Vertical blanking interval


Vertical blanking interval

The vertical blanking interval (VBI), also known as the vertical interval or VBLANK, is the time difference between the last line of one frame or field of a raster display, and the beginning of the next. It is present in analog television, VGA, and DVI and other signals. During the VBI the incoming data stream is not displayed on the screen. In cathode ray tubes the beam is blanked to avoid displaying the retrace line.

The VBI was originally needed because of the inductive inertia of the magnetic coils which deflect the electron beam vertically in a CRT; the magnetic field, and hence the position of the spot on the screen, cannot change instantly. For horizontal deflection, there is also a pause between successive lines, to allow the beam to return from right to left, called the "horizontal retrace" or horizontal blanking interval. While modern digital equipment does not require a long blanking time, it must be designed to retain compatibility with the broadcast standards intended for older equipment.

In analog television systems the vertical blanking interval can be used to carry digital data, since nothing sent during the VBI is displayed on the screen; various test signals, time codes, 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, or to provide a time reference for when switching such pointers is safe.

In video game systems the vertical blanking pulses are extensively used for timing, as they occur at an accurately known frequency. Most graphics operations on consoles up to and including the 16-bit era could be performed only during the VBI (which programmers generally referred to as the VBLANK), requiring programs to do all graphics processing rigidly within it.The need to synchronise game code this way made early video game systems such as the Atari 2600 difficult to program.Special raster techniques on the Atari 2600, Nintendo Entertainment System, and other consoles allowed extending this interval at the cost of some blank scanlines at the top or bottom of the screen, which may or may not end up in the overscan area.

Most consumer VCRs use the known black level of the vertical blanking pulse to set their recording levels. The Macrovision copy protection scheme inserts pulses in the VBI, where the recorder expects a constant level, on videotapes to disrupt recording.


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