Karbosguide.com - Module 7a.4

Refreshing the screen image

The contents:

  • Electronic beams
  • High refresh rate
  • The horizontal scan frequency
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  • Electronic beams

    In traditional CRT monitors, the 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. And the beam intensity is varied 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.

    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 screen works overtime

    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.

    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

    The screen image appears more steady, the higher the refresh rate. You see the same in 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.

    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.

    And please remember: You will have the monitor for an average of 5 years. It will serve more than one PC, so buy quality!

    More about screens

    Let us take a closer look at the monitors. If you read ads for monitors, you might see many hard to understand technical terms. They may mention many frequencies and dots and pitch?

    Note: In many ads, these terms (frequencies, etc.) can appear mixed and unclear. Therefore, be critical when you read monitor data.

    Trinitron or Invar

    When we talk about traditional CRT monitors, there are two primary types of tubes. The best use the so called Trinitron tube. That is a technological principle, which was patented by the Sony company. Since the patent has expired, there are now some clones (ChromaClear, SonicTron etc.).

    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.

    Invar for contrast

    The traditional screen can provide more contrast than the Trinitron screens, which is important in some technical applications. But for ordinary use in home and offices, where you would typically choose 17" or 19" screens the Trinitron screens are an obvious choice. Of course, they cost a little more than traditional types, but there is a marked difference in the visible quality. You will experience a much better screen image with a Trinitron tube, no doubt about that! But the best is a TFT display, as I'll show you later.

    The horizontal scan frequency

    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.

    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.

    NOTE: Using a digital interface for a TFT monitor, there is no horizontal scan frequency to concern about!

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    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 .

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