Vertical blanking interval
The vertical blanking interval (VBI) 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. 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, teletext, CGMS-A copy-protection indicators, content ratings for V-chip use, and other digital data can be sent during this time period. See also: broadcast television systems.
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