Video Hardware, Part 2 - Horizontal Frequency
(Page 5 of 10 )
Different video resolutions use different horizontal frequencies. For example, the standard VGA resolution of 640x480 requires a horizontal resolution of 31.5KHz, whereas the 800x600 resolution requires a vertical frequency of at least 72Hz and a horizontal frequency of at least 48KHz. The 1024x768 image requires a vertical frequency of 60Hz and a horizontal frequency of 58KHz, and the 1280x1024 resolution requires a vertical frequency of 60Hz and a horizontal frequency of 64KHz. If the vertical frequency increases to 75Hz at 1280x1024, the horizontal frequency must be 80KHz. For a super-crisp display, look for available vertical frequencies of 75Hz or higher and horizontal frequencies of up to 90KHz or more. My favorite 17'' NEC monitor supports vertical resolutions of up to 75Hz at 1600x1200 pixels, 117Hz at 1024x768, and 160Hz at 640x480!
Virtually all the analog monitors on the market today are, to one extent or another, multiple-frequency. Because literally hundreds of manufacturers produce thousands of monitor models, it is impractical to discuss the technical aspects of each monitor model in detail. Suffice it to say that before investing in a monitor, you should check the technical specifications to ensure that the monitor meets your needs. If you are looking for a place to start, check out some of the magazines that periodically feature reviews of monitors. If you can't wait for a magazine review, investigate monitors at the Web sites run by any of the following vendors: IBM, Sony, NEC-Mitsubishi, and ViewSonic. Each of these manufacturers creates monitors that set the standards by which other monitors can be judged. Although you typically pay a bit more for these manufacturers' monitors, they offer a known, high level of quality and compatibility, as well as service and support. Note that most monitor companies sell several lines of monitors, varying by refresh rates, CRT type, antiglare coatings, energy efficiency, and warranties. For best results at resolutions of 1024x768 and above, avoid the lowest-cost 17'' monitors because these models tend to produce fuzzy onscreen displays with low refresh rates.
Controls Most of the newer CRT monitors and LCD panels use digital controls instead of analog controls. This has nothing to do with the signals the monitor receives from the computer, but only the controls (or lack of them) on the front panel that enable you to adjust the display. Monitors with digital controls have a built-in menu system that enables you to set parameters such as brightness (which adjusts the black level of the display), contrast (which adjusts the luminance of the display), screen size, vertical and horizontal shifts, color, phase, and focus. A button brings the menu up onscreen, and you use controls to make menu selections and vary the settings. When you complete your adjustments, the monitor saves the settings in nonvolatile RAM (NVRAM) located inside the monitor. This type of memory provides permanent storage for the settings with no battery or other power source. You can unplug the monitor without losing your settings, and you can alter them at any time in the future. Digital controls provide a much higher level of control over the monitor and are highly recommended.
Tip -Digital video engineer Charles Poynton's notes on adjusting brightness and contrast controls provide an excellent tutorial on the use of these often misunderstood monitor adjustments. Find them online at http://www.vision.ee.ethz.ch/~buc/brechbuehler/
mirror/color/Poynton-color.html
Digital controls make adjusting CRT monitors suffering from any of the geometry errors shown in Figure 15.9 easy. Before making these adjustments, be sure the vertical and horizontal size and position are correct.
Tip -Get a monitor with positioning and image controls that are easy to reach, preferably on the front of the case. Look for more than just basic contrast and brightness controls; a good monitor should enable you to adjust the width and height of your screen images and the placement of the image on the screen. The monitor should also be equipped with a tilt-swivel stand so you can adjust the monitor to the best angle for your use.
Figure 15.9 Typical geometry errors in CRT monitors; these can be corrected on most models that have digital picture controls.
Although LCD panels aren't affected by geometry errors as CRT monitors can be, they can have their own set of image-quality problems, especially if they use the typical 15-pin analog VGA video connector. Pixel jitter and pixel swim (in which adjacent pixels turn on and off) are relatively common problems that occur when using an LCD monitor connected to your PC with an analog VGA connector.
 | 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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