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

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