Closed Captioning: Difference between revisions
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In North America, [http://www.fcc.gov/cgb/dro/caption.html US federal law] requires [[Wikipedia:Closed captioning|Closed captioning]] for all non-exempt television programs starting in 2006. |
In North America, [http://www.fcc.gov/cgb/dro/caption.html US federal law] requires [[Wikipedia:Closed captioning|Closed captioning]] for all non-exempt television programs starting in 2006. |
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==Analogue TV== |
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Some broadcasters are implementing [[Wikipedia:Extended Data Services|XDS]] (Extended Data Services). Both analogue TV CC and XDS fall under the scope of [[Wikipedia:EIA-608|EIA-608]]. |
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====ATSC==== |
====ATSC==== |
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Revision as of 18:12, 15 October 2011
In North America, US federal law requires Closed captioning for all non-exempt television programs starting in 2006.
Analogue TV
Some broadcasters are implementing XDS (Extended Data Services). Both analogue TV CC and XDS fall under the scope of EIA-608.
ATSC
The A/53 ATSC Digital Television Standard covers Closed Caption transport and the "digital" Closed Caption standard EIA 708-B. See the EIA-708 Wikipedia entry and Digital Television Closed Captioning FAQ for details.
EIA-708 specifies the transmission of CC bytes in a user data field following a picture header, inside the video elementary stream. If the driver doesn't extract CC data on its own, and the "broadcast flag" permits this, one could perhaps read video packets from the device, or a complete MPEG-2 program stream from disk, and demultiplex from there. A freestanding CC capture application would still need to tune in and choose a program ID. To complicate matters further, EIA-708 allows both old style EIA-608-compatible closed caption data as well as the newer DTVCC.
Scott Larson writes that "stations are required to pass both EIA-608 (VBI) and EIA-708 (DTVCC) data. Why? Because If an ATSC receiver is connected to an NTSC TV (which is a likely occurrence in the transition to digital broadcasting), the receiver is required to generate the old VBI Line 21 captions from the ATSC stream so the old TV will still have closed captions."
And while the EIA/CEA-708 spec allows for many improvements over the older EIA/CEA-608 analogue spec, it is, however, reported that "most closed captioning for DTV environments is done using tools designed for analog captioning (working to the CEA-608 NTSC spec rather than the CEA-708 DTV spec). The captions are then run through transcoders ... which convert the analog Line 21 caption format to the digital format. This means that none of the CEA-708 features are used unless they were also contained in CEA-608." [1]