Re: Closed Captioning on Playback
- From: Paul <nospam@xxxxxxxxxx>
- Date: Wed, 30 Jan 2008 21:16:01 -0500
Buck Turgidson wrote:
I have a USB TV device where the CC works just fine. However, when I capture something, there are no captions. Having a hearing impairment, this is a problem.
Is this due to the mpeg format not supporting it, or is it just a software thing?
Thanks.
I believe the capability is buried in both hardware and software. When
I tried to research this topic a few months back, I could find a lot
of hints that it existed, and it was a hot topic in some of the PVR software
forums a few years back. Which implies, the capability should be present
in some of the PVR software programs.
Using a WinTV tuner card, I was able to use MythTV to both record and
play back TV content with CC enabled. What I couldn't figure out, was
how to go from the NUV format file used in MythTV, to anything else.
I also tried going through the source code for MythTV, to see how
they put the CC in there. But couldn't find any code for the
function. I don't think it is implicit (automatically captured).
CC is stored in the VBI (vertical blanking interval - the lines
"above" the top of the screen). The BT878 capture chip on my WinTV
card, apparently has a DMA capability set aside for VBI, so the
appropriate bytes can be copied to system memory. I assume
that is how MythTV got access to the information in there. I believe
other capture chips also have VBI capabilities - some even handle
teletext, and have separate pins on the chip, for connection of
selected VBI info, to other chips on the capture card.
So the subject, apparently, is a "secret".
I guess it is easier for a programmer to watch TV the way he likes...
http://blogs.msdn.com/toub/archive/2005/09/17/470491.aspx
You may find the odd reference to GraphEdit. In the examples here,
there is a block called "Line 21 decoder", which has something to
do with Closed Caption. It is possible some research on the
GraphEdit side of things, may uncover the extent of standards
coverage for CC.
http://www.shspvr.com/graph_build/mepg_hwcc_old.html
More random links here from my sad attempt at research...
*******
"SageTV - Recording Closed Captions"
http://forums.freytechnologies.com/forums/showthread.php?postid=44757#post44757
"SageTV Who wants CC?"
http://forums.freytechnologies.com/forums/showthread.php?t=5308&page=5
Reply #9 on: November 20, 2005, 08:49:11 AM "Closed Caption Capture"
(Explains problems the programmers have. "VBI Pin scheme" refers to
GraphEdit and accessing Line21 in a MS environment.)
http://forum.byopvr.com/dvr/index.php/topic,392.0.html
(Claims to extract CC from the MythTV files)
http://ccextractor.sourceforge.net/
(Some details on converting MythTV files to other forms.
I couldn't get mplayer to work, so I'm not making any progress there.)
http://www.linuxjournal.com/article/8524
*******
I reformatted my Linux disk, and have given up on MythTV. Guide
data for it is no longer free.
Paul
.
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