Closed captions are broadcast at the same time as the video signal, but are hidden on a separate, invisible video line (line 21, as a matter of fact). Because they are hidden, they can be turned on and off at will. With a decoder, the information hidden in line 21 is "unlocked" and shows up on your television screen as captions. All newer TVs have built-in decoders. Older sets require plug-in decoders.

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