Chapter 16: Monitors and Displays^383
Most monitors refresh the display between 60 and 75 times per second. A CRT’s refresh
rate is expressed in Hertz (which means per cycles per second) and common refresh rates
for CRT monitors (and video cards) is 60 to 75Hz.
If the refresh rate of the CRT is set too low, the screen will flicker, which can cause eye
strain or headaches for the viewer. The refresh rate can be set higher to avoid flicker. Some
monitors also use a technique used on televisions to cut down on screen flicker. Televisions
use a 50Hz refresh rate. On a PC monitor this rate would likely cause the screen to flicker,
but because the television uses interleaving, the flicker is largely eliminated.
Interleaving divides the screen into two (or more) passes by refreshing every other
row as it sweeps down the display. On one pass it refreshes the odd numbered rows, and
on its second pass it refreshes the even numbered rows. When you consider that most
CRTs have at least 600 rows of pixels and 300 of the rows are refreshed in each pass, the
screen has an even balance of refreshed pixels. Without interleaving, the top of the screen
fades when the bottom is being refreshed, which causes the image to appear to flicker.Masking the Display
As fast as the electron beam is moving, it is hard for it to be precise. A CRT will include
one of two different types of guides to prevent the beam from lighting up the wrong
phosphormaterialsandproducingthewrongcolors:ashadowmaskoranaperturegrill.Shadow Mask
The shadow mask (see Figure 16-7) is a very fine screen that is mounted between the
electron gun and the pixels and has openings that permit each beam to hit only where it
should. Any phosphor material in its shadow is masked and will not be illuminated,
henceitsname.Theholesinthemaskarealignedperfectlywiththepixelsonthescreen.Aperture Grill
The alternative to the shadow mask method is the aperture grill (see Figure 16-8). On an
aperture grill display, pixels are masked into vertical stripes between fine metal wires,
which are held in place by thin wires that run horizontally across the display. The vertical
wires perform the same function as the shadow mask and keep the electron beam from
illuminating the wrong parts of the phosphor. Two popular types of CRTs that use this
method are the Sony Trinitron and the Mitsubishi Diamondtron, which are used in many
of the more popular monitor brands.
Aperture grill monitors have some advantages over those that use shadow mask
CRTs. The advantages include a brighter picture, a sharper image, and because the front
of the tube is vertically flat, less glare and less distortion. However, because the vertical