Closed Captioning - For the benefit of the hearing impaired, closed captions are subtitles transmitted as data in analog TV's Vertical Blanking Interval (VBI). It types out on the screen the words the characters are saying. The text is in/on line 21 of the Vertical Blanking Interval.
http://en.wikipedia.org/wiki/Closed_captioning   www.itvdictionary.com/closed_captioning.html

Closed Caption Decoder – An electronic device, such as a TV, that extracts previously encoded closed caption data from the video signal it receives. It then displays it.

Decoder