Minor tweaking will correct some monitor ailments

You turn on your monitor, it hums a short, tuneless little ditty as it
warms up, and then—uh-oh, the monitor looks funny. This can’t be right.


Ideally, the monitor’s electronic gun beams tiny light signals onto the phosphor
dots on the inside of the monitor tube, and then the resulting images are translated to
digital signals to be communicated to the host computer. The trouble is, a lot can go
wrong between these two steps.


Here are descriptions of some of the most common ailments. Fortunately, most of these
problems won’t occur if you’ve bought a high-quality monitor with good controls
in the first place.


Convergence error. It occurs when a misalignment of any of the three beams of light
from the light gun pass through the wrong aperture in the shadow mask and strike the wrong
phosphor dot. You’ll know it’s the problem if you spot a subtle blurring effect
wherever the misconvergence occurs. Specialized coils in the CRT’s deflection yoke
help to eliminate the problem, but not much can be done about it in a cheap monitor. This
condition is also sometimes referred to as misconvergence.


Distortions. The monitor’s viewing background is intended to be perfectly
rectangular. Deviations from this shape can lead to a number of troubling distortions in
the displayed image.


The most common distortions—named after their geometric patterns—are
barreling, pincushioning, trapezoidal, parallelogram and seagull. Usually you can easily
correct them using on-screen digital controls.


Electrostatic build-up. Common among older monitors, it contributes to a buildup of
dust and other allergenic materials. Anti-glare and anti-static screen coatings that come
on most new monitors ameliorate the problem.


Flicker. Often unnoticed, it’s a rapid variation in image intensity on the screen.
It’s usually caused by slow refresh rates and leads to eyestrain, headache, muscle
tension and other forms of user exhaustion. Flicker can usually be cleared up by setting
the monitor to a lower resolution.


North-south, east-west distortions. These distortions are common in older monitors and
are caused by the loss of deflection sensitivity at the center of the screen, leading to a
noticeable inward curvature around the screen boundaries.


Newer monitors automatically compensate for such distortions.


Moire. Most noticeable on blank screens, this effect is a pattern of curved and
straight lines that often slowly undulate near the edges of a screen. It’s usually
barely visible, and it’s most commonly seen on shadow mask CRTs. Unfortunately,
little can usually be done about it. It rarely affects the quality of graphical or text
images.

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