Hardware Fundamentals
© Copyright Brian Brown, 1992-2000. All rights reserved.

Monitor Screens
Monitor screens are devices used to display information from the computer. In fact, Television sets include monitor screens, though in general, the monitor screens used in computer systems are of a much higher quality.

Monitors come in various sizes, commonly starting at 14", then 15", as well as 17" and 19". This is the size of the viewable screen measured across the diagonal from corner to corner. The larger the size the more expensive the monitor. Larger sizes are required for higher resolutions.

Resolution refers to the number of dots displayed in the X (across) and Y (down) co-ordinates. Typical screens are capable of displaying 640x480 dots. Larger screens are required for higher resolutions, for example, 1024x768 resolutions displayed on a 14" screen look very small, but the same resolution on a 17" screen is much easier to read.

Another term used with monitors is refresh rate. This refers to the number of times per second that the image is redrawn on the monitor screen. A refresh rate of 60Hz means the images is redrawn 60 times a second. Typical refresh rates are 60Hz, 72Hz and 75Hz. Higher refresh rates are required for larger resolutions, else the viewer tend to notice the image on the screen flicker. The images on the screen have to be refreshed at a reasonable rate (greater than 50Hz), otherwise the human eye perceives the image as flickering.

Here is a list of some common features of monitor screens

• majority is based on Cathode Ray Tube technology (CRT), using the same principle as used in television sets
• the CRT consists of a vacuum tube enclosed in glass. At the end of the tube is an electron gun (which generates an electron beam), focusing and deflection assembly. At the other end of the tube is the screen surface which is coated in phosphorous which emits a dot of light when struck by the electronic beam.

  
  Fig 6.44: CRT Assembly

• the electron gun emits a high stream of electrons.
• the electron beam is deflected by signals applied to the deflection assembly.
• when the electron beam strikes the phosphorous coating on the inner surface of the screen, light is emitted.
• the electron beam is swept across the screen from left to right in horizontal lines, from top to bottom, in a pattern called a raster.

  
  Fig 6.45: Raster Image

• the screen image is repeated at the refresh rate, as the image dissipates quickly.
• color systems use three electron beams, one each for red, blue and green.
• in color systems, three different phosphors are used which match each of the three beams. This collection of three phosphors is called a triad.
• a shadow mask is used to prevent the beams from striking the wrong phosphors

  
  Fig 6.45: Shadow Mask and Triad

Interlaced Displays

• the screen picture is made up of two full passes of the electron beam from top to bottom
• the first pass builds up one screen doing only odd line numbers
• the second pass completes the screen doing all the even line numbers
• the technique relies upon the persistence of the image on the screen so that the human eye blends both passes together. Persistence is a measure of how long the image remains on the screen before it fades away. This is because phosphor only emits light when struck by electrons, when it stops being struck, it stops emitting light.
• this technique is commonly used for Television

Non-Interlaced Displays
In non-interlaced displays, the screen picture is made from one single pass of the electron beam from top to bottom.

Dot Pitch
This is the distance in millimeters between the phosphor triads. Typically it is .28mm, and the smaller the better, resulting in a much crisper sharper display. Obviously, the smaller dot pitch monitors like .26mm are very expensive.

Screen Resolutions

• is the amount of detail that the display can show
• it is expressed in a number of horizontal and vertical picture elements called pixels (dots)
• the higher the resolution, the more information that can be displayed on the screen, but also require larger screen sizes else the image looks too small
• the following table shows some common resolutions

  Standard	Resolution, x, y, colors
  CGA	640x200x2
  EGA	640x350x16
  VGA	640x480x256
  SVGA	800x600x256
  XGA	1024x768x65536

Summary
Most monitors today are based on CRT technology which has existed for many years. This explains the weight and size of monitors using CRT's.

Larger screen resolutions require larger a display area or the image being displayed is too small to read.

Dot pitch determines the sharpness of the display image, the smaller the better. Color systems use three electron guns and three phosphor dots called a triad to generate color.

The screen image must be periodically refreshed otherwise the image is lost. How many times a second this refresh occurs is called the refresh rate. A typical refresh rate is 60 times a second.

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© Copyright Brian Brown, 1992-2000. All rights reserved.