Captions are necessary for people who are deaf and hard of hearing to access the audio portion of video presentations. Captions are either open (part of the video, therefore always visible) or closed (a separate text track, which can typically be turned on or off). Open captions are displayed in all video players. However, in some context there are advantages to closed captions as described in the Knowledge Base article What is the difference between open and closed captioning? . Closed captions come in a variety of different file types, and media players vary as to which file types they support.
The following is a summary of some of the more common closed caption file types used to deliver captions for video on the Web:
- Timed Text or DFXP, short for "Distribution Format Exchange Profile", is a World Wide Web Consortium (W3C) draft standard. It is an XML markup language specifically designed for marking up timed text, or captions. It is the most common format used by Flash video players that support closed captions. For additional information consult the Timed Text 1.0 DFXP  specification.
- SMIL, short for "Synchronized Multimedia Integration Language", is also a W3C standard, and an XML markup language, designed to allow for the synchronized presentation of various media components such as video, text, images, and audio. SMIL is supported by both Quicktime and Real players for delivering closed captions. To display captions in either of these products, the player must open the SMIL file, which then references the other media files (i..e, the video and caption text file). The format of the caption text file is different in Quicktime than it is in Real Player. The two formats are described in Apple's Text Descriptors Tutorial  and Real's Text Tags Guide , respectively.
- SAMI, short for "Synchronized Accessible Media Interchange", is Microsoft's format for delivering closed captions. Unlike SMIL, which references captions that are stored in a separate file, SAMI files include the captions. SAMI is the format supported by Windows Media Player (WMP). To play closed captions in WMP, the video file and caption file must both be passed to the player. This can be done either using the HTML <object> element to embed a media player, as described in the MSDN document Understanding SAMI 1.0 ; or using an ASX file, as described in the WebAIM tutorial Captioning For Windows Media .
- SubRip (.srt) and SubViewer (.sub) - These formats are very simple, but slightly different, text formats. Both are officially supported by YouTube, as described on the YouTube help page Getting Started: Adding/Editing Captions . Although it is not specifically documented, YouTube also supports captions uploaded in DFXP and SAMI formats.
For information about how to turn captions on or off in various media players, consult the Knowledge Base article How do I turn on captions and descriptive audio in my media player? . For a general overview of multimedia accessibility, consult the Knowledge Base article How do I make multimedia accessible? .
-  What is the difference between open and closed captioning?
-  Timed Text 1.0 DFXP
-  Apple's Text Descriptors Tutorial
-  Real's Text Tags Guide
-  Understanding SAMI 1.0
-  Captioning For Windows Media
-  Getting Started: Adding/Editing Captions
-  How do I turn on captions and descriptive audio in my media player?
-  How do I make multimedia accessible?