Video Hardware, Part 1 - How CRT Display Technology Works
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A monitor can use one of several display technologies. The original display technology, and still the most popular, is cathode ray tube (CRT) technology—the same technology used in television sets. CRTs consist of a vacuum tube enclosed in glass. One end of the tube contains an electron gun assembly that projects three electron beams, one each for the red, green, and blue phosphors used to create the colors you see onscreen; the other end contains a screen with a phosphorous coating.
When heated, the electron gun emits a stream of high-speed electrons that are attracted to the other end of the tube. Along the way, a focus control and deflection coil steer the beam to a specific point on the phosphorous screen. When struck by the beam, the phosphor glows. This light is what you see when you watch TV or look at your computer screen. Three layers of phosphors are used: red, green, and blue. A metal plate called a shadow mask is used to align the electron beams; it has slots or holes that divide the red, green, and blue phosphors into groups of three (one of each color). Various types of shadow masks affect picture quality, and the distance between each group of three (the dot pitch ) affects picture sharpness.
Figure 15.1 illustrates the interior of a typical CRT.
The phosphor chemical has a quality called persistence , which indicates how long this glow remains onscreen. Persistence is what causes a faint image to remain on your TV screen for a few seconds after you turn off the set. The scanning frequency of the display specifies how often the image is refreshed. You should have a good match between persistence and scanning frequency so the image has less flicker (which occurs when the persistence is too low) and no ghost images (which occurs when the persistence is too high).
Figure 15.1 A typical CRT monitor is a large vacuum tube. It contains three electron guns (red, green, and blue) that project the picture toward the front glass of the monitor. High voltage is used to produce the magnetism that controls the electron beams that create the picture displayed on the front of the CRT.
The electron beam moves very quickly, sweeping the screen from left to right in lines from top to bottom, in a pattern called a raster. The horizontal scan rate refers to the speed at which the electron beam moves laterally across the screen.
During its sweep, the beam strikes the phosphor wherever an image should appear onscreen. The beam also varies in intensity to produce different levels of brightness. Because the glow begins to fade almost immediately, the electron beam must continue to sweep the screen to maintain an image—a practice called redrawing or refreshing the screen.
Most current CRT displays have an ideal refresh rate (also called the vertical scan frequency) of about 85 hertz (Hz), which means the screen is refreshed 85 times per second. Refresh rates that are too low cause the screen to flicker, contributing to eyestrain. The higher the refresh rate, the better for your eyes. Low-cost monitors often have flicker-free refresh rates available only at 640x480 and 800x600 resolutions; you should insist on high refresh rates at resolutions such as 1024x768 or higher.
It is important that the refresh rates expected by your monitor match those produced by your video card. If you have mismatched rates, you will not see an image and can actually damage your monitor. Generally speaking, video card refresh rates cover a higher range than most monitors. For this reason, the default refresh rate used by most video cards is relatively low (usually 60Hz) to avoid monitor damage. The refresh rate can be adjusted through the Windows display properties sheets.
Multiple Frequency Monitors Although a few very old monitors had fixed refresh rates, most monitors support a range of frequencies. This support provides built-in compatibility with a wide range of current and future video standards (described in the "Video Display Adapters" section later in this chapter). A monitor that supports many video standards is called a multiple-frequency monitor. Virtually all monitors sold today are multiple frequency, which means they support operation with a variety of popular video signal standards. Different vendors have used a variety of trade names to identify their multiple-frequency monitors, including multisync, multifrequency, multiscan, autosynchronous, and autotracking among others.
Note -Even though a monitor is capable of displaying a wide range of video standards, you usually need to fine-tune the display through its onscreen display (OSD) controls and Windows display properties sheets to achieve the best possible pictures.
 | This chapter is from Upgrading and Repairing PCs, 16th edition,by Scott Mueller. (Que Books, 2004, ISBN: 0789731738). Check it out at your favorite bookstore today. Buy this book now! |
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