VIDEO DISPLAY DEVICES

Typically, the primary output device in a graphics system is a video monitor.The operation of most video monitors is based on the standard cathode-ray* tube (CRT) design, but several other technologies exist and solid-state* monitors may eventually predominate.

REFRESH CATHODE-RAY TUBES

1.      Figure 2-2 illustrates the basic operation of a CRT. A beam of electrons (cathode rays), emitted by an electron gun, passes through focusing and deflection systems that direct the beam toward specified positions on the phosphor-coated screen.

2.      The phosphor then emits a small spot of light at each position contacted by the electron beam. Because the light emitted by the phosphor fades very rapidly,

3.      Some method is needed for maintaining the screen picture. One way to do this is to store the picture information as a charge distribution within the CRT.

4.      This charge distribution can then be used to keep the phosphors activated. However, the most common method now employed for maintaining phosphor glow is to redraw the picture repeatedly by quickly directing the electron beam back over the same screen points.

5.      This type of display is called a refreshCRT, and the frequency at which a picture is redrawn on the screen is referred to as the refresh rate. The primary components of an electron gun in a CRT are the heated metal cathode and a control grid (Fig. 2-3).

6.      Heat is supplied to the cathode by directing a current through a coil of wire, called the filament, inside the cylindrical cathode structure. This causes electrons to be “boiled off” the hot cathode surface.

7.      In the vacuum inside the CRT envelope, the free, negatively charged electrons are then accelerated toward the phosphor coating by a high positive voltage. The accelerating voltage can be generated with a positively charged metal coating on the inside of the CRT envelope near the phosphor screen, or an accelerating anode, as in Fig. 2-3, can be used to provide the positive voltage.

8.      Sometimes the electron gun is designed so that the accelerating anode and focusing system are within the same unit.

Intensity of the electron beam is controlled by the voltage at the control grid, which is a metal cylinder that fits over the cathode. A high negative voltage applied to the control grid will shut off the beam by

1.      repelling electrons and stopping them from passing through the small hole at the end of the control-grid structure.

2.      A smaller negative voltage on the control grid simply decreases the number of electrons passing through. Since the amount of light emitted by the phosphor coating depends on the number of electrons striking the screen, the brightness of a display point is controlled by varying the voltage on the control grid.

3.      The focusing system in a CRT forces the electron beam to converge to a small cross section as it strikes the phosphor. Otherwise, the electrons would repel eachother, and the beam would spread out as it approaches the screen. Focusing is accomplished with either electric or magnetic fields.

4.      With electrostatic focusing, the electron beam is passed through a positively charged metal cylinder so that electrons along the centerline of the cylinder are in an equilibrium position.

5.      This arrangement forms an electrostatic lens, as shown in Fig. 2-3, and the electron beam is focused at the center of the screen in the same way that an optical lens focuses a beam of light at a particular focal distance. Similar lens focusing effects can be accomplished with a magnetic field set up by a coil mounted around the outside of the CRT envelope, and magnetic lens focusing usually produces thesmallest spot size on the screen.

6.      Additional focusing hardware is used in high-precision systems to keep the beam in focus at all screen positions. The distance that the electron beam must travel to different points on the screen varies because the radius of curvature for most CRTs is greater than the distance from the focusing system to the screen center.

7.      Therefore, the electron beam will be focused properly only at the center of the screen. As the beam moves to the outer edges of the screen, displayed images become blurred. To compensate for this, the system can adjust the focusing according to the screen position of the beam.

8.      As with focusing, deflection of the electron beam can be controlled with either electric or magnetic fields. Cathode-ray tubes are now commonly constructed with magnetic-deflection coils mounted on the outside of the CRT envelope, as illustrated in Fig. 2-2.

9.      Two pairs of coils are used for this purpose. One pair is mounted on the top and bottom of the CRT neck, and the other pair is mounted on opposite sides of the neck. The magnetic field produced by each pair of coils results in a transverse deflection force that is perpendicular to both the direction of the magnetic field and the direction of travel of the electron beam.

10.  Horizontal deflection is accomplished with one pair of coils, and vertical deflection with the other pair. The proper deflection amounts are attained by adjusting the currentthrough the coils. When electrostatic deflection is used, two pairs of parallel plates are mounted inside the CRT envelope.