Specifications

Video Display Technologies
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Chapter 15
Red, green,
and blue
phosphors
Glass
panel
Shadow
mask
Integral implosion
protection
Internal
magnetic
shield
Electron gun
assembly
Deflection yoke
Glass
funnel
Figure 15.1 A typical CRT monitor is a large vacuum tube. It contains three electron guns (red, green,
and blue) that project the picture toward the front glass of the monitor. High voltage is used to produce the
magnetism that controls the electron beams that create the picture displayed on the front of the CRT.
The electron beam moves very quickly, sweeping the screen from left to right in lines from top to bot-
tom, in a pattern called a raster. The horizontal scan rate refers to the speed at which the electron
beam moves laterally across the screen.
During its sweep, the beam strikes the phosphor wherever an image should appear onscreen. The
beam also varies in intensity to produce different levels of brightness. Because the glow begins to fade
almost immediately, the electron beam must continue to sweep the screen to maintain an image—a
practice called redrawing or refreshing the screen.
Most current CRT displays have an ideal refresh rate (also called the vertical scan frequency) of about
85 hertz (Hz), which means the screen is refreshed 85 times per second. Refresh rates that are too low
cause the screen to flicker, contributing to eyestrain. The higher the refresh rate, the better for your
eyes. Low-cost monitors often have flicker-free refresh rates available only at 640×480 and 800×600
resolutions; you should insist on high refresh rates at resolutions such as 1024×768 or higher.
It is important that the refresh rates expected by your monitor match those produced by your video
card. If you have mismatched rates, you will not see an image and can actually damage your monitor.
Generally speaking, video card refresh rates cover a higher range than most monitors. For this reason,
the default refresh rate used by most video cards is relatively low (usually 60Hz) to avoid monitor
damage. The refresh rate can be adjusted through the Windows display properties sheets.
Multiple Frequency Monitors
Although a few very old monitors had fixed refresh rates, most monitors support a range of frequencies.
This support provides built-in compatibility with a wide range of current and future video standards
(described in the “Video Display Adapters” section later in this chapter). A monitor that supports many
video standards is called a multiple-frequency monitor. Virtually all monitors sold today are multiple fre-
quency, which means they support operation with a variety of popular video signal standards. Different
have a good match between persistence and scanning frequency so the image has less flicker (which occurs
when the persistence is too low) and no ghost images (which occurs when the persistence is too high).
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