I have a Korean cable box (from LG U+) with a USB stick input, which is convenient for watching movies but a pain for subtitles because it seems unable to play the common .SRT format. Instead, only subtitles in the SAMI (.SMI) format are supported.
Here's a very simple Ubuntu tool for converting between the two. It uses the command line, but wait wait don't go! It's super simple.
First, install libsubtitles-perl by running:
$ sudo apt-get install libsubtitles-perl
Next, navigate to the directory your subtitle file is in, then use one of these two commands to convert:
SMI -> SRT
$ subs -c srt filename.smi -o filename.srt
SRT -> SMI
$ subs -c smi filename.srt -o filename.smi
And that's it!
Also, if you're (in Korea and) looking for subtitle converters with a GUI, I suggest "Subtitle Editor" because neither Gaupol nor Gnome Subtitles can save into the SAMI format.