Closed captions are a text version of the spoken part of a television, movie, or computer presentation. Closed captioning was developed to aid hearing-impaired people, but it’s useful for a variety of situations. For example, captions can be read when audio can’t be heard, either because of a noisy environment, such as an airport, or because of an environment that must be kept quiet, such as a hospital.
Closed captioning information is encoded within the video signal, in line 21 of the vertical blanking interval. The text-only becomes visible with the use of a decoder, which may be built into a television set or available as a set-top box . In general, an onscreen menu on newer televisions allows you to turn closed captioning on or off. Open captions, in contrast, are an integral part of a transmission that cannot be turned off by the viewer.
Most programs are captioned in advance of transmission, but the nature of some programs, such as live news broadcasts, requires real time captioning. For real time captioning, a stenographer listens to the broadcast and types a shorthand version into a program that converts the shorthand into captions and adds that data to the television signal.