavaScript. Nothing is sent to a server, there is no sign-up, and there is no limit on how many subtitles you can sync.
Frequently asked questions
How do I automatically sync subtitles?
Drop in a subtitle file that is already correctly timed (the reference) and the file you want to fix. SubAlign compares the cue timings and works out the shift (and optionally the speed change) that lines the second file up with the first, then lets you download the corrected version. It all happens in your browser.
What do I need to auto-sync a subtitle?
You need a second subtitle file for the same video whose timing is already correct, for example a working English track when your translated subtitle is off. The tool aligns your file to that reference. If you do not have a correctly-timed file, use the manual sync tool to shift the timing yourself instead.
Can it sync subtitles in a different language to an English one?
Yes. The alignment uses only when each line appears, not the words, so a translated subtitle can be synced to a correctly-timed track in another language. Because the two files share the same scenes and dialogue rhythm, matching their timings lines the translation up with the video.
What is the difference between offset and linear (drift) mode?
Offset mode finds a single constant shift, which fixes subtitles that are early or late by the same amount throughout. Linear mode also corrects gradual drift by matching the first and last lines of both files, which fixes frame-rate speed differences where the gap grows over time.
Is automatic subtitle sync free and private?
Yes. The matching runs entirely in your browser using JavaScript, so neither file is uploaded to a server. There is no sign-up and no limit on how many subtitles you can sync.
Does this sync subtitles to the video audio?
No. This tool syncs one subtitle file to another correctly-timed subtitle file. It does not analyse the video or audio. That keeps it instant and fully private. For a video with no reference subtitle, use the manual sync tool and a timestamp you read from the audio.