A service, primarily for deaf people, that allows individual viewers to display the dialog of a television program in a readable form on the television screen. In the United Kingdom the service is called subtitling. Closed-caption television was introduced in the United States in 1980.
Captions for television programs can be prepared in three ways. (1) Off-line captioning is used when the television program has been prerecorded. This is the preferred method for creating captions in that they can be caused to appear on different parts of the home television screen, for example, under the individual speaking, and they can be displayed in close synchronism with the dialog. Also, the caption display can be edited if necessary to a comfortable reading rate of about 125 words per minute. (2) Live television broadcasts that are scripted in advance, such as presidential State of the Union speeches, employ a live display captioning method. Here the script is stored in a computer in advance of the broadcast. During the broadcast the script appears as roll-up captions at the bottom of the screen. (3) In live broadcasts, where a script is not available, real-time captions have to be generated during the broadcast. This is by far the most difficult way to caption a television program. Highly skilled court reporters using a stenograph keyboard and an associated computer create the captions, often at instantaneous rates of 200–250 words per minute. In this method, inevitably, typographical errors are often generated.
Prior to being broadcast, the captions are digitally encoded and inserted into the vertical blanking interval (VBI) of the television picture. In the United States and Canada, a single VBI line is allocated exclusively for the captioning service. In other countries such as the United Kingdom and Australia, the captions are integrated with other text material on a series of VBI lines that are used to deliver Teletext service to a more general audience. As the Teletext transmission data rate is about ten times the caption-only service data rate, it cannot be used in videocassette recorder (VCR) applications. This led to the caption-only system being introduced in home-television movies in the United Kingdom in the early 1990s.
The world's terrestrial television systems are in the process of converting from analog to digital broadcasting. In the United States, the Federal Communication Commission (FCC) has adopted the Advanced Television Standards Committee (ATSC) system. Europe has adopted the Digital Video Broadcasting–Terrestrial (DVB-T) system. Elsewhere in the world, those countries that previously adopted the United States or European analog television standards are typically adopting the respective digital television standards. However, the International Telecommunications Union (ITU) recommendation on a digital video, audio, and ancillary data multiplex model has been adopted worldwide. Captions have been classified as an ancillary service and will be multiplexed with other ancillary services such as text services.
Captioned program service growth
Legislative and regulatory actions are spurring the worldwide introduction and growth of captioned television service. In many countries, specific goals have been set as to the percentage of television programs that must be captioned by the broadcasters. In the United States, it is illegal to sell a television set with a screen size 12 in. (30 cm) or larger that does not contain caption decoder circuitry built in by the manufacturer. See also: Television