Caption Channels
The Line 21 data stream can consist of data from several data channels multiplexed together. Field 1 has four data channels: two Captions (CC1, CC2) and two Text (T1, T2). Field 2 has five additional data channels: two Captions (CC3, CC4), two Text (T3, T4), and Extended Data Services (XDS). 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. Similarly CC3 and CC4 share the second field of line 21. Since some early caption decoders supported only CC1 and CC2, captions in a second language were often placed in CC2. This led to bandwidth problems, however, and the current 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 English subtitles for many of its Spanish programs in CC3.
Read more about this topic: Closed Captioning
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