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ffsubsync vs alass: catalog facts
ffsubsync	alass
Free Free and open source.	Free Free and open source under the GPL-3.0-or-later license.
Windows, macOS, Linux, Command line	Windows, Linux, Command line
Automatic alignment without manual offsets Works from the command line	Corrects subtitle offsets, cuts, and frame-rate differences Can align subtitles in different languages
Requires Python and ffmpeg Cannot fix subtitles whose text does not match the audio	Command-line interface Linux use requires FFmpeg and ffprobe
Downloadable app	Downloadable app
Open source	Open source
License: MIT	License: GPL-3.0-or-later
No account needed	Not stated if an account is needed
Works offline	Not stated if it works offline
Official website Full listing and sources Checked October 1, 2026	Official website Full listing and sources Checked October 8, 2026

Catalog facts only. Anything “not stated” is unconfirmed. Check full listings for details.