- Fortis931Lv 51 十年前最愛解答
The vertical blanking interval (VBI), also known as the vertical interval or VBLANK, is a the time found between the last line of one frame or field and the beginning of the next. It is present in analog television, VGA, and DVI signals. During this time, the data that is transmitted is not displayed on the screen.
The VBI is needed because of the inductive inertia of the magnetic coils which deflect the electron beam vertically in a CRT; 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.
In analog television systems 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.
The pause between sending video data is used in real time computer graphics to perform various operations on the back buffer before copying it to the front buffer instead of just switching both pointers. Back Buffer may be briefly applied to a 3d mesh, allowing effects such as waves (vertex deformation) or broken glass (UVs deformation).
In early video game systems, the vertical blanking interval was extensively used for timing, since they occur at a constant and known rate. Most graphics operations on these systems could only be performed during this interval, so knowledge of the operation of the interval was absolutely required in order to produce the real-time graphics these games needed. The necessity of structuring the game code around this interval (which programmers generally referred to as the VBLANK) earned early video game systems, such as the Atari 2600, a reputation as difficult to program.
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.