Channels
EIA-608 defines four channels of caption information, so that a program could, for example have captions in four different languages. There are two channels, called 1 and 2 by the standard, in each of the two fields of a frame. However, the channels are often presented to users numbered simply as CC1-2 for the odd field and CC3-4 for the even field. However the first channel contains the captions related to the audio track and the second is any extra detail that is not in the audio track, such as visual descriptions or provider accreditation. So technically both could be displayed together however given most captions are generated from scripts and caption were designed primarily for the hearing impaired there has been little use for the second channel.
Within each channel, there are two streams of information which might be considered subchannels: one carries "captions" and the other "text." The latter is not in common use due to the lack of hardware support and bandwidth available. Text is signaled by the use of text commands and is used for a formatted URL string with a 16-bit checksum that is a web site that the captions relate to.
Read more about this topic: EIA-608
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