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| Older
monitors sometimes employed a scanning technique where odd lines were
first refreshed followed by the even lines. While this was an inexpensive
way of increasing screen resolution it caused flicker. For this reason,
most monitors are now non-interlaced. Non-interlaced monitors refresh
each line of the display in turn from top to bottom. |
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| Typical
computer monitors range in size from 14 to 21 inches. Most monitors sold
with Microsoft Windows come with either a 15 or 17-inch monitor. The size
of the monitor is determined by measuring the diagonal distance across
the surface of the cathode ray tube (CRT). Since the CRT is encased in
a plastic housing, also called a bezel, the viewable area of a monitor
is always less than the stated monitor size. The viewable area is then
the diagonal measurement of the portion of the CRT that is visible. Viewable area is often referred to as "vis" for viewable image size and sometimes "dvi" for diagonal viewable area. Typically, the viewable area of a monitor will be about one inch smaller than the stated CRT size. |
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| Pixels
or picture elements are the number of dots on the screen and are composed
of one red, green and blue glowing phosphor dot. The more dots,
the sharper the image. |
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| Dot
pitch refers to the distance between two same-colored red, green or blue
phosphor screen dots. Typical dot pitches range in distance from .25 to
.39 millimeters. A smaller DPI indicates that more pixels are present
in a measure of screen space and this yields a sharper screen image. |
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| Shadow-mask
monitors are sometimes called dot trio monitors because their design uses
phosphor dots arranged in red, green and blue triangles. The shadow-mask
name derives from the phosphor dots being separated by a metal mask.
Aperture-grille monitors, such as Sony Trinitron and Mitsubishi Diamondtron,
use a phosphor coating of alternating red, green and blue vertical stripes
instead of triangle dots. Aperture-grille is a more complicated
technology and requires 1, 2 or 3 wires be horizontally imbedded in the
monitor to act as image stabilizing reference points. Proponents
of aperture-grille monitors prefer their brighter colors while detractors
point out that the horizontal stabilizing wires are noticeable.
In general, shadow-mask monitors are more price-competitive, provide good
focus and use a nearly flat surface which reduces distortion. Aperture-grille
monitors are less heat sensitive which allows for higher color purity,
brighter due to increased phosphor surface and use darker glass which
enables increased contrast. |
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Degaussing is a process by which monitors reverse internal magnetization
by introducing a magnetic field that is stronger than the offending internal
magnetization. Most modern monitors are manufactured with a degaussing
coil and degauss at startup. Most modern monitors also have a manual
degaussing control. If your monitor develops a color problem on
part of the screen, it may be caused by either internal or external magnetic
interference. To correct the problem, degauss the monitor and remove
the offending external magnetic generator. |
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Resolution or desktop area as it is referred to in Windows is the measure
of dots on the screen. Higher resolutions result in more viewable information,
but the information will be smaller and more difficult to view. Windows
resolutions are stated in horizontal by vertical dimensions and maintain
an aspect ratio of 4 by 3. Typical Window resolutions are: 640 by 480,
800 by 600, 1024 by 768, 1152 by 864 and 1280 by 1024. In Windows, resolution
is set by double-clicking Control Panel, Display and then the Settings
tab.. |
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| Slow
refresh rates will cause a monitor to flicker. A refresh rate of 70Hz
is said to be flicker-free. The refresh rate is typically set by a utility
program furnished by the video card manufacturer.. |
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| The
number of colors that can be displayed on a monitor is actually limited
by video memory rather than the monitor. The terms 256 colors, high
colors and true colors are used to indicate the number of bits used to
store color information. The more bits, the more colors and the
more video memory required. 256 colors uses 8-bits and hence only
256 different colors are available for display if you select 256 colors.
High color uses 16-bits and hence 65,536 (64k) colors are available
for display by selecting high color. True colors uses 24-bits and
hence 16 million colors are available for display by selecting true colors.
Note, it is difficult for the human eye to distinguish a difference between
16 and 24-bit color. The difference between 8 and 16-bit
color is extremely noticeable. |
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| Aperture-grille
monitors such as Sony Trinitron, Mitsubishi Diamondtron and ViewSonic
SonicTron use a technology called aperture-grille that uses alternating
red, green and blue vertical phosphor stripes instead of dot triangles
to yield sharper images and brighter colors. Aperture-grille technology
uses 1, 2 or 3 (depending on monitor size) horizontal stabilizing wires
which can be seen as fine lines across the screen on bright images.
The visible horizontal stabilizing wires are a normal characteristic of
all aperture-grille monitors. |
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| Flickering
results when the "excited" phosphor begins to decay before being
refreshed. Typically, a vertical refresh rate above 70 Hz will eliminate
flicker. Remember, the vertical refresh rate, measured in Hertz,
represents the number of times per second the monitor/video card refreshes
the entire screen. Since our peripheral vision is more sensitive
to flicker, scan the edges of the display with your peripheral vision
to check for flicker. The Video Electronics Standards Association (VESA)
has set 85 Hz as the standard for flicker-free viewing, but it's generally
best to set the refresh rate at the lowest point at which flicker is not
noticeable. Excessive refresh rates degrade image quality and can
actually harm a monitor if the refresh rate is outside of the monitor's
supported range. |
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| External
electromagnetic influences can easily cause display distortion.
If your monitor suddenly displays distorted images or incorrect colors
on portions of the display, check for any magnetized objects around the
monitor. Monitors located next to each other, surge protectors,
overhead lights and speakers can negatively effect a monitor's display.
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| Monitors
typically degauss at startup often resulting in a snapping noise which
can be followed by a clicking noise that results from the monitor switching
into video mode after degaussing. |
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| A
failing monitor power supply will often cause images to shrink or expand
as the displayed image changes. |
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Unchanging colors on a portion of the display typically indicate a problem
with the random access memory of the video card. Try using the monitor
on another computer to ascertain if the problem resides with the video
card or the monitor. |
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| Monitors
use a DAC (digital to analog) circuit to convert the digital information
maintained in the random access memory of the video card into the analog
signals used to control the monitor's electron gun. If the entire
screen suddenly starts displaying colors incorrectly, it can indicate
a failure of the DAC circuit in the monitor. Try using the monitor
on another computer to ascertain if the problem resides with the video
card or the monitor. |
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