By Mary Bellis
Closed captions are captions that are hidden in the video signal, invisible without a special decoder. The place they are hidden is called line 21 of the vertical blanking interval (VBI). A law in the United States called the Television Decoder Circuitry Act of 1990 mandates since July 1993, that all televisions manufactured for sale in the U.S. must contain a built-in caption decoder if the picture tube is 13" or larger.
In 1970 the National Bureau of Standards (NBS) began to research the possibility of using a portion of the network television signal to send precise time information on a nationwide basis. The ABC-TV network agreed to be involved in the research and development. The project didn't pan out, but ABC suggested that it might be possible to send captions instead.
Captioning was first previewed to the public in 1971, at the First National Conference on Television for the Hearing Impaired in Nashville, Tennessee. A second preview of closed captioning was held at Gallaudet College on February 15, 1972. ABC and the NBS presented closed captions embedded within the normal broadcast of the television show The Mod Squad. The federal government funded the final development and testing of this system. The engineering department of the Public Broadcasting System started to work on the project in 1973, under contract to the Bureau of Education for the Handicapped of the Department of Health, Education and Welfare (HEW).
The Federal Communications Commission set aside line 21 in 1976, for the transmission of closed captions in the United States. PBS engineers then developed the caption editing consoles that would be used to caption prerecorded programs, the encoding equipment that broadcasters and others would use to add captions to their programs and also prototype decoders.
On March 16, 1980, the first, closed captioned television series was broadcast. The captions were seen in households that had the first generation of the closed caption decoder. The ABC Sunday Night Movie (ABC), The Wonderful World of Disney (NBC), Masterpiece Theatre (PBS) were all broadcast on March 16, 1980.
In 1982, the NCI developed real-time
captioning, a process for captioning newscasts, sports events or other
live broadcasts as the events are being televised.
Closed Captioning Web - How To Become a Captioner