Vertical Blanking Interval

A Vertical Blanking Interval (VBI) is the part of a composite video signal between the last visible line and the first visible line of the following field.

It contains the synchronising pulses which tell the receiver the next field is starting.

Several lines following the vertical sync are blanked, to allow time for the flyback of slow CRTs, but it was later realised that these "lost" lines could be used to carry data and test signals.

Teletext, Wide Screen Signalling, Closed Captioning, various Timecodes and Videocrypt are all transmitted as digital waveforms during the VBI.