The subtitle track is actually a 3gpp timed text track with the subtype atom set to ’sbtl’. It will work with all current-generation video-capable iPods (Classic, Nano, Touch and iPhone) as well as iTunes, Quicktime and even AppleTV (Take 2). Creating subtitled videos might be a bit complicated but it is tremendously easier than trying to create Closed Captions. This tutorial will help you to add any subtitles (as long as they are in SRT format) to your MP4 movies. It is tested with Quicktime 7.4.1, iTunes 7.6.1 and on an iPod Touch with firmware 1.1.4.

Note: I did not discover any of these features. All credits must go to entropic from the Handbrake forum and audiovizual from Doom9. This tutorial is written based on the following materials:

• http://forum.doom9.org/showthread.php?t=136170
• http://forum.handbrake.fr/viewtopic.php?f=14&t=5520&p=31577
• http://forum.handbrake.fr/viewtopic.php?f=4&p=31698

You will need:

• MP4Box (’sudo apt-get install gpac’ for Ubuntu or http://www.videohelp.com/tools/mp4box for Windows)
• Dumpster (http://developer.apple.com/quicktime/quicktimeintro/tools/)

First off, convert your .srt file to a .ttext file.
MP4Box -ttxt path_to_srt_file
You can edit the resulting ttext file to fit your needs. When you are done, add the track to your movie.
MP4Box -add path_to_converted_text_track:lang=language_code path_to_movie_file
Now your movie contains the 3gpp timed text track. Next step is to use Dumpster to set the correct atoms in the movie so that iPods and Quicktime will recognize it as a subtitle track.
Drag your movie file over the dumpster.exe to open it. Navigate to ‘moov -> trak -> mdia -> hdlr’ (make sure you choose the last ‘trak’ in the list). Click on ‘Component subtype’ and replace everything after the last $ sign in the text box in the bottom of the window with ‘7362746c’ (minus the quotes).
The result should look something like this:

Now Quicktime and iTunes will be able to display your newly-created subtitles. To be able to select the track on an iPod, an extra step is necessary.
Open the movie file in Dumpster again. Go to ‘moov -> trak -> tkhd’ and replace the ‘Alternate Group’ atom’s last zero with a 2 (see the screenshot below), apply th changes and close Dumpster.

Congratulations! Your subtitled movie is now ready. All you have to do now is enjoy
If you have any questions or problems, post them in the comments section, I’ll try to reply as soon as possible.