Hardware Reference
In-Depth Information
The phosphor chemical has a quality called
persistence
, which indicates how long this
glow remains onscreen. Persistence is what causes a faint image to remain on your TV
screen for a few seconds after you turn off the set. The vertical scanning frequency (also
called the
refresh rate
) of the display specifies how many times per second the image is
refreshed. You should have a good match between persistence and refresh rate so the im-
age has less flicker (which occurs when the persistence is too low) and no ghost images
(which occurs when the persistence is too high).
The electron beam moves quickly, sweeping the screen from left to right in lines from
top to bottom, in a pattern called a
raster
. The horizontal scan rate refers to the speed at
which the electron beam moves laterally across the screen, measured in the number of
lines drawn per second.
During its sweep, the beam strikes the phosphor wherever an image should appear on-
screen.Thebeamalsovariesinintensitytoproducedifferentlevelsofbrightness.Because
theglowbeginstofadealmostimmediately,theelectronbeammustcontinuetosweepthe
screen to maintain an image—a practice called
redrawing
or
refreshing
the screen.
Due to the lower persistence phosphors used in PC monitor CRTs, most have an ideal re-
fresh rate (also called the
vertical scan frequency
) of 85Hz, which means the screen is
