Computer monitors have evolved rapidly over the last decade. This
white paper takes an in-depth look at monitor technology and
construction.
The cathode ray tube
In simple terms, a monitor operates very similarly to how your
regular television set works. The principle is based upon the use
of an electronic screen called a cathode ray tube or CRT, which is
the major (and most expensive) part of a monitor. The CRT is lined
with a phosphorous material that glows when it is struck by a
stream of electrons. This material is arranged into an array of
millions of tiny cells, usually called dots. At the back of the
monitor is a set of electron guns, which produce a controlled
stream of electrons, much as the name implies. To produce a picture
on the screen, these guns start at the top of the screen and scan
very rapidly from left to right. Then they return to the left-most
position one line down and scan again, and repeat this to cover the
entire screen. In performing this scanning or sweeping type motion,
the electron guns are controlled by the video data stream coming
into the monitor from the video card that varies the intensity of
the electron beam at each position on the screen. This control of
the intensity of the electron beam at each dot is what controls the
colour and brightness of each pixel on the screen. This all happens
extremely quickly and, in fact, the entire screen is drawn in a
small fraction of a second.There are three electron guns (on a
colour monitor) that control the display of red, green and blue
light respectively. The surface of the CRT is arranged to have
these dots placed adjacently in a specific pattern. There are
separate video streams for each colour coming from the video card,
which allows the different colours to have different intensities at
each point on the screen. By varying the intensity of the red,
green and blue streams, the full rainbow of colours is made
possible.The surface of the CRT only glows for a small fraction of
a second before beginning to fade. This means that the monitor must
redraw the picture many times per second to avoid having the screen
flicker as it begins to fade and then is renewed. This rapid
redrawing is called "refreshing" the screen.
Controls and
internal electronicsThe monitor is controlled by a special
dedicated circuit board inside the monitor. It contains the
"smarts" for reading the input from the video card and controlling
the display of information on the CRT. It also interfaces with (and
is controlled by) the controls on the front of the monitor.The
controls on the front of the monitor generally come in one of two
styles: analog or digital. (Note by the way that this has nothing
to do with whether the monitor itself is analog or digital, since
that refers to the type of signal the monitor takes as input.) Each
of these has its advantages and disadvantages.Analog controls
(usually dials on the front of the monitor) provide fine control
over settings such as brightness and contrast, and are simple and
intuitive to use. Digital controls generally involve the use of
push buttons that control the settings using an on-screen display.
These can either be easy to use or difficult, depending on how they
are implemented. Some monitors, in an attempt to save money on
buttons, use one button to select which feature you want to change
(brightness, contrast, horizontal position) and then two buttons
for "up" and "down". These can be confusing to use. Others use more
buttons and are more intuitive. There is one major advantage
associated with monitors that use digital controls. Often when
using multiple resolutions (say, 640x480 and 800x600), the position
and size of the screen will change when changing resolutions. Many
newer monitors with digital controls will save the position and
size settings associated with each resolution that the monitor
supports. This "memory" feature will save you from having to adjust
the screen settings whenever you change the resolution on the
monitor.Finally, the more controls you have on the outside of the
monitor, the better. Virtually all monitors include adjustments for
brightness, contrast, vertical size and position, and horizontal
size and position. Some stop there, while others add controls for
focus, pin cushioning and other image factors. This is beneficial
because, for safety reasons, you should not open up your monitor to
make adjustments.There is now in fact, a new type of control that
attempts to marry the simplicity of analog instruments with the
precision, "memory" features and high adjustment of digital
controls. Some monitors now have a control that is simply a
finger-operated dial with a push button. Pressing the button brings
up an on-screen menu and the dial is used to select different
adjustments from the menu and to change their values. It can still
be confusing to operate compared to separate controls for each
function. On the other hand, you gain by this method control over
many more monitor parameters than older analog-instrumentation
monitors typically had.
Power supplyThe monitor is the only
component of a regular PC that uses its own power supply, built
into the monitor itself. Older monitors often used a special plug
that connected to a receptacle at the back of the PC's power
supply. This receptacle is just a pass-through from the power cord
that the PC uses ( these monitors do not get power from the PC's
internal power supply. The chief advantage of this connection is
that the power to the monitor is turned off when the PC is turned
off. This type of plug has fallen out of favour in recent years,
although you can buy cheap adaptors that will convert a standard
electrical cord to the type that connects to the back of the
PC.Many monitors have a switch to select between 110-volt and
220-volt power. Obviously, it's important to select the correct
voltage depending on where you live. Also, many monitors have a
fuse in the back that will blow if the monitor takes a voltage
surge or there is another electrical problem.
Interface and
cablingThe monitor is connected to the video card through a
cable that attaches to a connector on its back. There are a few
different connectors used for monitors:
15-pin VGA
(Standard): The standard for connecting monitors and video
cards today is the 15-pin video cable and connector, also called
"VGA" in reference to the standard that first used it. In fact, all
modern cards that call themselves VGA or SVGA use the standard
15-pin connector on the video card. Most standard monitors use the
same 15-pin connector as well.
9-pin (Older VGA, EGA, CGA):
Older displays used a smaller, 9-pin connector on the monitor,
meant to match with the 9-pin output of older video cards. Some of
these were VGA, but others employed older standards such as CGA or
EGA. There are 15 to 9-pin adaptors available that will let a
monitor with a 9-pin connector be used on a PC with a 15-pin video
connector, but most of these monitors are older and not up to
dealing with a modern video card in any event. Warning: Make sure
not to use an older digital monitor with a modern (analog) video
card or you risk damaging the monitor. BNC Connectors: Some
high-performance monitors use special coaxial cable that has a
standard 15-pin connector for the video card on one end, but breaks
out to separate wires with BNC connectors on them for the monitor.
There are usually 5 wires, one for each of red, green, blue,
horizontal sync and vertical sync. Some monitors that have BNC
connectors on them also have a standard 15-pin VGA, and either can
be used. The BNC cable can be expensive, but provides better
shielding from noise and interference than the standard VGA cable.
It is found much more often on larger, more expensive units. Some
older monitors require a sync signal from the PC in order to know
what mode to put themselves into when they are turned on. When
using this sort of monitor, you should turn the PC itself on
several seconds before powering up the monitor. This will ensure
that the monitor sees a video signal when it is turned on so it
will work properly.One distinguishing feature that many people
don't pay much attention to is whether the monitor uses an
integrated or a separate data cable. Monitors that use separate
data cables have a female data port on the back of the monitor,
much like the port on the video card, and use a male-to-male cable
to attach the two. Monitors using an integrated data cable have the
cable coming from the back of the monitor directly with a single
male plug for the video card. Both designs work the same, but if
the cable is ever damaged, the monitor with the integrated cable
will have to go to the repair shop, while the one with the separate
cable can be fixed with a £16 replacement.
Monitor caseThe
monitor is usually encased in a plastic box that protects it.
There's nothing too remarkable about the case and not much needs to
be said about it, except for one thing. The components inside the
monitor generates a lot of heat, and every monitor case has special
ventilation slots (usually many of them) to allow for cooling of
these components. It's important to make sure not to block these
slots off or the monitor could overheat and malfunction.
( PC
Guide 1999Compiled by Will Garside
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