JACOSub Converter
FileFlip converts JACOSub files into 6 other formats entirely inside your browser, using FFmpeg compiled to WebAssembly. The JACOSub file is never uploaded to a server: it is read off your disk, converted on your own machine, and saved back by the browser. There is no file-size limit and no account to create.
How do I convert a JACOSub file?#
- Drop your JACOSub file onto the converter at the top of this page, or click it to pick one from your computer.
- Choose the format you want out of it. FileFlip downloads the engine that conversion needs, once, and your browser caches it.
- Conversion starts the moment you pick the format, and the result is yours to download. The work happens in this tab, so nothing is uploaded and nothing is queued.
Is it safe to convert JACOSub files online?#
Yes, because nothing is uploaded. FileFlip converts JACOSub files inside the browser tab you already have open, using WebAssembly builds of the same engines a desktop converter would install. Your file is read from your own disk into the page's memory, and no request carrying it is ever made.
You do not have to take that on trust. Open your browser's network panel, run a conversion, and you will see the engine come down and your file go nowhere. There is no upload, no server-side copy, no retention window, and no account tied to what you converted.
How FileFlip works lists every engine, its version and its download size, and shows how to check for yourself that nothing leaves your machine.
What happens to a JACOSub file#
FileFlip converts JACOSub into 6 formats with FFmpeg, all of it inside the browser tab. Each conversion below names the engine it uses, what it downloads, and exactly what that conversion keeps or drops.
| Category | Subtitles |
|---|---|
| File extension | .jss |
| FileFlip support | Read only, so it is a source rather than a target |
| Conversions available | 6 |
| Engines | FFmpeg |
| Downloaded to your browser | 30.8 MB on the first conversion, then cached |
What is a JACOSub file?#
A JSS file is a JACOsub script: a plain-text subtitle track where each line gives a start and end time, an optional directional alignment code, and text that can carry inline styling codes for italics, bold, underline and on-screen positioning.
Recognize JSS as a legacy JACOsub script if it turns up in an old fansub archive; there's little reason to author new subtitles in it today.
Convert JSS to ASS or SRT to work with the subtitles in current editors and players, since JACOsub-native tooling is largely gone.
The JACOSub file format reference covers the specification, the programs that open one, and what each conversion keeps or drops.
JACOSub questions people ask#
What is a JSS file?
A JSS file is a JACOsub script, a plain-text subtitle format from the JACOsub editor for the Amiga that times each line by seconds rather than frames and supports inline styling and positioning codes. It's an early example of the typesetting approach later formats like ASS expanded on.
Can I still edit JACOsub files today?
Modern subtitle editors like Aegisub can open and convert JACOsub scripts, even though the format's own original editor is long out of use. Converting to ASS or SRT is the practical path once a JSS file needs further editing.
Does JACOsub timing break with a different video frame rate?
No. JACOsub times its events in seconds and a configurable sub-second unit, not by frame number, so it stays in sync with the video's clock time regardless of frame rate.