Hardware Reference
In-Depth Information
are physically smaller, they are often more readable with less eyestrain because they are
sharp and perfectly focused.
CRTsdon'thavea1:1relationship betweenresolutionandpixels.Therefore,whenyou're
comparing CRTs, the smaller the pixel pitch, the sharper the images will be. As an ex-
ample, the original IBM PC color monitor from the early 1980s had a pixel pitch of
.43mm, whereas newer color CRTs have a pixel pitch between .25mm and .27mm, with
high-end models offering .24mm orless. Toavoid grainy images onaCRT,lookforthose
with a pixel pitch of .26mm or smaller.
Horizontal and Vertical Frequency
Analog display connections such as VGA are designed to transmit signals that drive the
display to draw images. These signals tell the display to draw an image by painting lines
of pixels from left to right and from top to bottom. For example, if the display resolution
is1024×768,thatmeansthereare768linesthatwouldbedrawn,oneaftertheother,from
toptobottom.Oncethe768thlineisdrawn,theentireimagewouldbecompleted,andthe
process would repeat starting again from the top.
The speed at which this image drawing occurs has two components, called the horizontal
frequency and the vertical frequency . These frequencies are also called scan or refresh
rates . The horizontal frequency is the speed in which the horizontal lines are drawn, ex-
pressed as the total number of lines per second. The vertical frequency (or vertical refresh
rate)isthespeedinwhichcompleteimagesaredrawn,expressedinthenumberofimages
per second.
Using a 1024×768 display as an example, if the vertical refresh rate is 60Hz, then all of
the 768 lines that make up the image would need to be drawn 60 times per second, result-
inginahorizontalfrequencyof768linesperimage×60imagespersecond,whichequals
46,060 total lines per second, or a frequency of about 46KHz. If the vertical refresh rate
were increased to 85Hz, the horizontal frequency would be 768 × 85 = 65,280, or about
65.3KHz. The actual figures are technically a bit higher at 47.8KHz and 68.7KHz, re-
spectively, because there is about a 5% overhead called the vertical blanking interval that
was originally designed to allow the electron beam to move from the bottom back to the
topofthescreenwithoutbeingseen.AlthoughthereisnoelectronbeaminLCDdisplays,
theblankingtimeisstillusedforbackwardcompatibilityaswellastosendadditionaldata
thatisnotpartoftheimage.Theexactamountofverticalandhorizontalblankingtimere-
quired varies by the resolution and mode. It is governed by the VESA CVT (Coordinated
Video Timings) standard.
Displays with analog connections usually have a range of scan frequencies that they can
handle,whicheffectivelycontrolstheminimumandmaximumresolutionsthatcanbedis-
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