Vertical Blanking Interval
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In a
raster scan A raster scan, or raster scanning, is the rectangular pattern of image capture and reconstruction in television. By analogy, the term is used for raster graphics, the pattern of image storage and transmission used in most computer bitmap image s ...
display, the vertical blanking interval (VBI), also known as the vertical interval or VBLANK, is the time between the end of the final visible line of a frame or field and the beginning of the first visible line of the next frame. It is present in analog television,
VGA Video Graphics Array (VGA) is a video display controller and accompanying de facto graphics standard, first introduced with the IBM PS/2 line of computers in 1987, which became ubiquitous in the PC industry within three years. The term can no ...
,
DVI Digital Visual Interface (DVI) is a video display interface developed by the Digital Display Working Group (DDWG). The digital interface is used to connect a video source, such as a video display controller, to a display device, such as a comp ...
and other signals. In raster
cathode ray tube A cathode-ray tube (CRT) is a vacuum tube containing one or more electron guns, which emit electron beams that are manipulated to display images on a phosphorescent screen. The images may represent electrical waveforms ( oscilloscope), pictu ...
displays, the blank level is usually supplied during this period to avoid painting the retrace line — see
raster scan A raster scan, or raster scanning, is the rectangular pattern of image capture and reconstruction in television. By analogy, the term is used for raster graphics, the pattern of image storage and transmission used in most computer bitmap image s ...
for details; signal sources such as
television broadcasts Television Broadcasts Limited (TVB) is a television broadcasting company based in Hong Kong SAR. The Company operates five free-to-air terrestrial television channels in Hong Kong, with TVB Jade as its main Cantonese language service, and TVB ...
do not supply image information during the blanking period. Digital displays usually will not display incoming data stream during the blanking interval even if present. 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 blanking interval Horizontal blanking interval refers to a part of the process of displaying images on a computer monitor or television screen via raster scanning. CRT screens display images by moving beams of electrons very quickly across the screen. Once the be ...
. 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, often alternating between fairly steep diagonals from right to left and less-steep diagonals back from left to right, starting in the lower right of the display. In analog television systems the vertical blanking interval can be used for
datacasting Datacasting (data broadcasting) is the broadcasting of data over a wide area via radio waves. It most often refers to supplemental information sent by television stations along with digital terrestrial television (DTT), but may also be applied ...
(to carry digital data), since nothing sent during the VBI is displayed on the screen; various test signals,
time code A timecode (alternatively, time code) is a sequence of numeric codes generated at regular intervals by a timing synchronization system. Timecode is used in video production, show control and other applications which require temporal coordinatio ...
s,
closed captioning Closed captioning (CC) and subtitling are both processes of displaying text on a television, video screen, or other visual display to provide additional or interpretive information. Both are typically used as a transcription of the audio por ...
,
teletext A British Ceefax football index page from October 2009, showing the three-digit page numbers for a variety of football news stories Teletext, or broadcast teletext, is a standard for displaying text and rudimentary graphics on suitably equipp ...
,
CGMS-A Copy Generation Management System – Analog (CGMS-A) is a copy protection mechanism for analog television signals. It consists of a waveform inserted into the non-picture Vertical Blanking Interval (VBI) of an analogue video signal. If a compatib ...
copy-protection indicators, and various data encoded by the XDS protocol (e.g., the
content rating A content rating (also known as maturity rating) rates the suitability of TV shows, movies, comic books, or video games to this primary targeted audience. A content rating usually places a media source into one of a number of different categories, ...
s for
V-chip V-chip is a technology used in television set receivers in Canada, Brazil and the United States, that allows the blocking of programs based on their ratings category. It is intended for use by parents to manage their children's television view ...
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-canceling reference Ghost-canceling reference (GCR) is a special sub-signal on a television television channel, channel that receivers can use to compensate for the ghosting (television), ghosting effect of a television signal distorted by multipath propagation between ...
& line 21 was reserved for
NABTS NABTS, the North American Broadcast Teletext Specification, is a protocol used for encoding NAPLPS-encoded teletext pages, as well as other types of digital data, within the vertical blanking interval (VBI) of an analog video signal. It is standard ...
captioning data. The obsolete
Teletext A British Ceefax football index page from October 2009, showing the three-digit page numbers for a variety of football news stories Teletext, or broadcast teletext, is a standard for displaying text and rudimentary graphics on suitably equipp ...
service contemplated the use of line 22 for data transmission. The pause between sending video data is sometimes used in real time computer graphics to modify the
frame buffer A framebuffer (frame buffer, or sometimes framestore) is a portion of random-access memory (RAM) containing a bitmap that drives a video display. It is a memory buffer containing data representing all the pixels in a complete video frame. Modern ...
, or to provide a time reference for when switching the source buffer for video output can happen without causing a visible tear. This is especially true in video game systems, where the fixed frequency of the blanking period might also be used to derive in-game timing. On many consoles there is an extended blanking period, as the console opts to paint graphics on fewer lines than the television would natively allow, permitting its output to be surrounded by a border. On some very early machines such as the
Atari 2600 The Atari 2600, initially branded as the Atari Video Computer System (Atari VCS) from its release until November 1982, is a home video game console developed and produced by Atari, Inc. Released in September 1977, it popularized microprocessor- ...
, the programmer is in full control of video output and therefore may select their own blanking period, allowing arbitrarily few painted lines. On others such as the
Nintendo Entertainment System The Nintendo Entertainment System (NES) is an 8-bit third-generation home video game console produced by Nintendo. It was first released in Japan in 1983 as the commonly known as the The NES, a redesigned version, was released in America ...
, a predefined blanking period could be extended. Most consumer
VCR A videocassette recorder (VCR) or video recorder is an electromechanical device that records analog audio and analog video from broadcast television or other source on a removable, magnetic tape videocassette, and can play back the recording. ...
s use the known black level of the vertical blanking pulse to set their recording levels. The
Macrovision TiVo Corporation, formerly known as the Rovi Corporation and Macrovision Solutions Corporation, was an American technology company. Headquartered in San Jose, California, the company is primarily involved in licensing its intellectual property ...
copy protection Copy protection, also known as content protection, copy prevention and copy restriction, describes measures to enforce copyright by preventing the reproduction of software, films, music, and other media. Copy protection is most commonly found on ...
scheme inserts pulses in the VBI, where the recorder expects a constant level, to disrupt recording to videotapes.


Vertical blanking interval in digital video

While digital video interconnects (such as DVI and HDMI) generally do have a "vertical blanking" part of the datastream, they are unable to carry closed caption text or most of the other items that, in analog TV interconnects, are transmitted during the "vertical blanking interval". This can lead to .


See also

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Vertical blank interrupt {{unreferenced, date=February 2008 A vertical blank interrupt (or VBI) is a hardware feature found in some legacy computer systems that generate a video signal. Cathode-ray tube based video display circuits generate vertical blanking and vertical ...
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Datacasting Datacasting (data broadcasting) is the broadcasting of data over a wide area via radio waves. It most often refers to supplemental information sent by television stations along with digital terrestrial television (DTT), but may also be applied ...
*
Horizontal blanking interval Horizontal blanking interval refers to a part of the process of displaying images on a computer monitor or television screen via raster scanning. CRT screens display images by moving beams of electrons very quickly across the screen. Once the be ...
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Nominal analogue blanking Nominal analogue blanking is the outermost part of the overscan of a standard definition digital television image. It consists of a gap of black (or nearly black) pixels at the left and right sides, which correspond to the end and start of the h ...
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Raster scan A raster scan, or raster scanning, is the rectangular pattern of image capture and reconstruction in television. By analogy, the term is used for raster graphics, the pattern of image storage and transmission used in most computer bitmap image s ...
{{DEFAULTSORT:Vertical Blanking Interval Television technology