ffsubsync

ffsubsync is a tool that fixes out of sync subtitles by aligning them to a reference, either another subtitle file, a video file, or an audio track. You give it the broken subtitles and the reference, and it calculates the time shift needed to make them match. The algorithm uses speech detection and Fast Fourier Transform to find the optimal offset, which means it works regardless of language. You can sync English subtitles to a French audio track or align fan subs to an official video without caring about what language anyone is speaking.

The tool started as a command line Python project and won an honorable mention at HackIllinois 2019. Since then it has grown to over seven thousand stars on GitHub and become the go to open source solution for subtitle alignment. The browser version, hosted at smacke.net/ffsubsync, compiles the Python code to WebAssembly via Pyodide and runs entirely on your machine. Nothing is uploaded. You pick your files, wait for the processing, and download the fixed subtitles. For people who do not want to install Python or run terminal commands, this is the easiest way in.

The supported formats include SRT, ASS, and SSA, which covers the vast majority of subtitle files you will encounter. Advanced options let you correct framerate mismatches, handle mid file timing breaks, and use golden section search for tricky alignments. If you are syncing a rip where the video was sped up or the subtitles came from a different cut, these options help the algorithm find the right fit.

Audio decoding in the browser uses ffmpeg.wasm, which means you can sync against video files without extracting the audio track first. For command line users, the Python package supports remote files over HTTP, RTMP, RTSP, and FTP, which is useful for batch processing or automating workflows. Legacy character encodings are handled automatically, so you do not need to convert files to UTF 8 before processing.

The use case is straightforward. You downloaded a movie and the subtitles do not line up. Maybe they are three seconds off throughout, or maybe they drift over time because of a framerate mismatch. Either way, you drop the files into ffsubsync and get a synced version back. Media archivists, torrent communities, and anyone who deals with mismatched subtitles regularly will find this useful.

The project is open source under the MIT license, which means you can use it for personal or commercial purposes without restriction. The browser version is free to use with no signup. If you want to run it locally or integrate it into your own tooling, the Python package is available via pip. The creator, smacke, maintains the project on GitHub.