I think I know where the problem lies.
Apparently the idx & sub files go hand in hand - they comprise a bitmat image of the subtitle which is superimposed onto the video:
While the srt file is a stand-alone text file for that same avi's subtitles:VobSub's IDX and SUB subtitle format has become popular due in large part to its flexibility and nearly universal compatibility with media players. But there are still times when you need something different, especially when dealing with older file formats like AVI where subtitles need to be in text format rather than images which VobSub rips from a DVD. Fortunately VobSub also includes a tool capable of converting subtitles to text-based formats like SRT or SSA
So, for example, if I wanted to PERMANENTLY add the subtitles, apparently I have these options:The idx/sub format. This format uses a pair of files, one with extension idx and another with extension sub. These files go together and replace the srt in your player. Using idx/sub, because are external files (that is, not incorporated), you can subtitle any kind of video file. avi/mkv/mp4, you name it, if your player supports the combination.
- To incorporate XSUB subtitles in the avi from srt files.
- To incorporate XSUB subtitles in the avi converting idx/sub files.
- To incorporate XSUB subtitles in the avi coming from a combination of srt and idx/sub files.
- To generate idx/sub files from srt and use them as external subtitle files (in place of the original srt) in supporting players to subtitle any kind of video (avi/mkv/mp4 etc).
I'll look into these options in detail but won't be reporting back here because that task is off topic for this question.