How are closed captions transmitted to the viewers?
Analog broadcasts were transmitted with line 21 as part of the video signal. However, as of June 2009, virtually all broadcasts in North America have switched over to digital (DTV). For DTV broadcasts, the caption data is stored in metadata packets as part of the video stream, which is itself contained in the MPEG-2 transport stream. These captions must be decoded by the device that receives the DTV transmission, because once the image has been decoded, the metadata packets do not travel along with it. This will usually be the TV itself when using an antenna connection, or the cable/satellite/converter set-top box (STB) which is then connected to the TV by a baseband connection like HDMI or component video. Some STBs will re-generate the line 21 closed captions for standard definition analog connections such as composite or RF, so that the TV’s decoder may be used instead of the STB’s decoder. This is only possible for standard definition video.
