SUP files are subtitle bitmap files, while SRT files are SubRip subtitle files. SUP files are most commonly found on DVDs and Blu-ray discs. SRT files are most commonly used for computer video files and DVD and Blu-ray backups. Computer media players such as VideoLan Player natively read SRT files, so a conversion from SUP is necessary. A freeware utility is available to convert from one file type to the other.
Download and install SupRip (see References). It is a free, open-source software.
Open SupRip. Press the "Load" button on the top left portion of the screen. Navigate to the SUP file and double-click on it to open it.
Click the "Image" tab. Check "Automatically continue with next subtitle."
Click on the "SRT" tab. Press "Save." An SRT version of the original file is saved into the original directory.