The How and Why of PC Monitors

Media attention about advances in the PS2 platform has become a constant in our lives. Less attention has been paid to the equally significant advances that have occurred in the manufacture of the monitors we use to see the information generated by our PCs. This application note will attempt to explain what role the monitor must play as the primary interface between the PC and its operator, the demands placed on the monitor by advances in the PS2 platform, and how the monitor industry has reacted to keep pace with these demands.

The Difference Between a Cathode Ray Tube (CRT) Computer Monitor and a Television.

A common question asked by novice computer users is why the standard television display cannot be used as the monitor for a PS2 computer. The confusion here is easy to understand. In all CRT monitors, the image is painted on the screen by an electron beam that scans from one side of the display to the other. In television, the transitions in color, intensity and pattern as the beam scans across the screen tend to be very gradual. However, the transitions a computer monitor typically processes are more abrupt as areas of high intensity transform to areas of black as text is placed on the screen. Television uses a process that relies on the brain's ability to integrate gradual transitions in pattern that the eye sees as the image is painted on the screen. Each image on a television screen is composed of 525 lines, numbered from 1 to 525. During the first phase of screen drawing, even-numbered lines are drawn. During the next phase, the odd lines are drawn. The eye integrates the two images to create a single image. The scan is interlaced. An example of interlacing is shown in Figure 1.

However, the viewer of a computer monitor has different needs. Typically, the viewer is sitting within a foot or two of the screen, and is viewing a frequently changing text image. If a computer monitor used the same method of display as television, the many transitions would produce an annoying amount of flicker, because the brain is less able to integrate the dramatic transitions from bright to dark. Also, a secondary problem occurs due to the inability of the monitor to paint the interlaced images exactly in between the lines from the preceding scan. Text images make this much more visible to the eye at close range, and at the relatively slower speeds of an interlaced scan. Therefore, computer monitors use a technique that does not try to interlace two images into one, but rather paints one continuous image at a time and is said to be non-interlaced. Consequently, although the scan frequencies of the TV receiver and monitor are similar, computer monitors must be designed to paint every line during every write of the picture to prevent flicker. This requires electronics that operate at twice the speed (or bandwidth) as that of a television. The higher the bandwidth, the more expensive the display becomes.

Resolution: A Key Problem in Imaging

Resolution is the second area of advancement. Although many definitions of "resolution" exist, it is generally agreed that the word relates to the amount of information detail on the screen. When a high degree of detail is required, as with images generated by many of today's graphics software packages, greater resolution is needed. Because the screen image is made up of a series of individual lines, the total number of lines determines the resolution or "graininess" of the image. Painting the lines closer together requires more total lines to fill the screen. Thus, the higher the number of lines in a given resolution, the finer the resolution. Painting a greater variety of colors also requires more "screen memory" in the controller card. Several types of monitors have been designed to achieve finer resolution. The early monochrome displays were replaced by Color Graphic Adapter (CGA) monitors, the first controller card to support color graphics. CGA monitors supported two colors at 640x200 resolution. The medium-quality Enhanced Graphics Adapter (EGA) monitors painted a higher number of lines on the screen in nearly the same amount of time that was taken by the CGA monitors, making the CGA monitors obsolete. The majority of monitors sold today are Video Graphics Array (VGA) or Super Video Graphics Array (SVGA)-compatible and display an even higher color quality. True VGA supports 16 colors at 640x480 resolution or 256 colors at 320x200 resolution. There are many derivatives of VGA, including the Extended Graphics Array (XGA) monitor, designed to handle the video and animation requirements of modern multimedia packages and games. Because more lines must be drawn in the same amount of time, the time taken to draw each line must be shorter. In other words, the so-called "scan rate" (or rate at which lines are drawn across the screen) is higher. Higher scan rates require electronics that are more sophisticated, more powerful, and consequently more costly.

Analog Verses Digital Interfaces

The last area of advancement is the physical interface, which allows communication between the PC and the monitor. There are two signal standards existing today: analog and digital. The analog style of connector most commonly uses three individual BNC-terminated cables to carry the individual video/color information to the monitor, conforming to the RS170 standard. The majority of modern monitors accept analog signals as required by VGA, SVGA, XGA and other high-resolution color standards. In the digital interface, a connector carries Transistor-Transistor Logic (TTL)-compatible signals to the monitor. The TTL signals can be converted to analog, but resolution is lost and an additional adapter is required. Manufacturers responded to this problem with the creation of a monitor that can self-adjust to the different scan rates and accept either analog or digital information. These Multiscan monitors can display images at different resolutions, depending on the information sent to them by the video adapters.

Liquid Crystal Displays

Liquid Crystal Display (LCD) monitors are rapidly becoming popular because of their compactness. They take up less space and are lighter than most CRT monitors. LCDs can be used many places where a larger CRT monitor cannot fit. Other advantages to LCDs are their absence of electromagnetic waves, lower heat emission and power savings over the CRT. LCD monitors come in monochrome or color. The screen is perfectly flat, with no distortions due to a curved screen like CRTs. Most new LCDs connect to standard video cards, although a few require their own graphics adapters. Disadvantages to the LCD monitor are the higher cost, image dimness, small viewing angle and slower response times. All of these disadvantages are being addressed as new technology is applied. LCD display is based on a totally different technology than CRTs. An electric current is passed through a liquid crystal solution between two sheets of polarizing material. As the current passes through, it causes the crystals to align so that light cannot pass through them. The image appears as some crystals allow light to pass through and others do not. There are two basic techniques for producing color on these displays: passive matrix and active matrix. Passive matrix is the less expensive of the two technologies and the most popular display used in notebook computers today. The active matrix display refreshes the screen more frequently than the passive. Network Technologies Inc is committed to supplying monitor testers designed for all types of displays and to integrating new standards as they become available.

About Video Products Inc:
Video Products Inc (VPI), based in Aurora, OH (USA), is dedicated to supplying the highest quality connectivity products to integrators, distributors, IT professionals, and tech-savvy home-users. VPIs product line includes a wide variety of monitor testers, cables, adapters, switches and splitters. All products are rigorously tested and are backed with a one-year warranty on all parts and labor, and a 30-day satisfaction guarantee. For more information, visit www.vpi.us.