JACOSub File Format (.jss)
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.
JACOSub at a glance#
| Full name | JACOSub script |
|---|---|
| File extension | .jss |
| Category | Subtitles |
| Compression | Uncompressed |
| FileFlip support | FileFlip reads this format but does not write it |
What a .jss file can hold#
These are the things a JACOSub file either supports or does not, and they are what decides whether a conversion keeps everything or quietly drops something.
| Selectable text | Yes |
|---|---|
| Human readable | Yes |
Strengths and limitations of JACOSub#
What JACOSub does well
- Timing is measured in seconds plus a configurable sub-second unit rather than frames, so it doesn't drift when the video's frame rate changes
- Supports overlapping events, letting more than one line be on screen at once
- Carries inline styling and visual positioning codes that plain formats like SRT can't hold
Where JACOSub falls short
- Predates modern typesetting formats like ASS and has largely been replaced by them in fansubbing
- No modern muxer produces it, so it survives mostly as scripts made with the original JACOsub editor
- Sparsely documented outside its original specification, making third-party tooling rare
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.
How to open a .jss file#
A .jss file opens in the programs below, grouped by the platform you are on. If you would rather not install anything, FileFlip converts JACOSub into a format your machine already opens, and it does it in the browser tab rather than on a server.
| Platform | Programs that open it |
|---|---|
| Windows | Aegisub, Subtitle Edit |
| Web | FileFlip |
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.
Converting JACOSub files#
Convert JACOSub to another format
FileFlip does not write .jss files, so nothing on the site converts into this format. It works as a source only.
Every conversion runs inside your browser. The file is read from your disk, the work happens on your own machine, and nothing is uploaded. How FileFlip works names the engine behind each family and how to verify that for yourself.