Vertical blanking interval
Encyclopedia
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 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. 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 for datacasting
(to carry digital data), since nothing sent during the VBI is displayed on the screen; various test signals, time code
s, closed captioning
, teletext
, CGMS-A
copy-protection indicators, and various data encoded by the XDS
protocol (e.g., the content rating
s 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 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.
Raster graphics
In computer graphics, a raster graphics image, or bitmap, is a data structure representing a generally rectangular grid of pixels, or points of color, viewable via a monitor, paper, or other display medium...
display, and the beginning of the first line of the next frame. It is present in analog television, VGA
Video Graphics Array
Video Graphics Array refers specifically to the display hardware first introduced with the IBM PS/2 line of computers in 1987, but through its widespread adoption has also come to mean either an analog computer display standard, the 15-pin D-subminiature VGA connector or the 640×480 resolution...
, DVI
Digital Visual Interface
The Digital Visual Interface is a video interface standard covering the transmission of video between a source device and a display device. The DVI standard has achieved widespread acceptance in the PC industry, both in desktop PCs and monitors...
and other signals. During the VBI the incoming data stream is not displayed on the screen. In raster cathode ray tube
Cathode ray tube
The cathode ray tube is a vacuum tube containing an electron gun and a fluorescent screen used to view images. It has a means to accelerate and deflect the electron beam onto the fluorescent screen to create the images. The image may represent electrical waveforms , pictures , radar targets and...
displays the beam is blanked to avoid displaying the retrace line; see raster scan
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 systems...
for details.
The VBI was originally needed because of the inductive inertia of the magnetic coils which deflect the electron beam vertically in a CRT
Cathode ray tube
The cathode ray tube is a vacuum tube containing an electron gun and a fluorescent screen used to view images. It has a means to accelerate and deflect the electron beam onto the fluorescent screen to create the images. The image may represent electrical waveforms , pictures , radar targets and...
; the magnetic field, and hence the position being drawn, 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
Horizontal blank
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 back and forth from the left to right side of the screen...
. 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 for datacasting
Datacasting
Datacasting 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 television, but may also be applied to digital signals on analog TV or radio...
(to carry digital data), since nothing sent during the VBI is displayed on the screen; various test signals, time code
Vertical interval timecode
Vertical Interval TimeCode is a form of SMPTE timecode embedded as a pair of black-and-white bars in a video signal. These lines are typically inserted into the vertical blanking interval of the video signal...
s, closed captioning
Closed captioning
Closed captioning is the process of displaying text on a television, video screen or other visual display to provide additional or interpretive information to individuals who wish to access it...
, teletext
Teletext
Teletext is a television information retrieval service developed in the United Kingdom in the early 1970s. It offers a range of text-based information, typically including national, international and sporting news, weather and TV schedules...
, CGMS-A
CGMS-A
Copy Generation Management System - Analog is a copy protection mechanism for analog television signals. It consists of a waveform inserted into the non-picture Vertical Blanking Interval of an analogue video signal...
copy-protection indicators, and various data encoded by the XDS
Extended Data Services
Extended Data Services , is an American standard classified under Electronic Industries Alliance standard CEA-608-E for the delivery of any ancillary data to be sent with an analog television program, or any other NTSC video signal.XDS is used by TV stations, TV networks, and TV program...
protocol (e.g., the content rating
Content rating
Content rating most often means the suitability-to-audience of TV broadcasts, movies, comic books, or computer games.The introduction of Internet to the global population has revolutionized the way of consuming and gaining access to the virtually unlimited variety of contents...
s for V-chip
V-chip
V-chip is a generic term for technology used in television set receivers in the USA, Canada, and Brazil which allows the blocking of programs based on their ratings category. It is intended for use by parents to manage their children's television viewing...
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 the 16-bit era
History of video game consoles (fourth generation)
In the history of computer and video games, the fourth generation began on October 30, 1987 with the Japanese release of Nippon Electric Company's PC Engine...
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
Atari 2600
The Atari 2600 is a video game console released in October 1977 by Atari, Inc. It is credited with popularizing the use of microprocessor-based hardware and cartridges containing game code, instead of having non-microprocessor dedicated hardware with all games built in...
difficult to program.
Special raster techniques on the Atari 2600
Atari 2600
The Atari 2600 is a video game console released in October 1977 by Atari, Inc. It is credited with popularizing the use of microprocessor-based hardware and cartridges containing game code, instead of having non-microprocessor dedicated hardware with all games built in...
, Nintendo Entertainment System
Nintendo Entertainment System
The Nintendo Entertainment System is an 8-bit video game console that was released by Nintendo in North America during 1985, in Europe during 1986 and Australia in 1987...
, 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
Overscan
Overscan is extra image area around the four edges of a video image that may not be seen reliably by the viewer. It exists because television sets in the 1930s through 1970s were highly variable in how the video image was framed within the cathode ray tube .-Origins of overscan:Early televisions...
area. The use of double buffering
Double buffering
In computer science, multiple buffering is the use of more than one buffer to hold a block of data, so that a "reader" will see a complete version of the data, rather than a partially-updated version of the data being created by a "writer"...
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
Macrovision
Rovi Corporation is a globally operating, US-based company that provides guidance technology, entertainment data, copy protection, industry standard networking and media management technology for digital entertainment devices and services...
copy protection
Copy protection
Copy protection, also known as content protection, copy obstruction, copy prevention and copy restriction, refer to techniques used for preventing the reproduction of software, films, music, and other media, usually for copyright reasons.- Terminology :Media corporations have always used the term...
scheme inserts pulses in the VBI, where the recorder expects a constant level, to disrupt recording to videotapes.
See also
- DatacastingDatacastingDatacasting 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 television, but may also be applied to digital signals on analog TV or radio...
- Horizontal blanking interval
- Nominal analogue blankingNominal analogue blankingNominal analog blanking or nominal analogue blanking is the outermost part of the overscan of a standard definition digital television image...
- Raster scanRaster scanA 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 systems...