Hardware Reference
In-Depth Information
For example, let's say you have a CRT display that supports a maximum
horizontal fre-
quency
of 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. Be-
cause 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 sup-
port,andinsomecaseswitholderCRTs,selectingarefreshrateinexcessofthemonitor'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.
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 import-
antly, LCDs have a CCFL (cold cathode fluorescent lamp) or LED backlight that for all
intentsandpurposesdoesn'tflicker.(Itoperateseithercontinuouslyorathighfrequencies
of 200Hz or more.) In other words, although a vertical refresh rate setting of 60Hz is con-
sidered bad for CRTs, that is the standard rate used by most LCDs because they don't ex-
hibit 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
Ifyoutry to use a refresh rate higher than yourdisplay can support, youmight 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
youhavebothaCRTandanLCDconnectedtosuchavideocard,usethehighestrefreshrate
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,thescreenisdrawnfromtoptobottom,one
line after the other, completing the screen in one pass. In
interlaced
mode, the screen is
