Closed Captioning: Difference between revisions
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{Insert CC description here}} |
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This is a record of my to date failed attempt to make closed captioning work on my FlyVideo3000FM card. |
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In the United States, starting in 2006, federal law requires Closed Captioning for all non-exempt television programs. [http://www.fcc.gov/cgb/dro/caption.html] |
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In the zvbi tarball, available on [http://zapping.sourceforge.net/cgi-bin/view/ZVBI/Download#Source_Code zvbi's project page], there are several useful utilities for testing [[text_capture | text capture]]. |
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==Analogue TV== |
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Using the "capture" utility, I'm picking up something, but it looks like the title of the program rather than the conversation: |
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===PAL & SECAM=== |
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===NTSC=== |
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zvbi-0.2.15/test# ./capture -v --ntsc --dump-cc |
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In North America, which follows the NTSC standard, closed captioning uses line 21 of the [[VBI]]. |
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Try to open V4L2 0.20 VBI device, libzvbi interface rev. |
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$Id: io-v4l2.c,v 1.31 2004/12/30 02:24:11 mschimek Exp $ |
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Opened /dev/vbi |
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Try to open V4L2 2.6 VBI device, libzvbi interface rev. |
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$Id: io-v4l2k.c,v 1.27 2005/01/20 01:40:14 mschimek Exp $ |
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Opened /dev/vbi |
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/dev/vbi (LifeView FlyVIDEO3000) is a v4l2 vbi device, |
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driver saa7134, version 0x0000020c |
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Using streaming interface |
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Current scanning system is 525 |
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Querying current vbi parameters... success |
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VBI capture parameters supported: format 59455247 [GREY], 27000000 Hz, |
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2048 bpl, offs 256, F1 10...21, F2 272...283, flags 00000000 |
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Attempt to set vbi capture parameters |
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VBI capture parameters requested: format 59455247 [GREY], 27000000 Hz, |
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1457 bpl, offs 256, F1 10...21, F2 272...284, flags 00000000 |
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Successfully set vbi capture parameters |
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VBI capture parameters granted: format 59455247 [GREY], 27000000 Hz, |
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2048 bpl, offs 256, F1 10...21, F2 272...283, flags 00000000 |
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Nyquist check passed |
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Request decoding of services 0x40000060, strict level 0 |
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Will capture services 0x00000060, added 0x60 commit:1 |
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Requesting 5 streaming i/o buffers |
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Mapping 5 streaming i/o buffers |
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Successful opened /dev/vbi (LifeView FlyVIDEO3000) |
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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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p waiitn.h . Ja. y ).Le.,no.,.@./.../HE...^.........*......n@.. |
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^L..........NB..C...........HE...^......Th..e..To..ni..gh..t .. |
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Sh..ow..h?h? w..it..h ..Ja. y . ????Le.Xno.X.@."..."SA.*NT..IAH |
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EGO.^:....xNB.xC.."..."..Hen@y,^L h..ey.., Thheey.To.,ni.,gh./t |
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./Sh...?ow.. w..it..h ..Ja..y..Le..no...@......HE...^.........* |
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......NB..C...........HE...^......Th..e ..To..ni..gh..t..????Sh |
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..ow.. w..it..h ..Ja..y ..Le. no. .@.P...Pn@Yo^Lu'..ll.. b e.* |
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ok..ayHE...^.p...pNBLoC.ok... ..LoThoke aTot nimegh..t .,Sh.,o |
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w./w./it..h ..Ja..y ..Le..no...@......HE...^...... ...*......n@ |
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..^L..........NB..C...........HE..??.^ |
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==Digital TV== |
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On a bttv card, things look much better: |
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====ATSC==== |
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# ./capture -v --ntsc --dump-cc |
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The [http://www.atsc.org/standards/a53.html A/53 ATSC Digital Television Standard] covers Closed Caption transport and the "digital" Closed Caption standard [http://global.ihs.com EIA 708-B]. See the [http://en.wikipedia.org/wiki/EIA-708 EIA-708 Wikipedia entry] and [http://www.robson.org/capfaq/dtv.html Digital Television Closed Captioning FAQ] for details. |
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Try to open V4L2 0.20 VBI device, libzvbi interface rev. |
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$Id: io-v4l2.c,v 1.31 2004/12/30 02:24:11 mschimek Exp $ |
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Opened /dev/vbi |
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Try to open V4L2 2.6 VBI device, libzvbi interface rev. |
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$Id: io-v4l2k.c,v 1.27 2005/01/20 01:40:14 mschimek Exp $ |
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Opened /dev/vbi |
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/dev/vbi (BT878 video (AVerMedia TVCaptur) is a v4l2 vbi device, |
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driver bttv, version 0x0000090f |
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Using streaming interface |
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Current scanning system is 525 |
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Querying current vbi parameters... success |
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VBI capture parameters supported: format 59455247 [GREY], 28636363 Hz, |
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2048 bpl, offs 244, F1 10...25, F2 273...288, flags 00000000 |
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Attempt to set vbi capture parameters |
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VBI capture parameters requested: format 59455247 [GREY], 27000000 Hz, |
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1457 bpl, offs 256, F1 10...21, F2 272...284, flags 00000000 |
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Successfully set vbi capture parameters |
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VBI capture parameters granted: format 59455247 [GREY], 28636363 Hz, |
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2048 bpl, offs 244, F1 10...21, F2 273...284, flags 00000000 |
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Nyquist check passed |
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Request decoding of services 0x40000060, strict level 0 |
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Will capture services 0x00000060, added 0x60 commit:1 |
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Requesting 5 streaming i/o buffers |
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Mapping 5 streaming i/o buffers |
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Successful opened /dev/vbi (BT878 video (AVerMedia TVCaptur) |
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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, |
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....................................................... . .P.P....AND THE BRAVES |
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EIA-708 allows both old style EIA-608-compatible closed caption data as well as the newer DTVCC. |
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HAVE JUST WON.t.t....THE WORLD SERIES!..,.,././................................. |
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................................................................................ |
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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." |
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.......................... . .R.R....OH, WHAT A TRAVESTY!...,././............... |
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............................................. . .T.T....THAT, MY FRIENDS,..p.p.. |
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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." [http://en.wikipedia.org/wiki/Closed_caption#DTV_standard_captioning_improvements] |
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..WILL FOREVER BE KNOWN AS.....,.,././.......................................... |
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.......................T.T...."A GOOCHER.".,.,././.............................. |
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===DVB=== |
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==Also See== |
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* [[Text capture]] |
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* [[Teletext|Teletext/Videotext]] |
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==External Links== |
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* [[Wikipedia:Closed captioning|Wikipedia's Closed captioning article]] |
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* [http://www.robson.org/gary/a-nv1.php A Gary Robson article on CC information] |
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** [http://www.robson.org/gary/cchandbook.php Link to Gary Robson's "The Closed Captioning Handbook"] |
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[[Category:Technology]] |
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Latest revision as of 23:51, 17 October 2011
{Insert CC description here}}
In the United States, starting in 2006, federal law requires Closed Captioning for all non-exempt television programs. [1]
Analogue TV
PAL & SECAM
NTSC
In North America, which follows the NTSC standard, closed captioning uses line 21 of the VBI.
Some broadcasters are implementing XDS (Extended Data Services). Both analogue TV CC and XDS fall under the scope of EIA-608.
Digital TV
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." [2]