Monitor Frequencies

One essential buying decision is to choose a monitor that works with your selected video adapter. Today, virtually all monitors are multiple-frequency (also called multiscanning and multifrequency) units that accommodate a range of standards, including those that are not yet standardized.

However, big differences exist in how well various monitors cope with various video adapters. With multiple-frequency CRT monitors, you must match the range of horizontal and vertical frequencies the monitor accepts with those generated by your video adapter.

The wider the range of signals, the more expensive—and more versatile—the monitor. Your video adapter's vertical and horizontal frequencies must fall within the ranges your monitor supports. The vertical frequency (or refresh/frame rate) determines the stability of your image (the higher the vertical frequency, the better).

Typical vertical frequencies range from 50Hz to 160Hz, but multiple-frequency monitors support different vertical frequencies at different resolutions. You might find that a bargain monitor has a respectable 100Hz vertical frequency at 640x480 but drops to a less desirable 60Hz at 1024x768.

The horizontal frequency (or line rate) typically ranges from 31.5KHz to 90KHz or more. By default, most video adapters use a 60Hz default vertical scan frequency to avoid monitor damage.

Although LCD monitors use lower vertical frequencies than CRTs, they avoid problems with screen flicker because of their design. Because they use transistors to activate all the pixels in the image at once, as opposed to a scanning electron beam that must work its way from the top to the bottom of the screen to create an image, LCD panels never flicker.

Refresh Rates (Vertical Scan Frequency)

The refresh rate (also called the vertical scan frequency) is the rate at which the screen display is rewritten. This is measured in hertz. A refresh rate of 72Hz means that the screen is refreshed 72 times per second. A refresh rate that is too low causes CRT screens to flicker, contributing to eyestrain. The higher the refresh rate, the better for your eyes and your comfort during long sessions at the computer.

A flicker-free refresh rate is a refresh rate high enough to prevent you from seeing any flicker. The flicker-free refresh rate varies with the resolution of your monitor setting (higher resolutions require higher refresh rates) and must be matched by both your monitor and display card.

Because a refresh rate that is too high can slow down your video display, use the lowest refresh rate that is comfortable for you. One important factor to consider when purchasing a CRT monitor is the refresh rate, especially if you are planning to use the monitor at 1024x768 or higher resolutions.

Low-cost monitors sometimes have refresh rates that are too low to achieve flicker-free performance for most users and thus can lead to eyestrain. Table below compares two typical 17'' CRT monitors and a typical mid-range graphics card.


ATI RADEON 9000 Pro Video Card Vertical Refresh

ViewSonic E70 (17'') Monitor Vertical Refresh (Maximum)

ViewSonic P75f (17'') Monitor Vertical Refresh (Maximum)











Not supported


Note the differences in the refresh rates supported by the ATI RADEON 9000 (based on the popular ATI RADEON 9000 Pro GPU [graphics processing unit] chip) and two 17'' CRT monitors from ViewSonic: the E70 and P75f. The E70 sells for around $140, and the P75f sells for about $185.

The P75f offers flicker-free refresh rates at higher resolutions than the cheaper E70. Although the ATI RADEON 9000 video card supports higher refresh rates than either monitor, these rates can't be used safely. Use of video adapter refresh rates in excess of the monitor's maximum refresh rate can damage the monitor!

To determine a monitor's refresh rates for the resolutions you're planning to use, check out the monitor manufacturer's Web site. During installation, Windows 2000, Windows 98, Windows 95B (OSR 2.x), Windows Me, and Windows XP support Plug and Play (PnP) monitor configuration if both the monitor and video adapter support the Data Display Channel (DDC) feature.

When DDC communication is available, the monitor can send signals to the operating system that indicate which refresh rates it supports, as well as other display information; this data is reflected by the Display Properties sheet for that monitor.

Monitors that don't support PnP configuration via DDC can be configured with an .INF (information) file, just as with other Windows-compatible devices. This might be supplied with a setup disk or can be downloaded from the monitor vendor's Web site.

A 60Hz vertical scan frequency (frame rate) is the minimum anybody should use, and even at this frequency, most people notice a flicker. Especially on a larger display, onscreen flicker can cause eyestrain and fatigue. If you can select a frame rate (vertical scan frequency) of 72Hz or higher, most people are not able to discern any flicker; 72Hz is the minimum refresh rate I recommend.

Most modern mid-range or better displays easily handle vertical frequencies up to 85Hz or more at resolutions up to 1024x768. This greatly reduces the flicker a user sees. However, note that increasing the frame rate, although it improves the quality of the image, can also slow down the video hardware because it now needs to display each image more times per second.

If you're a gamer, slower frame rates can reduce your score. In general, I recommend that you set the lowest frame rate you find comfortable. To adjust the video card's refresh rate with Windows 9x/Me/2000/XP, use the Display icon in Control Panel. Depending on your flavor of Windows, the refresh rates supported by the video card will appear on one of the Display tabs.

Optimal is the default setting, but this really is a "safe" setting for any monitor. Select a refresh rate of at least 72Hz or higher to reduce or eliminate flicker. Click Apply for the new setting to take effect. If you choose a refresh rate other than Optimal, you might see a warning about possible monitor damage.

This is a warning you should take seriously, especially if you don't have detailed information about your monitor available. You can literally smoke a monitor if you try to use a refresh rate higher than the monitor is designed to accept. Before you try using a custom refresh rate, do the following:

  • Make sure Windows has correctly identified your monitor as either a Plug and Play monitor or by brand and model.

  • Check the manual supplied with the monitor (or download the statistics) to determine which refresh rates are supported at a given resolution. As in the example listed earlier, low-cost monitors often don't support high refresh rates at higher resolutions.

Click OK to try the new setting. The screen changes to show the new refresh rate. If the screen display looks scrambled, wait a few moments and the screen will be restored to the previous value; you'll see a dialog box asking whether you want to keep the new setting. If the display was acceptable, click Yes; otherwise, click No to restore your display. If the screen is scrambled and you can't see your mouse pointer, just press the Enter key on your keyboard because No is the default answer. With some older video drivers, this refresh rate dialog box is not available. Get an updated video driver, or check with the video card vendor for a separate utility program that sets the refresh rate for you.

If you have a scrambled display with a high refresh rate, but you think the monitor should be capable of handling the refresh rate you chose, you might not have the correct monitor selected. To check your Windows 9x/Me/2000/XP monitor selection, check the Display Properties dialog box. If your monitor is listed as Standard VGA, Super VGA, or Default Monitor, Windows is using a generic driver that will work with a wide variety of monitors. However, this generic driver doesn't support refresh rates above 75Hz because some monitors could be damaged by excessive refresh rates.

In some cases, you might need to manually select the correct monitor brand and model in the Windows Display Properties dialog box. If you don't find your brand and model of monitor listed, check with your monitor vendor for a driver specific for your model. After you install it, see whether your monitor will safely support a higher refresh rate.

Horizontal Frequency

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.