Closed captioning

Closed captioning is a system of television broadcasting for deaf or hearing-impaired individuals. Spoken words comprising the television program's soundtrack are transcribed by a court reporter and encoded into line 21 of the television picture, which is the last line of the vertical blanking interval. Sometimes music or sound effects are also described using words or symbols within the closed caption.

Under normal conditions, the vertical blanking interval containing the captioning information is invisible to television viewers (this is why it's called closed-captioning). However, a person may activate the closed-captioning feature found on most modern television receivers, thereby causing the program's spoken-word dialogue to be displayed alphabetically, like subtitles, on a portion of the screen. Certain video presentations actually have the captions permanently burned into the video instead of encoded in the vertical blanking interval; this is known as open-captioning.

Since approximately 1990, when the Americans With Disabilities Act became U.S. law, manufacturers of television receivers sold in the U.S. have been required to include closed captioning.

Because closed-captions of live performances are necessarily done by live typists, they often include minor typographical errors or misquotes.