Peripherals: Cathode ray tubes

Cathode ray tubes

Most computer monitors are based on cathode ray tubes (CRTs), similar to those used in TV sets. They consist of one or more 'guns' which fire streams of electrons at a special chemical which backs the surface of the monitor. These electron streams repeatedly scan the screen from top to bottom, dot by dot and line by line, each scan taking only a fraction of a second. In colour screens, there are three guns, one for each of the colours red, green and blue. These cause each dot on the screen to generate red, green, or blue light, the combination of these three giving the full colour spectrum. CRTs give a bright picture, with good colours, but they are bulky and consume a relatively large amount of power.

Monochrome models display text and graphics as either green on a black background, orange on a black back­ ground, or white on a black background. (The foreground and background colours can be reversed by software.) Monochrome monitors are quite cheap, and they give a sharp picture. They are well suited for office applications, such as word processing, which do not require colour.

The more expensive colour or RGB monitors give colour displays, but the picture sharpness on the cheaper models will not be as good as that achieved by monochrome monitors. RGB stands for Red-Green-Blue, and the CRTs in these monitors contain the three electron guns described above, each directly controlled by the computer.

Flat screens

Portable computers require screens which are light, occupy little space, and, in the case of battery-operated models, don't consume much power. The various types of flat screen that are on the market meet this need. Unlike CRTs, though, the brightness of the image is generally limited, and, at the time of writing, very few flat screen monitors give colour displays.

Liquid crystal display (LCD) monitors are used on bat­tery-operated portable computers, as these consume very little power. (This is the type of display used on digital watches.) However, compared to CRTs the displays tend to lack contrast and in some light conditions are not very legible, though the newer 'supertwist' LCD displays are quite good.

In an LCD, the image is formed by so-called liquid crystals. These are long rod-like molecules which, though solid, can flow like a liquid. Each pixel on the screen consists of a microscopic electrode positioned below several of these molecules. As the output from the computer scans the screen a row at a time, it activates each of these pixels in turn, switching it on or off. When a pixel is 'on', the crystals twist in such a way that they block out the light. When it is 'off', they let the light through. In early LCDs, the crystals were not very 'liquid', i.e. they did not respond very rapidly to the signal, but modern types of liquid crystal are much faster.

Colour LCDs require very fast liquid crystals. In what's called passive matrix systems there are three screens, col­oured red, green, and blue, which are placed on top of each other. Below the three screens are the thousands of pixel electrodes which generate the light. The pixels in each screen act as filters for this light. If a pixel is to be red, for example, the green and blue filters above it are turned on, blocking out those colours, but the red filter is off, allowing the red light through. By turning the filters on and off in other combinations, other colours can be obtained.

Passive matrix displays are not very bright, however, since to display red, for example, light from only one out of the three crystals is allowed through the filters. They also consume too much power to be viable for battery-powered computers, and the screen refresh rate is rather slow. So active matrix displays are being developed, which use thin film transistor (TFf) screens. In these, there is a single screen, each pixel of which has three sub-pixels coloured red, green, and blue. These sub-pixels are separately acti­vated to produce the colour output. Active matrix screens are, at the present time, difficult to manufacture and very expensive, so they are available on only a few top-of-the­ range portables.

Two other types of flat screen are sometimes used for computer monitors, namely the gas plasma display, and the electroluminescent display. These are both light-emitting displays, and give a very clear, legible output, but they consume almost as much power as a conventional monitor. They are therefore suitable for portable computers which are designed to run off the mains, but not for battery­ powered models. These displays are monochrome only -typically orange on a black or brown background, as this is reckoned to be both highly legible and restful on the eyes.