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There are many types of monitors, but the most popular is the raster display monitor that displays information as illuminated points of varying intensities located on horizontal lines. In this chapter, the word monitor refers to raster display monitors unless otherwise indicated.
A monitor illuminates the essentially rectangular portion of a cathode ray tube (CRT) surface that is visible to viewers. The illuminated area may represent graphical or text data that human eye-brain systems translate to information using spatial integration and perceptual processes. Producing a desired illumination pattern requires exploiting numerous electrical, electromagnetic, chemical, and physical principles.
As illustrated in Figure 6-1, computer systems place monitor data in a special memory area called a frame buffer. The frame buffer can be in the computer’s memory or on a special card, or the VGA card memory can be used as a frame buffer (see Chapter 6).
A display controller circuit interprets this data and generates various signals that travel to the monitor over the monitor cable. The monitor receives these signals from the display controller, interprets them, and generates coordinated signals for other monitor components that produce the desired illuminations. To understand the coordination that produces these illuminations requires understanding the individual monitor components.
Whether you have a gray scale or color monitor, it is important to adjust the monitor’s brightness and contrast settings to obtain the best visual output from your display subsystem. The easiest method is to:
An incorrect brightness setting usually is the source of picture reproduction problems though contrast ratio is a major determinant of perceived picture quality.
A CRT is a bottle-shaped transparent glass tube with a narrow neck and large, relatively flat bottom called the face. In the simplest case, the inside surface of the CRT face is coated with a single special material called a phosphor. In the narrow-neck end, the CRT has a special assembly called the electron gun which emits electrons that bombard the phosphor surface, causing it to glow. To achieve this effect, the electron gun contains a heating element which heats a cathode. When completely assembled, the air pressure within a CRT is reduced to minus 14 pounds per square inch—a near vacuum. Heating the cathode in such a vacuum causes electrons to “boil” or “spray” off the cathode in a relatively chaotic cloud. These electrons appear at a relatively constant rate.
Near the CRT face, a component called the anode produces a strong (10,000 to 30,000+ volts) positive voltage charge which strongly attracts the negatively charged electrons. As the electrons begin accelerating from the cathode towards the anode in a relatively wide electron stream, electron gun components called control grids produce electrostatic fields that modulate the beam intensity (number of electrons allowed to pass by) channel. Under control, the various control grids narrow and focus the streaming electron spray into a focused electron beam. This determines the monitor’s spot size. A monitor’s spot size is the minimum size area the monitor can illuminate.
Looking at a cross section of a focused beam reveals a near Gaussian distribution of energy within the beam that leaves the beam center with more energy than the outside perimeter. Thus, an illuminated spot exhibits a blur which tends to make the spot size larger than it should be, causing blurry images.
Dynamic focusing varies the focusing depending on intended impact position. Because the beam strikes the sides of the viewing area at an oblique angle, the circular beam produces an elliptical impact area towards the CRT screen edges. To reduce the blurring effect caused by elliptical spots, dynamic focusing can reduce the spot size at the display area edges.
As the electron beam continues its journey, it passes inside a special circular electrical coil made of precision-wound thin wire, called the yoke. The yoke is located outside the CRT where the neck joins the rest of the CRT and consists of two independent parts. The yoke components generate vertical and horizontal magnetic fields, respectively, which divert the straight path of the focused beam to a desired impact point on the CRT face. Varying these magnetic fields varies the impact point with great precision but slightly defocuses the electron beam as it passes through the yoke’s magnetic field. Avoiding unnecessary beam distortion requires careful yoke designs and careful alignment procedures.
As electrons approach the phosphor-coated CRT face, their velocity continually increases. When an electron eventually impacts a phosphor molecule, the phosphor molecule absorbs the tremendous energy and enters an excited state. Excited phosphor molecules eventually, but not simultaneously, release their energy in the form of light. As excited phosphors release this energy they are said to fluoresce. A fluorescing phosphor area provides a specific chromaticity (color and saturation) that depends on its chemical composition. The resulting light passes through the CRT glass face and is visible to the viewer. Some CRT faces have anti-glare coatings to reduce reflections and these treatments diffuse (slightly blur) the fluorescing light as well.
Bombarding phosphors with more electrons produces greater excitement and increases light intensity. The excitement level decreases, or decays, at a predicable rate once the electron stream is removed. This decay rate is called the phosphor’s persistence—different phosphors have different decay rates as well as different chromaticities. The human eye-brain system’s temporal integration characteristic and retinal afterimage make decay rates appear slower to viewers than they really are.
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