A feature utilized in television and video media that displays dialog and sound effects cues in text on the viewer's screen. Originally developed as an aid to the deaf and hearing impaired.
In most cases, the captions are inserted in a special scan line in a video image. To view the captions, one must use an external decoder or a television with a built-in decoder. Changes in federal law require all televisions with 13-inch or larger screens sold in the US to be equipped with closed-captioning decoders.
Closed captioning can be utilized in two ways. In realtime captioning, a typist trained in stenography uses a special keyboard device to enter the text, dialog and cues. The typed stream uses a complex form of phonetics, and the typist can enter the text at speeds of up to 250 words per minute. The stream is sent to a computer, where it is converted to plain text and sent to an encoder for broadcast.
In offline captioning, the captions are added to recorded events in post-production. Doing so requires staff to enter the text, encode it to the media, and check it for accuracy.
A fellow named Gary Robson has designed a site with extensive FAQs and information about closed captioning, laws and requirements, methods, and equipment. You'll find it at http://www.robson.org/capfaq/.
Also handy for watching late night programs and not waking the family.