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.