View source for Engineering:Horizontal blanking interval

{{short description|Part of the process of displaying images on a raster scan monitor}}

'''Horizontal blanking interval''' refers to a part of the process of displaying images on a computer monitor or [[Engineering:Television|television]] [[Engineering:Display device|screen]] via [[Raster scan|raster scan]]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.<ref name="tveng" /><ref name="analogtv" /> 

In detail, the Horizontal blanking interval consists of:
* [[Engineering:Analog television#Structure of a video signal|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".
* [[Engineering:Analog television#Structure of a video signal|back porch]] – blank while moving right again, before the start of the next scanline. [[Engineering:Colorburst|Colorburst]] occurs during the back porch, and unblanking happens at the end of the back porch.

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

Some modern monitors and video cards support ''reduced blanking'', standardized with [[Coordinated Video Timings]].<ref>
[http://tvone.crmdesk.com/answer.aspx?aid=448&back= "What does 'Rb' mean?"].
</ref>

In the PAL television standard, the blanking level corresponds to the [[Engineering:Black level|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 is [[Raster bar|raster bar]]s.

In video games, the horizontal blanking interval was used to create some notable effects. Some methods of [[Parallax scrolling|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 ''[[Software:Castlevania: Rondo of Blood|Castlevania]]'', which was written for the [[Engineering:TurboGrafx-16#TurboGrafx-CD/CD-ROM²|PC Engine CD-ROM]] which does not support multiple background layers. The [[Engineering:Super Nintendo Entertainment System|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 ==
*[[Engineering:Nominal analogue blanking|Nominal analogue blanking]]
*[[Engineering:Vertical blanking interval|Vertical blanking interval]]

==References==
{{Reflist |refs= 
<ref name="tveng">{{Cite book
 | last = Gupta
 | first = R. G.
 | title = Television Engineering and Video Systems
 | publisher = Tata McGraw-Hill
 | year = 2006
 | page = 62
 | isbn = 0-07-058596-2
 | url = https://books.google.com/books?id=P6BlcWaizHUC
 | access-date = 25 September 2010
 }}
</ref>

<ref name="analogtv">{{Cite web
 | url = http://www.pembers.freeserve.co.uk/World-TV-Standards/index.html
 | title = World Analogue Television Standards and Waveforms
 | first = Alan
 | last = Pemberton
 | date = 30 November 2008
 | work = Pembers' Ponderings
 | location = Sheffield, England
 | archive-url = https://web.archive.org/web/20080220113153/http://www.pembers.freeserve.co.uk/World-TV-Standards/index.html
 | archive-date = Feb 20, 2008
 | access-date = 25 September 2010
 }}
</ref>

}}

{{Analogue TV transmitter topics}}

{{DEFAULTSORT:Horizontal blanking interval}}
[[Category:Video signal]]
[[Category:Television technology]]
[[Category:Television terminology]]

{{Sourceattribution|Horizontal blanking interval}}