Since I watch a lot of Japanese anime, which comes in MKV files with Japanese soundtrack and English subtitles, this is something I'd really like to do.

Normal MKV->AVI converters simply lose the subtitles - no good for me. Some MKVs can be transferred without conversion but again lose subtitles.

I've heard of some MKV->AVI converters which render the subtitles into the video, though not found any that are free. These are good, but it would be best to have subtitles as a true subtitle stream.

Apparently there are numerous little utilities that can extract the subtitle stream, then another converts the mkv to a mp4, then the extracted subtitles can be changed to another format, then added in to the MP4 as a subtitle stream which the TiVo HNP will correctly transfer over... but I've not found anything that can do this in one go, not anything to do the dmuxing or adding MP4 subtitles stream without a lot of cost.

I wonder if anyone has anythingto do this? It would be a great utility if you could simply drop an MKV into a tool and it output an mp4 with subtitle stream, but I suspect I'd have to write this myself - and I've nowhere near enough knowledge of video to do this. At least you'd be able to assume 4:3 or 16:9 screen ratio... there'd be the issue of picking a subtitle and audio track though.

Anyone have any ideas?