Captioning for RealPlayer
http://www.webaim.org/techniques/captions/real/

WebAIM
2003

Abstract:

RealPlayer uses SMIL to combine media content with a RealText (.rt) file. The .rt file contains the captions themselves and information about how and when they should appear. The SMIL file is really just a pointer file. It contains information about where and how your captions and media content should display.