Horizontal blanking interval

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 across the screen. Once the beam of the monitor has reached the edge of the screen, the beam 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.^[1]^[2]

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 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 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.

References

^ Gupta, R. G. (2006). Television Engineering and Video Systems. Tata McGraw-Hill. p. 62.
^ Pemberton, Alan (30 November 2008). "World Analogue Television Standards and Waveforms". Pembers' Ponderings.

Analog television broadcasting topics

Systems	180-line 405-line ( System A ) 441-line 525-line ( System J , System M ) 625-line ( System B , System C , System D , System G , System H , System I , System K , System L , System N ) 819-line ( System E , System F )

Color systems	NTSC PAL PAL-M PAL-S PALplus SECAM

Video	Back porch and front porch Black level Blanking level Chrominance Chrominance subcarrier Colorburst Color killer Color TV Composite video Frame (video) Horizontal scan rate Horizontal blanking interval Luma Nominal analogue blanking Overscan Raster scan Safe area Television lines Vertical blanking interval White clipper

Sound	Multichannel television sound NICAM Sound-in-Syncs Zweikanalton

Modulation	Frequency modulation Quadrature amplitude modulation Vestigial sideband modulation (VSB)

Transmission	Amplifiers Antenna (radio) Broadcast transmitter/Transmitter station Cavity amplifier Differential gain Differential phase Diplexer Dipole antenna Dummy load Frequency mixer Intercarrier method Intermediate frequency Output power of an analog TV transmitter Pre-emphasis Residual carrier Split sound system Superheterodyne transmitter Television receive-only Direct-broadcast satellite television Television transmitter Terrestrial television Transposer

Frequencies & Bands	Frequency offset Microwave transmission Television channel frequencies UHF VHF

Propagation	Beam tilt Distortion Earth bulge Field strength in free space Knife-edge effect Noise (electronics) Null fill Path loss Radiation pattern Skew Television interference

Testing	Distortionmeter Field strength meter Vectorscope VIT signals Zero reference pulse

Artifacts	Dot crawl Ghosting Hanover bars Sparklies

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Horizontal blanking interval