FFsubsync is a command-line tool that automatically synchronizes subtitles with video by detecting and correcting timing misalignment.
The tool solves the problem of out-of-sync subtitles by using voice activity detection and speech recognition to align subtitle timing with audio. It works language-agnostically by analyzing the audio track to identify speech patterns, then matching those patterns against the subtitle text to determine the correct starting point. The approach uses fast Fourier transforms and string alignment algorithms to find the optimal synchronization offset.
FFsubsync suits anyone dealing with misaligned subtitles across any language pair. It works with any subtitle format that ffmpeg can read and accepts video, audio, or correctly-synced reference subtitles as input. The tool offers both a command-line interface for scripted or bulk operations and a browser-based version that requires no installation and processes files entirely client-side. For users with multiple subtitle files in the same directory, the tool can auto-detect and sync them all at once. Remote references are supported via HTTP, RTMP, RTSP, and FTP protocols, though local files are more reliable for large sources.
Development activity shows consistent maintenance with regular updates addressing edge cases and user-reported issues. The project maintains backward compatibility across Python versions and actively incorporates user feedback into refinements of the synchronization algorithm. Documentation is thorough, covering both common workflows and advanced use cases like using reference subtitles instead of audio. The maintainer has made the tool accessible through multiple interfaces—command-line, browser, and programmatic—suggesting responsiveness to different user needs.
