Many database systems or specific applications create .idx files to speed up searches.
Converting an .idx file to another format depends on the application that created it and the desired output format. Here are some possible conversion scenarios: what is .idx extension
It is almost always accompanied by a .SUB file . While the .SUB file contains the actual graphical images of the subtitle text, the .IDX file tells the media player exactly when to show each image. Many database systems or specific applications create