Text capture

Teletext is popular in Europe and provides both informational pages and captions or subtitles to television programs. In 1992, teletext was provided in 18 countries. In the PAL standard, the text is digitally encoded in the vertical blanking interval (vbi) on lines 17 through 20.

In North America, closed captioning uses line 21 of the vertical blanking interval (NTSC standard). US federal law requires closed captioning of all non-exampt programs starting in 2006. Some broadcasters are implementing XDS, or Extended Data Services.

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

To visualize the Vertical Blanking Interval on NTSC, issue

ntsc-cc -d /dev/vbi -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
• zapzilla
• 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.

zapzilla

Zapping has a built-in teletext viewer called Zapzilla. In addition, Zapping

Unsupported cards

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