Horizontal blanking interval refers to a part of the process of displaying images on a computer monitor or
television
Television (TV) is a telecommunication medium for transmitting moving images and sound. Additionally, the term can refer to a physical television set rather than the medium of transmission. Television is a mass medium for advertising, ...
screen via
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 ...
ning. CRT screens display images by moving beams of electrons very quickly across the screen. Once the beam of the monitor has reached the edge of the screen, it is switched off, and the deflection circuit voltages (or currents) are returned to the values they had for the other edge of the screen; this would have the effect of retracing the screen in the opposite direction, so the beam is turned off during this time. This part of the line display process is the Horizontal Blank.
In detail, the Horizontal blanking interval consists of:
*
front porch – blank while still moving right, past the end of the scanline,
*
sync pulse
Analog television is the original television technology that uses analog signals to transmit video and audio. In an analog television broadcast, the brightness, colors and sound are represented by amplitude, phase and frequency of an analog s ...
– blank while rapidly moving left; in terms of amplitude, "blacker than black".
*
back porch – blank while moving right again, before the start of the next scanline.
Colorburst
Colorburst is an analog and composite video signal generated by a video-signal generator used to keep the chrominance subcarrier synchronized in a color television signal. By synchronizing an oscillator with the colorburst at the back p ...
occurs during the back porch, and unblanking happens at the end of the back porch.
In the
NTSC
NTSC (from National Television System Committee) is the first American standard for analog television, published and adopted in 1941. In 1961, it was assigned the designation System M. It is also known as EIA standard 170.
In 1953, a second ...
television standard, horizontal blanking occupies out of every scan line (17.2%). In
PAL
Phase Alternating Line (PAL) is a color encoding system for analog television. It was one of three major analogue colour television standards, the others being NTSC and SECAM. In most countries it was broadcast at 625 lines, 50 fields (25 ...
, it occupies out of every scan line (18.8%).
Some modern monitors and video cards support ''reduced blanking'', standardized with
Coordinated Video Timings.
"What does 'Rb' mean?"
In the PAL television standard, the blanking level corresponds to the black level, whilst other standards, most notably some variants of NTSC, may set the black level slightly above the blanking level on a ''pedestal'' or "set up level".
HBlank effects
Some graphics systems can count horizontal blanks and change how the display is generated during this blank time in the signal; this is called a raster effect, of which an example is raster bars.
In video games, the horizontal blanking interval was used to create some notable effects. Some methods of parallax scrolling use a raster effect to simulate depth in consoles that do not natively support multiple background layers or do not support enough background layers to achieve the desired effect. One example of this is in the game '' Castlevania: Rondo of Blood'', which was written for the PC Engine CD-ROM which does not support multiple background layers. The Super Nintendo Entertainment System
The Super Nintendo Entertainment System, commonly shortened to Super Nintendo, Super NES or SNES, is a Fourth generation of video game consoles, 16-bit home video game console developed by Nintendo that was released in 1990 in Japan, 1991 in No ...
's Mode 7
Mode 7 is a graphics mode on the Super Nintendo Entertainment System video game console that allows a background layer to be rotated and scaled on a scanline-by-scanline basis to create many different depth effects. It also supports wrapping eff ...
uses the horizontal blanking interval to vary the scaling and rotation, per scan line, of one background layer to make the background appear to be a 3D plane.
See also
*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 ...
*Vertical blanking interval
In a raster scan 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 fra ...
References
{{DEFAULTSORT:Horizontal blanking interval
Video signal
Television technology
Television terminology