Text capture

The PAL standard uses teletext, while the NTSC standard uses closed captioning. TV capture chipsets implement these in different ways, and the code to support text capture is still missing or incomplete on some chipsets. Finally, different applications implement support in different ways. The following notes are just a start to describe the current state of affairs.

Text is transmitted in the Vertical Blanking Interval, or vbi. To visualize the vbi on NTSC, issue

ntsc-cc -d /dev/vbi1 -c -w -r 27

bttv and saa7134 cards show orderly signals, while at least on my end, cx88 shows just noise.

Applications

• alevtv
• gstreamer
• ntsc-cc
• tvtime
• zvbi

gstreamer

The application gstreamer has incorporated support for closed captioning (they also mention some tweaks for Canadian English and French television); see Freedesktop's repository.

ntsc-cc

The application ntsc-cc handles closed captioning on bttv devices. For ntsc-cc to work, you typically need to be running an application for viewing or recording television, such as xawtv and mencoder. If no such application is running, ntsc-cc tends to produce garbled output.

tvtime

tvtime has built-in support for closed captioning for bttv and saa7134 cards (also others?).

In early 2004, Kevin Ko wrote a patch with useful comments to tvtime's vbidata.c; see his detailed account and the tvtime bugreport.

Unsupported cards

There is currently no code supporting closed captioning on cx88 cards.