International Captioning Project
Description of Line-21 Closed
Captions
Closed captions data are carried with the normal broadcast signal. Specifically, they are recorded (or encoded) in the VBI. The VBI is made up of 21 lines, some of which are assigned by the FCC to carry non-video information. Closed-caption data are specified by the FCC to be carried on the 21st line of the VBI; hence the formal name, line-21 closed captions. (The actual picture seen by the viewer at home begins on line 22.) If the viewer is watching television with a set-top or built-in decoder, the data on line 21 are converted into captions which appear on the television screen. Because line-21 closed captions are transmitted at a relatively slow rate (60 characters per second, or 600 baud), they may be recorded and played back by the viewer at home using consumer-level VCRs. A more technical advantage is that the data may pass through the entire broadcast transmission process and suffer very little degradation.
Line-21 captions appear on the television screen as white letters in a black box (although the use of color captions is now facilitated under the new FCC rules). The captions may appear in whole units of one, two, three or four lines (known as pop-on captions), much like foreign-language subtitles, or may be a continually scrolling display of two, three or four rows of text (roll-up captions). Pop-on captions may be positioned nearly anywhere in the picture to indicate not only what is being said, but who is speaking. Roll-up captions usually appear in the lower third of the picture, but the new FCC specifications allow them to be repositioned vertically to avoid covering important visual information.
