CTA-708

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CTA-708 (formerly EIA-708 and CEA-708) is the standard for closed captioning for ATSC digital television (DTV) streams in the United States and Canada. It was developed by the Consumer Electronics sector of the Electronic Industries Alliance, which is now a standalone organization Consumer Technology Association.

Unlike RLE DVB and DVD subtitles, CTA-708 captions are low bandwidth and textual like traditional EIA-608 captions and EBU Teletext subtitles. However, unlike EIA-608 byte pairs, CTA-708 captions are not able to be modulated on an ATSC receiver's NTSC VBI line 21 composite output and must be pre-rendered by the receiver with the digital video frames, they also include more of the Latin-1 character set, and include stubs to support full UTF-32 captions, and downloadable fonts. CTA-708 caption streams can also optionally encapsulate EIA-608 byte pairs internally, a fairly common usage.[1]

CTA-708 captions are injected into MPEG-2 video streams in the picture user data. The packets are in picture order, and must be rearranged just like picture frames are. This is known as the DTVCC Transport Stream. It is a fixed-bandwidth channel that has 960 bit/s typically allocated for backward compatible "encapsulated" Line 21 captions, and 8640 bit/s allocated for CTA-708 captions, for a total of 9600 bit/s.[2] The ATSC A/53 Standard contains the encoding specifics. The main form of signalling is via a PSIP caption descriptor which indicates the language of each caption and if formatted for "easy reader" (3rd grade level for language learners) in the PSIP EIT on a per event basis and optionally in the H.222 PMT only if the video always sends caption data.

CTA-708 caption decoders are required in the U.S. by FCC regulation in all 13" (33 cm) diagonal or larger digital televisions. Further, some broadcasters are required by FCC regulations to caption a percentage of their broadcasts.


References[edit]

  1. ^ https://www.adobe.com/content/dam/acom/en/devnet/video/pdfs/introduction_to_closed_captions.pdf (2015) "The majority of premium content produced for the United States today still contains 608 captions embedded in the 608 over 708 digital format."
  2. ^ https://ecfsapi.fcc.gov/file/6008646915.pdf "NTSC...captions...must always be placed in the User datastream before any DTVCC caption data" and "On average, NTSC captions are allocated 960 bps, and DTVCC captions (EIA-708-A) are allocated 8640 bps" 4 captions are possible as in EIA 608

External links[edit]