Vertical blanking interval
From Free net encyclopedia
The vertical blanking interval (VBI) or vertical interval for short is an interval in a television or VDU 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 VBI is necessary mostly because of the inductive inertia of the magnetic coils which deflect the electron beam vertically: the magnetic field cannot change instantly. For horizontal deflection, there is also a gap between successive lines, to allow the beam to return from right to left, called the horizontal retrace or the horizontal blanking interval.
The vertical blanking interval can be used to carry data, since anything sent during the VBI would naturally not be displayed; various test signals, time code, closed captioning, teletext, CGMS-A copy-protection indicators, and various data encoded by the XDS protocol (e.g., the content ratings for V-chip use) and other digital data can be sent during this time period.
Most consumer VCRs use the known black level during the vertical blanking interval to set their recording levels. The Macrovision copy protection scheme uses pulses in the VBI on videotapes to disrupt recording.
See also: broadcast television systems.