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 first line of the next frame. It is present in analog television, VGA, DVI and other signals. During the VBI the incoming data stream is not displayed on the screen. In raster cathode ray tube displays, the beam is blanked to avoid displaying the retrace line; see raster scan for details. The signal source, such as a television broadcast, does not supply image information during the blanking period.

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 being drawn, cannot change instantly. Additionally, the speed of older circuits was limited. 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. Modern CRT circuitry does not require such a long blanking interval, and thin panel displays require none, but the standards were established when the delay was needed (and to allow the continued use of older equipment). Blanking of a CRT may not be perfect due to equipment faults or brightness set very high; in this case a white retrace line shows on the screen, from bottom right to top left.

In analog television systems the vertical blanking interval can be used for datacasting (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. In U.S. analog broadcast television, line 19 was reserved for a ghost-cancelling signal; line 21 was reserved for captioning data. The obsolete Teletext service contemplated the use of line 22 for data transmission.

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 the 16-bit era could be performed only during the blanking interval. 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. The use of double buffering in modern graphics hardware has rendered these techniques obsolete.

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, to disrupt recording to videotapes.

Other articles related to "vertical blanking interval, vertical, blanking":

Rovi Corporation - Technology Details - Content Protection (RipGuard and Analog Copy Protection)
... Known as the vertical blanking interval (VBI), these extra lines are used to convey different things in different countries, like closed captioning ... On some TVs that do not properly blank the vertical retrace, dotted white lines additionally appear near the top of the picture ... CGMS-A consists of a "flag" within the vertical blanking interval (essentially data, like closed captioning) which digital recording devices search for ...
NAPLPS - History
... Ceefax encoded character data into the lines in the vertical blanking interval of normal television signals where they could not be seen on-screen, and then used a ... For instance, it could use the vertical blanking interval to send data to the user, and a modem to return selections to the servers ... sent to users' televisions by encoding them as dot patterns in the vertical blanking interval of the video signal ...
Analogue Television Synchronization - Synchronization - Vertical Synchronization
... Vertical synchronization (Also vertical sync or VSYNC) separates the video fields ... In PAL and NTSC, the vertical sync pulse occurs within the vertical blanking interval ... The vertical sync pulses are made by prolonging the length of HSYNC pulses through almost the entire length of the scan line ...
Blanking And Piercing
... Blanking and piercing are shearing processes in which a punch and die are used to modify webs ... two, only the terminology is different in blanking the punched out piece is used and called a blank in piercing the punched out piece is scrap ...

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