I have a television set that displays the text of what people say. Often the text lags the spoken words by several seconds and contains hilarious mistakes. Written in English, Spanish, French, or German, they aid kids learning to read, immigrants learning a language, or the hearing impaired. The captions are hidden because they go between video frames, in the non-displayed part of the signal.
Captions can be funny and some are better than others.
The captioner creates the text on a computer, synchronizes it to the video dialogue, and inserts it into the videotape. He reviews his work to eliminate slip-ups. Television game shows and movies use this procedure. Live shows are more difficult. The captioner works as the event occurs either from a script or by typing as he listens.