Normally, when you rip the subtitles from a DVD, they are exported in an image-based format that is very close to the one used in the DVD itself. This format consists of two files: a SUB file that contains subtitle images, and an IDX file that contains essential information such as subtitle timing. To convert subtitles from this format to the text-based SRT format, use a program that's capable of performing optical character recognition on the subtitle images.
Ensure you have both the IDX and the SUB files for the subtitles you want to convert. Because the IDX file doesn't contain the actual subtitle images, you cannot convert an IDX file to SRT without the matching SUB file.
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Download, install and launch the SubRip program (see Resources).
Click "File" and select "Open VOB(s)."
Click "Open IFO," navigate to the location of the subtitles you want to convert and double-click on the IDX file to load it into the program.
Select the language stream you want to convert using the drop-down menu at the top of the window. Depending on the DVD they were extracted from, many IDX files may contain subtitles not only in English, but in other languages as well.
Ensure the "SubPictures to Text via OCR" option is selected and click "Start." The program opens a window labeled "Subtitles" at the bottom of the screen, to hold the converted subtitles and timing information, and start converting the IDX file, prompting you for input every time it finds a character it can't recognize.
Type the character in question and click "OK" whenever you are prompted. Although you may have to do this very frequently at the beginning of the process, the program learns what the characters look like and recognizes them when they reoccur without prompting you again.
Continue monitoring the program and typing characters until the process is complete.
Click "File" and select "Save As" in the Subtitles window. Select a location and name for your converted SRT file and click "Save."