Crossing the boundary: analogue universe, digital worlds
Crossing the boundary: analogue universe, digital worlds

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5.5.1 Monitors

Nearly all computers are supplied complete with a monitor which opens a window onto the machine's digital world. Without one we could have little idea about what the computer was doing, or even whether it was working at all.

There are two main types of monitor: the CRT (cathode ray tube) and LCD (liquid crystal display). A CRT monitor looks like a television screen, and works in a similar way to a TV or a scanner. A beam of electrons is fired from a gun at the back of the tube onto a glass screen on the front. The beam sweeps across this screen in the same raster scanning pattern illustrated in Figure 25. The front of the tube is coated in a phosphorescent material that glows as the electron beam hits it. In this way the picture is built up.

SAQ 21

Do you think your monitor gives a digital or analogue image? Look closely at the image on your own monitor if need be.


If you look very closely at your screen while it is displaying an image, you will see the pixels. So the monitor supplies a digital display.

Usually this digital image is of high enough resolution to trick the eye into interpreting it as analogue. High-quality professional monitors can give resolutions of up to 1920×1440 pixels or more, although 1024×768 and 1280×1024 are the most common. However, watching even a good quality CRT monitor for a long time can be tiring, as the scanning of the electron beam gives a flickering sensation.

You will probably have seen LCD monitors, though you may not own one. They are completely flat, because they have no electron gun. They work on the principle of passing light through a special material, the molecules of which change their orientation when an electrical voltage is applied to them. This is the same principle on which the screens of mobile phones and calculators work. On a computer LCD monitor, there is one tiny unit of such material for each pixel on the screen, so once again a digital picture is produced.

Although professional graphic artists still favour CRT monitors for their colour accuracy, LCDs are to be preferred for general use. They use much less power than CRTs; they take up little space; and they are much less tiring to work on – every pixel is always on, so they do not flicker. Resolutions vary, but 1280×1024 pixels is common.

Finally, though, note that the resolution actually displayed on the screen is not decided by the monitor itself, but by the program that prepares the digital encodings for display. This program allows a user to set the resolution to any one of a range of possibilities, depending on how much memory the computer has available. For example, the monitor I use at work can resolve up to a maximum of 1920×1440, but I usually opt for a lower resolution than this.

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