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Closed captioning allows deaf, hard of hearing / hearing_impaired, and other people to read, through captions, a transcript of the audio portion of a video that they cannot hear. As the video plays, captions showing what is being said and by who, as well as other relevant sounds, are displayed along with the video. Closed captioning is so called because they are optional: hearing viewers need not see them.
Captions are distinct from subtitles: subtitles assume the viewer can hear but cannot speak the language, so they only note dialogue. Captions describe all the audio content, as well as information about the identity of speakers and their tone of voice. The distinction between subtitles and closed captions is not made in the UK, where the term "subtitles" is used for both.
Television For live programs in countries that use the NTSC television system, like Canada and the U.S., spoken words comprising the television program's soundtrack are transcribed by a court reporter using stenotype or stenomask techniques. Alternatively, in some cases the transcript is available beforehand and captions are simply typed out in advance.
For prerecorded programs and home videos, audio is transcribed and captions are prepared, positioned, and timed in advance.
For all types of NTSC programming, captions are "encoded" into Line 21 of the vertical blanking interval – a part of the TV picture that sits just above the visible portion and is usually invisible. Sometimes music or sound effects are also described using words or symbols within the closed caption. For ATSC programming, three streams are encoded in the video, two are backward compatible Line 21 captions, and the third is a set of up to 63 additional caption streams encoded in EIA-708 format.
Captioning is transmitted and stored differently in PAL and SECAM countries, where teletext is used rather than Line 21, but the methods of preparation are similar. (Note that, for home videotapes, a variation of the Line 21 system is used in PAL countries. Teletext captions can't be stored on a standard VHS tape. Further, only NTSC DVDs may carry closed captions, all of which are in the Line 21 format.)
Since 1993, when the Television Decoder Circuitry Act became U.S. law, manufacturers of most television receivers sold in the U.S. have been required to include closed captioning. High-definition TV sets, receivers, and tuner cards are also covered, though the technical specifications are different. Canada has no similar law, but receives the same sets as the U.S. in most cases.
Certain video presentations (now very rare) actually have the captions permanently burned into the video instead of encoded in the vertical blanking interval; this is known as open-captioning.
There are three styles of Line 21 closed captioning:
- Roll-Up or scroll-up: The words appear from a left corner, up to one line at a time; when a line is filled, the whole line scrolls up to make way for a new line, and the line on top is erased. The captions usually appear at the bottom of the screen, but can actually be placed anywhere to avoid covering graphics or action. This method is used for live events, where a sequential word-by-word captioning process is needed.
- Pop-on or pop-up: A caption appears anywhere on the screen as a whole, followed by another caption or no captions. This method is used for most pre-taped television and film programming.
- Paint-on: The caption, whether it be a single word or a line, appear on the screen letter-by-letter from left to right, but ends up as a stationary block like pop-on captions. Rarely used; most often seen in very first captions when little time is available to read the caption or in "overlay" captions added to an existing caption.
A single program may include scroll-up and pop-on captions (e.g., scroll-up for narration and pop-on for song lyrics).
For live programs, some soap operas, and other shows captioned using scroll-up, caption text includes the symbols '>>' to indicate a new speaker, and '>>>' in news reports to identify a new story. Capitals are frequently used because there are often no descenders for the lowercase letters g, j, p, q, and y on home decoder fonts, though that is rapidly changing. Text can be italicized, among a few other parameters. There are many shortcomings in the Line 21 specification from a typographic standpoint, since, for example, it lacks enough characters for French and Spanish.
Movies First-run movies in theatres can be captioned using the Rear Window Captioning System. A sheet of translucent plastic sits on a stalk in front of the viewer's seat. The panel reflects an LED display at the back of the theater, which shows the movie's captions in mirror-image. The movie is watched through the panel, which also shows the reflected captions. (The panel can be positioned to minimize overlap with the movie screen.)
A few other technologies are available to caption first-run films, but Rear Window is the only one in wide use that enables captions at all times without special prints or screenings.
Telephones Closed captioning is now starting to be applied to telephones for the hard-of-hearing and deaf. See Captioned telephone.
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