Sub/Idx or VobSub format subtitles are usually extracted from DVDs. These subtitles always come in a pair: a sub file that contains picture of the subtitle text, and an idx file that contains information of when to show each picture. You need both the sub file and the idx file to use them, the subtitles are unusable if you only have one or the other.
Sub/idx files contain pictures of text.
These types of subtitles are commonly called subpictures.
This tool uses OCR to read the text in each picture, and builds up an srt file with the results.
OCRing pictures is hard work for the server, converting a file can take few minutes.
If the website is busy, your file will be queued.
Because it takes a long time to convert a VobSub file, they can only be uploaded one by one, and not inside an archive file.
If you have a lot of sub/idx files and you want to convert them all at once, you can take a look at the sub/idx batch tool.
If you have a .sub file, but no .idx file, this tool will not work. But there is a chance that your .sub file is a MicroDVD file because they share the same file extension. To find out if it is a MicroDVD file, you can try converting it using the convert to srt tool.
Sub/idx type subtitles use pictures of text to display subtitles on the screen.
This works great if you want your subtitles to work on every tv in the world, regardless of what font or text encoding the tv uses.
Using pictures to display the text also has downsides: editing the text is very hard, the files are relatively big, and the display quality can be low (especially on big screens).
Nowadays the VobSub format isn't used that much anymore. A lot of newer devices such as smartphones and some smart TVs don't support sub/idx subtitles at all. This converter tool can be used to convert VobSub subtitles to srt so you can use them on any device.
Sub/Idx files can contain multiple languages.
On the result page you can select the languages you want to extract.
Sometimes a single sub/idx file can contain the same language multiple times. Usually one of them are "normal" subtitles, and the others are subtitles for the deaf and hard of hearing. You can't see which one is which, so it is easiest to extract all of them and then choose the one you want to keep.