How are closed captions stored in SD video prior to broadcast?
Most analog (e.g. VHS, BetaSP) and full raster digital (e.g. DigiBeta, IMX, D-1) formats store the CEA-608 caption data in line 21, which is a line of video in the VBI (Vertical Blanking Interval). Line 21 is outside of the viewable area of the video image, but still behaves like part of the image. The caption data is represented as white dashes which blink on and off, similar to Morse code. These dashes convey the 1s and 0s which make up the encoded CEA-608 caption data. This data is very sensitive to dropped frames and other kinds of video distortion, which may cause the data to be unintelligible to the decoder. Some non-full raster digital formats such as DV (miniDV, DVCAM, DVCPRO) and DVD do not store the VBI data, so they do not have a line 21. Instead, they store the CEA-608 captions separately as metadata packets in the digital data stream. Many (but not all) devices will convert these packets into a line 21 when playing the data back through the analog outputs, and vice versa w
