The Zapping VBI library, in short ZVBI, provides functions to
capture and decode VBI data. It is written in plain ANSI C with few
dependencies on other tools and libraries, licensed under GPL.

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.