In detail, the Horizontal blanking interval consists of:

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

In the NTSC television standard, horizontal blanking occupies {{val|10.9|ul=us}} (17.2%) out of every {{val|{{#expr:572/9 round 1}}|u=us}} scan line. In PAL, it occupies {{val|12|u=us}} (18.8%) out of every {{val|64|u=us}} scan line.

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'.

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 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 Rondo of Blood, which was written for the PC Engine CD-ROM which does not support multiple background layers. The Super Nintendo Entertainment System's Mode 7 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
• Raster scan
• Vertical blanking interval

References