Horizontal blank
Encyclopedia
Horizontal blanking interval refers to a part of the process of displaying images on a computer monitor or television
Television
Television is a telecommunication medium for transmitting and receiving moving images that can be monochrome or colored, with accompanying sound...

 screen
Display device
A display device is an output device for presentation of information in visual or tactile form...

 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. However, the beams are positively-charged during part of the trip: once the beam of the monitor has reached the right edge of the screen, it is quickly moved back to the left side of the screen. As the beam is being retraced (directed back), it is negatively-charged, and this part of the display process is the Horizontal Blank. Phosphors only react to positive charge. Switching the line output transformer on and off during the horizontal scanning will result in some undesirable side-effects on the picture and greatly reduce the reliability of the line output transformer, which is unreliable enough as it is!

In detail, the Horizontal blanking interval consists of:
  • front porch
    Front Porch
    Front Porch, Inc. provides services to Internet Service Providers. Front Porch technology enables an Internet Service Provider to insert its own messages to be presented to users as they use their web browsers, such as customer service notices or online advertising...

     – blank while still moving right, past the end of the scanline,
  • sync pulse – 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
    Colorburst is a analog video, composite video signal generated by a video-signal generator used to keep the chrominance subcarrier synchronized in a color television signal...

     occurs during the back porch, and unblanking happens at the end of the back porch.


In the PAL
PAL
PAL, short for Phase Alternating Line, is an analogue television colour encoding system used in broadcast television systems in many countries. Other common analogue television systems are NTSC and SECAM. This page primarily discusses the PAL colour encoding system...

 television standard, the blanking level corresponds to the black level
Black level
Video black level is defined as the level of brightness at the darkest part of a visual image or the level of brightness at which no light is emitted from a screen, resulting in a pure black screen....

, whilst other standards, most notably NTSC
NTSC
NTSC, named for the National Television System Committee, is the analog television system that is used in most of North America, most of South America , Burma, South Korea, Taiwan, Japan, the Philippines, and some Pacific island nations and territories .Most countries using the NTSC standard, as...

, set the black level slightly above the blanking level on a 'pedestal'.

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 are raster bar
Raster bar
The raster bar is an effect used in demos that displays animated horizontal bars of colour that extend into the overscan area of the display...

s.

In video games, the horizontal blanking interval was used to create some notable effects. Some methods of parallax scrolling
Parallax scrolling
Parallax scrolling is a special scrolling technique in computer graphics, popularized in the 1982 arcade game Moon Patrol. In this pseudo-3D technique, background images move by the camera slower than foreground images, creating an illusion of depth in a 2D video game and adding to the immersion...

 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
Castlevania: Rondo of Blood
Castlevania: Rondo of Blood, known in Japan as is a platform video game developed by Konami for the PC Engine. The tenth installment of the Castlevania video game series and a 2D side-scroller, it acts as a middleground between the earlier, typically linear Castlevania games and the later...

, which was written for the PC Engine CD-ROM which does not support multiple background layers. The Super NES
Super Nintendo Entertainment System
The Super Nintendo Entertainment System is a 16-bit video game console that was released by Nintendo in North America, Europe, Australasia , and South America between 1990 and 1993. In Japan and Southeast Asia, the system is called the , or SFC for short...

's Mode 7
Mode 7
Mode 7 is a graphics mode on the Super NES video game console that allows a background layer to be rotated and scaled on a scanline-by-scanline basis to create many different effects. The most famous of these effects this can create is the application of a perspective effect on a background layer...

 used the horizontal blanking interval to vary the scaling and rotation of one background layer on a scanline by scanline basis to create many different effects. The most famous and hyped effect of Mode 7 was to turn the background layer into a planar texture map
Texture mapping
Texture mapping is a method for adding detail, surface texture , or color to a computer-generated graphic or 3D model. Its application to 3D graphics was pioneered by Dr Edwin Catmull in his Ph.D. thesis of 1974.-Texture mapping:...

.
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