Closed Captioning

Just as a caption in a book is the text under a picture, captions on video are text located somewhere on the picture. Since there is no way for a television to put text outside the area of the picture tube, captions do end up covering a portion of the picture (there are non-broadcast applications where this can be done).

The vertical blanking interval (VBI) of a television signal is the part "between frames." If you have an older television with a vertical hold adjustment, and you tweak it until a black bar rolls across the screen, that black bar is the VBI.

The VBI consists of a number of "lines" of video. The 21st line has been allocated to closed-caption information. The method of encoding used in North America allows for two characters of information to be placed in each frame of video, and there are 30 frames in a second. This corresponds (roughly) to 60 characters per second, or about 600 words per minute. That sounds like a lot, until you realize that it also takes one frame to transmit a command (like "go to a new line of roll-up"), and position information on pop-on captions take more than one frame. Also, that 600 words per minute includes everything required for caption 1, caption 2, text 1, and text 2, because they're all stored in the same place, so there are decided limitations on the capacity of Line 21 for captions.

More FAQ's in:
http://www.robson.org/capfaq/overview.html