Video Hardware, Part 2 - Refresh Rates (Vertical Scan Frequency)
(Page 4 of 10 )
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 you use with a CRT display, 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 15.6 compares two typical 17'' CRT monitors and a typical mid-range graphics card.
Note the differences in the refresh rates supported by the ATI RADEON 9000 Pro and two 17'' CRT monitors from ViewSonic: the E70 and P75f.
The E70 sells for around $130, 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 Pro 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!
Table 15.6 Refresh Rates Comparison Refresh Rates |
Resolution | ATI Radeon 9000 Pro | Viewsonic E70 | Viewsonic P75f+ |
1024x768 | 60Hz–200Hz* | 87Hz* | 85Hz* |
1280x1024 | 60Hz–160Hz* | 66Hz | 89Hz* |
1600x1200 | 60Hz–120Hz* | Not supported | 77Hz* |
*Rates above 72Hz will be flicker-free for many users; the VESA standard for flicker-free refresh is 85Hz or above.
To determine a monitor's refresh rates for the resolutions you're planning to use, check out the monitor manufacturer's Web site.
Note -Many manufacturers use the term optimal resolution to refer to the highest CRT monitor resolution that supports the VESA standard for flicker-free viewing (85Hz or higher). I recommend that you consider the monitor's optimal resolution as its highest practical resolution because higher resolutions, which don't support flicker-free viewing, are likely to provide poor visual quality.
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 CD or can be downloaded from the monitor vendor's Web site.
Note -Because CRT monitors are redrawing the screen many times per second, the change in a noninterlaced CRT screen display is virtually invisible to the naked eye, but it is very obvious when computer screens are photographed, filmed, or videotaped. Because these cameras aren't synchronized to the monitor's refresh cycle, it's inevitable that the photo, film, or videotape will show the refresh in progress as a line across the picture.
If you need to capture moving images from a monitor to videotape, use a video card with a TV-out option to send your picture to a VCR. If you need to take still photos of a monitor (for example, to record BIOS/CMOS setup information), use an LCD display instead of a CRT monitor.
In my experience, a 60Hz vertical scan frequency (frame rate) is the minimum anybody should use with a CRT, 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 CRT 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 in Table 15.6, 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.
 | 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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