Editing Closed captioning (section)

=== Caption channels ===
[[File:Cc3tout.jpg|thumb|150px|right|A [[bug (television)|bug]] touting CC1 and CC3 captions (on [[Telemundo]])]]
The Line 21 data stream can consist of data from several data channels [[multiplexed]] together. Odd field 1 can have four data channels: two separate synchronized captions (CC1, CC2) with caption-related text, such as website [[URL]]s (T1, T2). Even field 2 can have five additional data channels: two separate synchronized captions (CC3, CC4) with caption related text (T3, T4), and [[Extended Data Services]] (XDS) for Now/Next [[EPG]] details. XDS data structure is defined in CEA-608.

As CC1 and CC2 share bandwidth, if there is a lot of data in CC1, there will be little room for CC2 data and is generally only used for the primary audio captions. Similarly, CC3 and CC4 share the second even field of line 21. Since some early caption decoders supported only single field decoding of CC1 and CC2, captions for [[Second audio program|SAP]] in a second language were often placed in CC2. This led to bandwidth problems, and the U.S. [[Federal Communications Commission]] (FCC) recommendation is that bilingual programming should have the second caption language in CC3. Many Spanish television networks such as [[Univision]] and [[Telemundo]], for example, provides [[Telemundo#English subtitles|English subtitles]] for many of its [[Spain|Spanish]] programs in CC3. [[Canada|Canadian]] broadcasters use CC3 for French translated SAPs, which is also a similar practice in South Korea and Japan.

Ceefax and Teletext can have a larger number of captions for other languages due to the use of multiple VBI lines. However, only [[Europe|European countries]] used a second subtitle page for second language audio tracks where either the [[NICAM]] dual mono or [[Zweikanalton]] were used.