The vertical blanking interval (VBI) is an interval in a television
signal that temporarily suspends transmission of the signal for
the electron gun to move back up to the first line of the television
screen to trace the next screen field. The vertical blanking interval
can be used to carry data, since anything sent during the VBI would
naturally not be displayed; various test signals, closed captioning,
and other digital data can be sent during this time period.

The zvbi library provides functions to read from Linux V4L, V4L2
and FreeBSD BKTR raw VBI capture devices, from Linux DVB devices
and from a VBI proxy to share V4L and V4L2 VBI devices between
multiple applications.

It can demodulate raw to sliced VBI data in software, with support
for a wide range of formats, has functions to decode several popular
services including Teletext and Closed Caption, a Teletext cache
with search function, various text export and rendering functions.

Basically zvbi offers all functions needed by VBI applications
except for the user interface. The library was written for the
Zapping TV viewer http://zapping.sourceforge.net.