CRT Monitors
A monitor is a box containing a CRT (Cathode Ray Tube) and its power
supplies. The CRT contains a gun that can shoot an electron beam against a phosphorescent
screen near the front of the tube, as shown in Fig. 2-4(a). (Color monitors
have three electron guns, one each for red, green and blue.) During the horizontal
scan, the beam sweeps across the screen in about 50 msec, tracing out an
almost horizontal line on the screen. Then it executes a horizontal retrace to get
back to the left-hand edge in order to begin the next sweep. A device like this that
produces an image line by line is called a raster scan device.
(a) (b)
Electron gun
Grid
Screen
Spot on
screen
Vacuum
Vertical
deflection
plate
Horizontal scan
Vertical retrace Horizontal retrace
Figure 2-4. (a) Cross section of a CRT. (b) CRT scanning pattern.
Horizontal sweeping is controlled by a linearly increasing voltage applied to
the horizontal deflection plates placed to the left and right of the electron gun.
Vertical motion is controlled by a much more slowly linearly increasing voltage
applied to the vertical deflection plates placed above and below the gun. After
somewhere between 400 and 1000 sweeps, the voltages on the vertical and horizontal
deflection plates are rapidly reversed together to put the beam back in the
upper left-hand corner. A full-screen image is normally repainted between 30 and
60 times a second. The beam motions are shown in Fig. 2-4(b). Although we
have described CRTs as using electric fields for sweeping the beam across the
screen, many models use magnetic fields instead of electric ones, especially in
high-end monitors.
To produce a pattern of dots on the screen, a grid is present inside the CRT.
When a positive voltage is applied to the grid, the electrons are accelerated, causing
the beam to hit the screen and make it glow briefly. When a negative voltage
is used, the electrons are repelled, so they do not pass through the grid and the
screen does not glow. Thus the voltage applied to the grid causes the corresponding
bit pattern to appear on the screen. This mechanism allows a binary electrical
signal to be converted into a visual display consisting of bright and dark spots.
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