Hardware Reference
In-Depth Information
75KHz. That display would be capable of an 85Hz refresh rate at 1024×768, but it would only be
capable of a 60Hz refresh at either 1280×1024 or 1600×1200. Because the flickering on a CRT at
60Hz is unacceptable, using those resolutions would be out of the question. By comparison, a monitor
capable of supporting a 110KHz horizontal frequency could handle even the highest 1600×1200
resolution at an 85Hz refresh rate, which would mean no flicker. Premium CRT displays offer
flicker-free refresh rates at higher resolutions. Note that you can't use refresh rates higher than the
display can support, and in some cases with older CRTs, selecting a refresh rate in excess of the
monitor's maximum can actually cause damage to the display circuitry!
Windows supports Plug and Play (PnP) monitor configuration if both the monitor and video adapter
support the DDC feature. Using DDC communication, Windows can read the VESA EDID from the
display and use it to configure the graphics controller to match the display's capabilities, such as
supported resolutions and refresh rates. This normally prevents the user from selecting refresh rates
that the monitor cannot support.
Why LCD and LED Screens Don't Flicker
LCD and backit LED monitors aren't affected by vertical refresh rates like CRTs; LCDs avoid
problems with flicker because of their design. LCDs 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. But most importantly, LCDs have a CCFL (cold cathode
fluorescent lamp) or LED backlight that for all intents and purposes doesn't flicker. (It operates either
continuously or at high frequencies of 200Hz or more.) In other words, although a vertical refresh rate
setting of 60Hz is considered bad for CRTs, that is the standard rate used by most LCDs because they
don't exhibit visible flicker. Although most LCDs can accept refresh rates of up to 75Hz, in most
cases selecting rates higher than 60Hz will only force the video card to work harder and won't
actually affect what you see on the display.
Tip
If you try to use a refresh rate higher than your display can support, you might see a message
that you have selected an out-of-range frequency. If you use a dual-head video card, keep in
mind that some models don't permit you to assign different refresh rates to each monitor. If you
have both a CRT and an LCD connected to such a video card, use the highest refresh rate
supported by both displays (usually 75Hz) to minimize flicker on the CRT.
Interlaced Versus Noninterlaced
Some monitors and video adapters can support both interlaced as well as noninterlaced modes. In
noninterlaced
(conventional) mode, the screen is drawn from top to bottom, one line after the other,
completing the screen in one pass. In
interlaced
mode, the screen is drawn in two passes—with the
odd lines first and the even lines second. Each pass takes half the time of a full pass in noninterlaced
mode.
Early high-resolution CRT monitors used interlacing to reach their maximum resolutions; as with
interlacing, the vertical and horizontal scan frequencies were cut in half. Unfortunately, this usually
introduces noticeable flicker into the display, so in most cases you should avoid interlacing where
possible. Fortunately, most modern monitors support noninterlaced modes at all supported
resolutions, thus avoiding the slow screen response and potential flicker caused by interlacing.
