CRT Monitors
A monitor operates fundamentally like a TV set, except that it is designed to receive signals from a display
adapter card in the computer, rather than a broadcast signal. A variety of design factors and the features of
the display adapter card influence the quality of a monitor's display. The CRT (cathode-ray tube) is the main
component of a traditional monitor. The rear of the CRT holds a cylinder that contains one or more electron
guns. Most color monitors have three electron guns for red, green, and blue (RGB) which allows the visual
production of all colors. The wide end of the CRT is the display screen, which has a phosphor coating that can emit light when hit with radiation. When
active, the electron guns beam a stream of
charged electrons onto the phosphorus
coating. When the coating is hit with the right
amount of energy, light is produced in a
pattern of very small dots. This same
technology is used in X-ray imaging,
oscilloscopes, and other CRT devices.
Similarly, monitors emit X-radiation. There is
one dot for each RGB color, and the dots are
grouped in patterns close together. The name
for a collection of all dots in a specific
location is a pixel, which stands for picture
element. The human eye perceives
the collection of pixels on the front
of a CRT as a compound image, in
much the same way as it interprets
he pattern of ink dots in a newspaper halftone as a photograph
Screen Resolution and Dot Pitch: Screen RESOLUTION refers to the degree of detail offered in the presentation of an image on the screen and is expressed in pixels per inch. The greater the number of pixels per inch, the smaller the detail that can be displayed and the sharper the picture. Monitor resolution is expressed as the number of horizontal pixels x the number of vertical pixels. DOT PITCH defines the diagonal distance between the two closest dots of the same color and is expressed in hundredths of millimeters. the smaller the dot pitch, the greater the number of dots, and the sharper the image.
The image on the screen is not painted all at once. The stream is directed in rows starting in an upper left corner. A series of rasterlines are drawn down the face of the screen until the beam reaches the lower right, whereupon the process starts over. A given line must be visible for long enough to allow the formation of a complete image, but must not blur the dots painted in the next pass. Thetermpersistence is used to define how long the phosphors on the screen remain excited and emit light. The time the stream takes to complete a vertical pass is called the vertical refresh rate (VRR) and the time the stream takes to pass once from left to right is known as the horizontal refresh rate (HRR). If the vertical rate is too slow, it can cause flickering and larger CRTs require higher, i.e., quicker, refresh rates. At a resolution of 640 × 480, the minimum refresh rate is 60 Hz; at a resolution of 1600 × 1200, the minimum refresh rate is 85 Hz. Both the monitor and the display adapter produce the refresh rate. The first monitors had fixed refresh rates. In 1986, NEC introduced the first multi-frequency monitor that could automatically adjust the refresh rate to take advantage of the highest rate supported by the display adapter cards of that time. Today, this feature is standard on most monitors. Some older monitors use interlacing. Interlacing refreshes the monitor by painting alternate rows on the screen and then returning to paint the rows that were skipped the first time around. This increases the effective refresh rate but can lead to eye strain.
The monitor is the highest consumers of electrical current in the average computer therefore most new monitors provide some level of power-saving technology. The Video Electronics Standards Association (VESA) has established a standard set of power economy controls to reduce power use when the monitor is idle. These are collectively referred to as Display Power Management Signaling (DPMS) modes. DPMS technology uses monitors to gauge activity levels of the display. If there is no change in the data stream from the adapter, as set in either the BIOS or operating system controls, the monitor is switched to inactive status. The goal is to reduce power consumption while minimizing the amount of time required to restore the display to full intensity when needed. Frequently turning a monitor on and off places stress on the components. DPMS reduces the need to use the mechanical switch to turn the device on or off. You should advise clients without power-saving systems in place to turn on the display only when it is first needed and to turn it off at the end of each workday.
In most cases, the monitor must be adjusted for a proper picture when the screen resolution or refresh rate is
changed or a new display card is added to the system. Table below lists the typical monitor adjustments.
