Today, two types of captions exist: closed and open. To understand the difference, you first need to understand how the terms closed and open originated.
Captions were initially developed for television. Like movie subtitles, television captions display spoken dialogue as printed words on the screen. Unlike subtitles, these captions are specifically designed for deaf and hard-of-hearing viewers. Television captions are carefully placed to identify speakers, on- and off-screen sound effects, music, and laughter.
The terms closed and open arose from the technology used to deliver TV captions whereby the captions are hidden in line 21 of the analog video signal (also known as the vertical blanking interval (VBI)). A set-top decoder must be used to decode, or open, the captions. Television captions are called closed because they start out turned off and, after they have been decoded to become part of the television picture, they are called open.
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