The Keys to Image Quality

Although the monitor is the most visible part of your display system, it's only one third of the imaging puzzle. The primary piece is the video graphics adapter, which takes the data from the CPU and processes it into the signals that feed the image to the monitor.

VIDEO ADAPTERS

Video adapters are growing in type and number (see next month's Lab Report for a comparison of video adapters). Conventional VGA adapters convert the digital data from the CPU to analog information before sending the data to the monitor. Accelerated video adapters use special chip sets to speed up the processing of GUI operations by off-loading some of these operations from the central CPU. Many high-end adapters also offer 24-bit color rendering, which requires additional on-board RAM for storing processed data.

Fo r highest quality, match a powerful adapter to a monitor capable of receiving the data at high vertical and horizontal frequencies. Carefully review the refresh-rate specifications for both monitors and adapters, even when vendors claim VESA compliance. An adapter may use a standard refresh rate of 70 Hz for 1024- by 768-pixel resolution but use one of a number of different horizontal frequencies. If the horizontal frequency doesn't match that preset by the monitor manufacturer, you'll have to adjust the image for size and placement. In most cases, the problem is temporary, since you can save the adjustment as a user-definable setting. But you'll need to exercise care in matching an adapter and monitor when buying a fixed-frequency monitor. If you find that you can accept the limitations of these monitors, the reward is economic: Often they are the lowest-priced displays.

DISPLAY DRIVER

The final piece in the puzzle is the display driver. In the standard VGA resolution mode of 640 by 480 pixels, a ll adapters should function without special drivers. At higher resolutions, drivers provide the programming interface between the operating system and the video adapter's hardware.

Most adapters now provide specialized drivers for Windows 3.1. If you use OS/2 or applications that aren't Windows-based, such as CAD/CAM programs, make sure that proper drivers exist before making a purchase. If you have had your adapter for a while, it may pay for you to contact the manufacturer for a driver update. Updated drivers are often available from a vendor's BBS. The time you invest in getting a new driver may pay off. We found significant differences in performance from the same adapter simply by using updated display drivers.

Terms

Aperture grill

In Sony's Trinitron picture tubes, a thin piece of metal with long vertical slots through which electron beams pass before striking the phosphors (see Shadow mask).

Blooming

A monitor image that appears to grow in size or "bloom" when it is br ightly illuminated. This occurs most often with bright white characters or objects and causes illegible text, because characters become unfocused as the pixels spread and diffuse.

Degaussing

The realigning of monitor electron beams to reduce screen discoloration. Discoloration occurs when a monitor's electromagnets succumb to interference from the earth's magnetic field after a monitor is tilted or moved. Many monitors automatically degauss at power on; others offer a manual degauss control. Caution: Repeated degaussing within a short time span can damage a monitor's control circuitry.

Dot pitch

The distance between the centers of red, green, and blue phosphor dots that make up the color triad. Smaller distances generally mean greater resolution, since smaller dots produce more dense characters and graphics. In products using the Trinitron aperture grill, the dot pitch refers to the center-to-center distance of the vertical grill openings.

Fixed-frequency

Describes monitors that support video signals at one of a limited number of combinations. These combinations must match frequencies supported by the display adapter used with the monitor.

Flicker

A visual effect that results when an image starts to fade before the monitor's electron beam scans the screen. High refresh rates reduce flicker and keep images sharp and stable. Higher resolutions require high refresh rates to avoid flicker.

Horizontal frequency

The number of lines illuminated on a monitor screen in 1 second. Increasing resolution requires greater horizontal frequency.

Interlaced displays

Interlacing divides the display screen into two fields of odd and even scan lines. In one pass, the electron beams refresh the even lines, and on the next pass, it illuminates odd lines. Interlacing is principally used on devices supporting the 8514A display mode, and the technique can produce noticeable flicker.

Misconvergence

Occurs when the electron beams scan the dots of the wrong red-green-blu e triad. The result is colored edges on white objects or characters. Large amounts of misconvergence appear as shifted colors, as when printed four-color images are misaligned.

Multiscanning

A multiscanning monitor supports an unlimited range of horizontal and vertical frequencies within the limits of its controlling electronics. Multiscanning monitors offer the most flexibility in working with a variety of video display adapters. Most manufacturers offer some number of preset modes--that is, combinations of vertical and horizontal frequencies where the image is pretuned to fill the screen and center itself. The more presets offered, the less chance there is that the user will have to adjust the image manually. (See Fixed-frequency.)

Noninterlaced displays

In noninterlaced displays, electron beams scan every line of the display on each vertical sweep. Noninterlaced displays are generally more visually pleasing and are less likely to cause eyestrain than interlaced monitors. (See Interlaced displays.)

Resolution

Image quality based on the size and amount of pixels used to display an image. Increasing resolution requires more and smaller pixels. Standard VGA resolution is 640 pixels horizontally and 480 pixels vertically (we tested monitors capable of resolutions as high as 1600 by 1200 pixels).

Shadow mask

A piece of thin metal with minute holes that allow electron beams to pass through before striking the phosphors (see Aperture grill). These masks help the electron beams to strike the correct phosphor dots. For example, the mask blocks the electron beam for red from illuminating blue phosphors; in other words, it creates a shadow over the blue and green dots when the beam for red passes.

Vertical refresh rate

The speed at which the electron beams scan across the screen from top to bottom.