Dot-pitch is the most universally touted specification when it comes to
buying a CRT display. Friends, relatives, magazines, and manufacturers dish out
recommended values to look for.
Unfortunately, dot-pitch is not what it's cracked up to be and you're actually
much better off by simply ignoring it.
That's because the interpretation of dot-pitch depends on the tube design,
so a 0.25 mm pitch for brand A may not be as good as 0.30 for brand B.
There are also a number of other inter-related factors that are actually more
important than dot-pitch.
Size and Distance
A good analogy is that comparing displays using dot-pitch is like trying to
compare automobile engines by using the cubic inch or liter displacement size:
it really doesn't tell you which engine is better.
In fact, it doesn't even tell you which engine is more powerful because of
factors like fuel injection, turbocharging, number of valves, valve timing, etc.
So what should you do instead of going with dot-pitch? Use your eyes.
They're accurate, reliable and the results are easy to interpret.
In fact, the only good way to determine the sharpness, resolution,
and graininess of a display is to carefully and critically look at
one or more test images. DisplayMate includes the most sensitive and powerful
test images available, plus every screen includes loads of guidance and advice.
Below is a general discussion that explains dot-pitch and why it isn't useful
for selecting or comparing displays. If you need a lot of convincing,
make sure that you read the last paragraph of this article.
The screen is made up of groups of tiny red, green and blue phosphors,
called triads, that produce the colored light patterns that makeup the image.
The pitch is the spacing between the groups, expressed in millimeters.
Values typically range between 0.22 and 0.40 mm, going as high as 0.80 mm
for the largest displays.
The smaller the value the finer and sharper the image can be.
But you can't directly compare the dot-pitch values between displays
to figure out which is better.
There are many factors that affect image sharpness and resolution that are
more important than dot-pitch:
- Color Registration or Convergence
A color display actually has 3 separate red, green, and blue beams,
which need to be critically aligned so that the image is not blurred.
The misalignment is generally much larger than the dot-pitch,
so it's more of a determining factor.
- Beam Size and Focus
The size of beam that hits the phosphors represents the finest detail that
can be displayed. The beam size should always be larger than the dot-pitch,
so it's more of a determining factor.
- Video Bandwidth
How quickly and accurately the display electronics can control the beam
is a major factor in sharpness and image quality.
Video bandwidth, also a much abused specification, describes this capability.
- Video Board Signal Quality, Video Cable Quality and Length
Poor signal quality at any point will degrade image sharpness and image quality.
What Kind of CRT Tube Is It? Compare Dot, Slot and Grille Pitch?
There are three major tube designs that are found in CRT displays:
a shadow mask tube with round dots, a shadow mask tube with slots,
and an aperture grille tube.
The term dot-pitch actually applies only to a shadow mask tube with round dots.
The terms slot-pitch and grille-pitch apply to the other two designs.
Note that pitch values between different designs cannot be compared
to one another.
Which Tube Design Is Best?
The answer to that question depends on your application and visual preferences
and prejudices. Each design has its own particular set of advantages and
disadvantages, so there is no clear winner. That's why all of the current
designs have survived in the marketplace. Most monitors sold use shadow mask
tubes made by a variety of manufacturers. Sony and Mitsubishi make
aperture grille tubes under the Trinitron and Diamondtron names.
You can find plenty of advice favoring each design, but you're much better
off comparing them for yourself. You'll need to carefully and critically
look at several test images. DisplayMate includes the most sensitive and
powerful test images available, plus every screen includes loads of guidance
Guard Bands and Black Matrix
While pitch specifies the distance between phosphor triads,
there are also spaces and borders between adjacent phosphors elements,
which are the guard band and black matrix areas,
and vary significantly with tube design.
The guard band is used to protect against beam misalignment and the black
matrix improves contrast.
The type and size of the guard band and black matrix can have a considerable
effect on image purity, contrast, brightness and Moiré,
which is discussed below.
Guard bands and black matrix specifications are not generally available
to consumers because their interpretation in terms of overall image quality
is difficult even for tube designers.
What Size Screen and How Close Are You Sitting?
The dot-pitch is often larger for bigger screen sizes.
That's fine if you simply want a bigger screen to show the same image to
a larger audience from a greater distance.
Under these circumstances dot-pitch can be proportional to screen size,
all other factors being equal.
On the other hand, if you are going to a larger screen in order get higher
resolution images, then the dot-pitch shouldn't increase, again all other
factors being equal.
Finally, if you are looking at the screen from the typical user distance
of 20-25 inches (50-65 cm), then you may begin noticing the graininess
of the phosphor elements themselves when the pitch is greater than 0.35 mm.
Moiré patterns appear as ripples, waves, and wisps of intensity variations
that are superimposed on the screen image.
They appear to varying degrees in all color CRT displays and arise from the
interference of the beam and image pixels with the screen phosphor elements.
For more information see:
Moiré Patterns: What Are They and How Do I Get Rid of Them?
Horizontal, Vertical, Variable, and Nearest-Neighbor Pitch
So far we've only mentioned a single pitch size. Actually there are several.
One is the horizontal pitch, which is the distance between triads that are
on the same horizontal row. Another is the vertical pitch, which is always
different from the horizontal pitch, and it varies with tube design.
The question is which is the one used in the specifications?
The answer is, believe it or not, that it depends on the tube design.
For dot-pitch it's always the vertical pitch and for grille-pitch it's
always the horizontal pitch. For the slot-pitch it can go either way.
Finally, for some tube designs the pitch varies with position on the screen.
By now you should be thoroughly convinced that pitch specifications are a
bad, even an incorrect way to select a display.
As it turns out, the pure vertical or horizontal pitch values are not the best
values to consider anyway. In terms of image quality, a better figure of merit
is the nearest-neighbor pitch, which is the distance to the closest phosphor
element of the same color. These are not generally available but can be
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