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. 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.[1][2] 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:
In the PAL television standard, the blanking level corresponds to the black level, whilst other standards, most notably NTSC, 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 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 NES's Mode 7 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.
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