Feature

White Paper: Looking inside the display

Computer monitors have evolved rapidly over the last decade. This white paper takes an in-depth look at monitor technology and construction.

The cathode ray tube

In simple terms, a monitor operates very similarly to how your regular television set works. The principle is based upon the use of an electronic screen called a cathode ray tube or CRT, which is the major (and most expensive) part of a monitor. The CRT is lined with a phosphorous material that glows when it is struck by a stream of electrons. This material is arranged into an array of millions of tiny cells, usually called dots.

At the back of the monitor is a set of electron guns, which produce a controlled stream of electrons, much as the name implies. To produce a picture on the screen, these guns start at the top of the screen and scan very rapidly from left to right. Then they return to the left-most position one line down and scan again, and repeat this to cover the entire screen. In performing this scanning or sweeping type motion, the electron guns are controlled by the video data stream coming into the monitor from the video card that varies the intensity of the electron beam at each position on the screen. This control of the intensity of the electron beam at each dot is what controls the colour and brightness of each pixel on the screen. This all happens extremely quickly and, in fact, the entire screen is drawn in a small fraction of a second.

There are three electron guns (on a colour monitor) that control the display of red, green and blue light respectively. The surface of the CRT is arranged to have these dots placed adjacently in a specific pattern. There are separate video streams for each colour coming from the video card, which allows the different colours to have different intensities at each point on the screen. By varying the intensity of the red, green and blue streams, the full rainbow of colours is made possible.

The surface of the CRT only glows for a small fraction of a second before beginning to fade. This means that the monitor must redraw the picture many times per second to avoid having the screen flicker as it begins to fade and then is renewed. This rapid redrawing is called "refreshing" the screen.

Controls and internal electronics

The monitor is controlled by a special dedicated circuit board inside the monitor. It contains the "smarts" for reading the input from the video card and controlling the display of information on the CRT. It also interfaces with (and is controlled by) the controls on the front of the monitor.

The controls on the front of the monitor generally come in one of two styles: analog or digital. (Note by the way that this has nothing to do with whether the monitor itself is analog or digital, since that refers to the type of signal the monitor takes as input.) Each of these has its advantages and disadvantages.

Analog controls (usually dials on the front of the monitor) provide fine control over settings such as brightness and contrast, and are simple and intuitive to use. Digital controls generally involve the use of push buttons that control the settings using an on-screen display. These can either be easy to use or difficult, depending on how they are implemented. Some monitors, in an attempt to save money on buttons, use one button to select which feature you want to change (brightness, contrast, horizontal position) and then two buttons for "up" and "down". These can be confusing to use. Others use more buttons and are more intuitive.

There is one major advantage associated with monitors that use digital controls. Often when using multiple resolutions (say, 640x480 and 800x600), the position and size of the screen will change when changing resolutions. Many newer monitors with digital controls will save the position and size settings associated with each resolution that the monitor supports. This "memory" feature will save you from having to adjust the screen settings whenever you change the resolution on the monitor.

Finally, the more controls you have on the outside of the monitor, the better. Virtually all monitors include adjustments for brightness, contrast, vertical size and position, and horizontal size and position. Some stop there, while others add controls for focus, pin cushioning and other image factors. This is beneficial because, for safety reasons, you should not open up your monitor to make adjustments.

There is now in fact, a new type of control that attempts to marry the simplicity of analog instruments with the precision, "memory" features and high adjustment of digital controls. Some monitors now have a control that is simply a finger-operated dial with a push button. Pressing the button brings up an on-screen menu and the dial is used to select different adjustments from the menu and to change their values. It can still be confusing to operate compared to separate controls for each function. On the other hand, you gain by this method control over many more monitor parameters than older analog-instrumentation monitors typically had.

Power supply

The monitor is the only component of a regular PC that uses its own power supply, built into the monitor itself. Older monitors often used a special plug that connected to a receptacle at the back of the PC's power supply. This receptacle is just a pass-through from the power cord that the PC uses ( these monitors do not get power from the PC's internal power supply. The chief advantage of this connection is that the power to the monitor is turned off when the PC is turned off. This type of plug has fallen out of favour in recent years, although you can buy cheap adaptors that will convert a standard electrical cord to the type that connects to the back of the PC.

Many monitors have a switch to select between 110-volt and 220-volt power. Obviously, it's important to select the correct voltage depending on where you live. Also, many monitors have a fuse in the back that will blow if the monitor takes a voltage surge or there is another electrical problem.

Interface and cabling

The monitor is connected to the video card through a cable that attaches to a connector on its back. There are a few different connectors used for monitors:

15-pin VGA (Standard): The standard for connecting monitors and video cards today is the 15-pin video cable and connector, also called "VGA" in reference to the standard that first used it. In fact, all modern cards that call themselves VGA or SVGA use the standard 15-pin connector on the video card. Most standard monitors use the same 15-pin connector as well.

9-pin (Older VGA, EGA, CGA): Older displays used a smaller, 9-pin connector on the monitor, meant to match with the 9-pin output of older video cards. Some of these were VGA, but others employed older standards such as CGA or EGA. There are 15 to 9-pin adaptors available that will let a monitor with a 9-pin connector be used on a PC with a 15-pin video connector, but most of these monitors are older and not up to dealing with a modern video card in any event.

Warning: Make sure not to use an older digital monitor with a modern (analog) video card or you risk damaging the monitor.

BNC Connectors: Some high-performance monitors use special coaxial cable that has a standard 15-pin connector for the video card on one end, but breaks out to separate wires with BNC connectors on them for the monitor. There are usually 5 wires, one for each of red, green, blue, horizontal sync and vertical sync. Some monitors that have BNC connectors on them also have a standard 15-pin VGA, and either can be used. The BNC cable can be expensive, but provides better shielding from noise and interference than the standard VGA cable. It is found much more often on larger, more expensive units.

Some older monitors require a sync signal from the PC in order to know what mode to put themselves into when they are turned on. When using this sort of monitor, you should turn the PC itself on several seconds before powering up the monitor. This will ensure that the monitor sees a video signal when it is turned on so it will work properly.

One distinguishing feature that many people don't pay much attention to is whether the monitor uses an integrated or a separate data cable. Monitors that use separate data cables have a female data port on the back of the monitor, much like the port on the video card, and use a male-to-male cable to attach the two. Monitors using an integrated data cable have the cable coming from the back of the monitor directly with a single male plug for the video card. Both designs work the same, but if the cable is ever damaged, the monitor with the integrated cable will have to go to the repair shop, while the one with the separate cable can be fixed with a £16 replacement.

Monitor case

The monitor is usually encased in a plastic box that protects it. There's nothing too remarkable about the case and not much needs to be said about it, except for one thing. The components inside the monitor generates a lot of heat, and every monitor case has special ventilation slots (usually many of them) to allow for cooling of these components. It's important to make sure not to block these slots off or the monitor could overheat and malfunction.

( PC Guide 1999

Compiled by Will Garside

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This was first published in October 1999

 

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