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Refreshing the Screen Image
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In traditional screens, an electron gun continually sends out very
precisely aimed beams of electrons, moving from pixel to pixel. The beam
actually flickers as it sweeps the screen. Each dot on the screen receives
a quick flash of electrons before the beam moves on to the next dot with
the beam intensity varying from dot to dot.
The phosphor coating on the screen has the peculiar ability to light
up when hit by electrons but the light quickly fades away. In practice
the electron beam "visits" again before there is any visible fading of
the light. It looks to us as a steady picture on the screen, but actually
it flickers every time the electron beam hits the phosphor coated dots.
The video card issues the refresh signals, thus controlling the refresh
rate. Typically, each pixel is hit 60, 70, 75, or 80 times per second.
Thus, the electron gun must move extremely fast to make 18 million or more
hits per second. If the image is refreshed 75 times per second, we talk
about a refresh rate of 75 Hz. Therefore a screen with a resolution of
1280 x 1024 and a refresh rate of 75 Hz requires the electron gun to make
98 million pixel hits per second which can sometimes result in beam contamination. |
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High Refresh Rate
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| The screen image appears more steady the higher the refresh rate. You
see the same in a TV, where traditional sets have a refresh rate of only
50 Hz. Some manufacturers now produce TV sets with 100 Hz refresh rate.
Some claim that they cannot notice the difference. However, once you have
been used to 100 Hz refresh rate, it is uncomfortable to return to 50 Hz.
Similarly with PC monitors, only here we have more options.
Older and inferior screens can only work at 60 Hz which produces a low
quality flickering image which is not suitable for Windows. The general
consensus is that 70 Hz produces an acceptable image.
Note: refresh rate is also called vertical frequency or vertical refresh
rate.
The higher the refresh rate, the better quality monitor you need. If
you want both high resolution and high refresh rate, you will need both
a high quality monitor and a high quality video card. The bigger the screen,
the more it must be able to produce. Screens can always run with higher
refresh rates in lower resolutions. Here are three examples, showing how
the screen performance drops with resolution.
| Screen |
800 x 600 |
1024 x 768 |
1280 x 1024 |
1600 x 1200 |
| Standard 15" |
75 HZ |
70 Hz |
60 Hz |
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| 15" Trinitron |
90 Hz |
80 Hz |
75 Hz |
- |
| 17" Trinitron |
110 Hz |
100 Hz |
90 Hz |
85 Hz |
For the screen to deliver images at the desired refresh rate, both screen
and video card must be matched to the correct specifications - the higher
your demands are, the higher the cost will be. A good monitor usually is
expensive. Cheap monitors may function at high refresh rates, but the image
may not be good, Always check a new monitor visually before buying it! |
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Horizontal Scan Frequency
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The most important factors are maximum resolution and refresh rate.
The screen must be able to deliver an image in a suitable resolution (depending
on screen size) and at a good refresh rate (75 Hz or more). The screen
can display many different image types – in various resolutions and refresh
rates. The interesting point is the maximum refresh rate at different
resolutions and this data is often reported together in a number, called
the horizontal scan frequency. The number is measured in KHz and
it is very important. Basically, the horizontal scan frequency is calculated
from resolution and refresh rate. As an example, an 800 x 600 resolution
at 75 Hz gives a horizontal scan frequency of 60 KHz. You cannot calculate
the number yourself. Also it varies slightly from screen to screen. Here
are examples of horizontal scan frequency. The numbers can vary slightly
from screen to screen:
| Different performances |
| Resolution |
Refresh rate |
Horizontal scan frequency |
| 640 x 480 |
60 Hz |
31.5 KHz |
| 640 x 480 |
72 Hz |
37.8 KHz |
| 800 x 600 |
75 Hz |
46.9 KHz |
| 800 x 600 |
85 Hz |
53.7 KHz |
| 1024 x 768 |
75 Hz |
60.0 KHz |
| 1024 x 768 |
85 Hz |
68.8 KHz |
| 1152 x 864 |
85 Hz |
77.6 KHz |
| 1280 x1024 |
75 Hz |
80.0 KHz |
| 1280 x 1024 |
85 Hz |
91.2 KHz |
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Multisync Screen with Digital Control
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| All modern screens are of the multisync type. This means, that
the screen adjusts itself to the signals received. The individual model
has a minimum and maximum horizontal scan frequency. As long as the signals
are received within that spectrum, it adjusts itself to the signals.
When the screen receives signals at any given frequency, these signals
must be adjusted to fill the screen 100%. That is done through the digital
controller found in modern screens. Older screens would show a clear
black border surrounding the image, whenever the resolution was changed
to, lets say 800 x 600.
To enable adjustment to maximum screen utilization, the screen must
have digital control electronics. These adjustments are made on the screen
control panel. We are talking about:
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Horizontal and vertical size, to have the image fill the maximum usable
screen area.
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Horizontal and vertical positioning, to center the image.
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Compensation for trapezoid and pin cushioning.
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Colors and light intensity.
Often screens are preset to a choice of different possible adjustments.
In these preset conditions, the image will immediately appear perfect.
However, when you set up a monitor to work under non preset conditions
you have to adjust the image yourself. Once that is done the monitor will
remember your settings. There are no international standards for the design
of these digital controllers. They are quite different from monitor to
monitor and not all easy to work with. However, working with adjustments
is a minor problem, relative to other monitor qualities. Often you see
the term dot pitch or aperture grill pitch. It is measured in millimeters.
The numbers indicate the average distance between individual screen dots.
The smaller the better. That provides a finer grain screen. For large screens
(21"), the dot pitch can be 0.31 mm or 0.28 mm. Otherwise, a dot pitch
of 0.28 mm or 0.25 mm is considered sufficiently good for ordinary 15"
and 17" screens. A few monitors offer 0.22 mm dot pitch. |
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