Vertical Blanking Interval
The vertical blanking interval (VBI) is a very brief period of time during some video signals through which transmission of visual field/frame information is temporarily suspended. This interval corresponds to the time that was required for the electron guns of old CRT display technologies to move their beam's position back up to the left side of the first line of the television/display screen in order to begin to trace out the next screen field. Though this requirement is no longer relevant to modern digital display technologies, the VBI itself has been retained in video signals for compatibility purposes.
As nothing is displayed onscreen during the VBI, this portion of the video signal can be used for the purposes of carrying other data information. Indeed, various test signals, Closed Captioning, Teletext, copy-protection mechanisms, and various forms of digital data all exploit the opportunity afforded by the VBI