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Horizontal blanking interval refers to a part of the process of displaying images on a television screen via raster scanning. These 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 only on during part of the trip: once the beam of the monitor has reached one side 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 always off, and this part of the display process is the Horizontal Blank.
In detail, it consists of:
- front porch – blank while still moving right, past the end of the scanline,
- sync pulse – blank while rapidly moving left,
- back porch – blank while moving right again, before the start of the next scanline.
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, some methods of parallax scrolling use a raster effect to simulate depth.
See also
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