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.
The result is that the it looks to us as a steady screen image. But actually the pixels of the image flickers every time the electron beam hits the phosphor coated dots.
The video card issues the refresh signals, thus controlling the refresh rate. Thus, the video card has to match the monitor, so the two units can interface with a suitable electronical signal.
Let us think of a monitor with a resolution of 1280 x 1024 and a refresh rate of 75 Hz. That requires the electron gun to make 98 million pixel hits per second! That screen works at a very hectic pace – which can sometimes result in beam contamination.
High refresh rate
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.
I find 75 Hz acceptable, but 80 or 85 Hz may be better when you have to work many hours daily in front of the screen. You have to try these rates to find the best on your gear. Not unoften 75 Hz is the best refresh rate.
Here you see a dump from settings of a ATI Radeon graphics controller. It can deliver 11 different refresh rates (from 43 Hz to 160 Hz) in the the 1280 x 1024 resolution:
Note: refresh rate is also called vertical frequency or vertical refresh rate, but I have chosen to use the term 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.
|CRT Screen||800 x 600||1024 x 768||1280 x 1024||1600 x 1200|
|Standard 15"||75 HZ||70 Hz||60 Hz||-|
|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. Normally the CRT monitors have a feature called multisync. This means, that they automatically adapt to the signal coming from the video controller.
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.
More about screens
Trinitron or Invar
In the Trinitron screens, the light sensitive pixels on the inside of the tube are placed in a vertical grid, while traditional screens have round masks for the color dots. With the grid mask, you can achieve denser coverage and thus more color saturated images. Here is an attempt to illustrate the difference between those masks:
The Trinitron screens are generally very high quality. Since the Trinitron tube is more expensive than the traditional Invar tubes, manufacturers also include better control electronics in the Trinitron tubes. That increases their price somewhat, but that money is well spent!
The only disadvantage of the Trinitron (besides price) is the thin lines, which run across the screen. They are visible wires, which contain a grid. In daily work, you will not notice them, but rather enjoy the pleasure of an extremely fine and sharp image.
The horizontal scan frequency
These data are 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. As I said, the numbers can vary slightly from screen to screen, but they are still in the same ball park:
|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|
Ususally you get the best performance using the highest refresh rate available. The resolution depends on screen size and user habits. In all cases, it would be foolish to run the screen at 31.5 KHz.
To learn more
Read about video cards in Module 7b .
Read about sound cards in Module 7c .
Read about digital sound and music in Module 7d .