Monitors
THE
DIGITAL CORNER
While
a digital output device is probably the most critical component to producing
high quality digital images, you spend a lot more time looking at your
computer's display. This month I'll
give some tips on selecting a display controller and monitor.
A
display sub-system consists of a "monitor" and a display controller
(or "graphics card"). Both
are important to the end result and are closely related.
Most display controllers are sold on the basis of speed, resolution, and
special features such as the ability to handle 3D animation or motion video.
The special features are almost never important for imaging applications.
Speed is always important, but the common metrics for speed are pretty
much irrelevant for imaging: Graphics
accelerators are very useful for drawing lines and figures, but they can do
little to accelerate the display of photographs.
What DOES help is raw bandwidth. That
is, you need fast, wide paths for the image data.
PCI bus used to be pretty much the only reasonable choice for either PC
or Macintosh platforms. Today, many
PCs have a single "AGP" (accelerated graphics port) slot specifically
for high performance graphics [now enhanced to 2X and 4X
AGP versions]. AGP
was developed specifically for 3D animation, but works by providing tremendous
bandwidth between the system memory and display controller, which helps us as
well. Within the graphics board the
data path to the frame buffer is typically 64 or 128 bits wide, but considerably
faster than even the AGP bus. This
is necessary because additional bandwidth is needed within the confines of the
graphics board. For other kinds of
graphics this bandwidth is used to achieve very fast drawing where many pixels
are drawn in response to a relatively small amount of data (a command) sent over
the bus. But in most cases a lot of
bandwidth is also consumed just to refresh the display.
That is, data is constantly being read out of the frame buffer to drive
the monitor. The higher the
resolution, color depth, and refresh rate, the more bandwidth is needed for
display refresh. The exception to
this rule is that some graphics boards use dual ported memory ("VRAM")
where one port is dedicated to display refresh and has almost no affect on the
bandwidth available on the other port. Until
recently this was the only practical way to achieve the kind of resolution and
color depth needed for serious color imaging (for example, 1280 by 1024 with 24
bits per pixel or 16.7 million colors). But
VRAM was always expensive and has not improved as rapidly as other technologies [I
don't believe anyone is using VRAM anymore].
The current generation of controllers mostly use synchronous dynamic
memory (SDRAM or SGRAM) and provide enough bandwidth for all but the top end
display modes (1600 by 1200 in true color).
Synchronous memory is much cheaper than VRAM for both technical and
market reasons.
As
noted above, serious color imaging requires a resolution on the order of 1280 by
1024 or greater. Color depth is the
number of bits used to represents each pixel (sometimes expressed as the number
of distinct colors possible, which is 2 raised to the Nth power where N is the
number of bits). "True
color" means 16.7 million colors, which is 24 bits, or 8 bits each for the
red, green, and blue channels. In
some cases there are 32 bits of memory for each pixel, but in almost all cases
only 24 are actually used to hold the primary image, so the display is still
limited to 16.7 million colors. "Hi
color" means 64 or 32 thousand colors, which is 16 or 15 bits.
In many images it is difficult to see the difference between hi color and
true color, but in other cases the difference is obvious, and ugly, so true
color is definitely recommended. The
amount of memory needed on a graphics board is defined by the resolution and
color depth: M = (X x Y x B) where M is the minimum memory needed in
bytes, X and Y are the resolution, and B is the number of bytes per pixel.
Note that in some cases additional memory is needed for optimal
performance, or due to architectural limitations.
4 [32-64] Megabytes is common today. This
is just enough to support 1280 by 1024 at 24 bits, but not at 32 bits [today's
controllers have more than enough memory for resolution beyond the capability of
the best monitors - the extra memory is used to accelerate rendering operations
for gaming].
Monitors
are sold on the basis of size (measured diagonally) and resolution.
Unfortunately, resolution in a color monitor is difficult to define.
In most cases an ad will state a specific maximum resolution, such 1024
by 768, and/or a "dot pitch". Dot
pitch is the space between the phosphor dots on a color CRT. People generally assume that there should be one such
"dot" per pixel, and that is generally how manufacturers and retailers
determine resolution, but it is far from the truth.
The "dots" (actually three dots each:
red, green, and blue), are
not individually addressable. That
is, you cannot precisely control where any given pixel will appear on the
screen. So there should be many dot
trios for each pixel. Unfortunately,
the dot trios are defined by a mechanical device (a "shadow mask"),
which has limited precision. Other
factors that affect resolution are amplifier bandwidth and "spot
size". Spot size is the
diameter of the electron beam spot focused on the screen surface.
It is closely related to subjective sharpness, but is difficult to
quantify as it is affected by many variables.
Another
common metric for monitors is the refresh rate. This is usually expressed simply as X hertz, but this number
is meaningless when taken out of context: It's
fairly easy to make a monitor with a vertical sweep rate much greater than that
needed to avoid flicker. But the
horizontal sweep period multiplied by the number of lines usually limits the
refresh rate to a much lower value. In
other words, you need to know what the refresh rate will be at the resolution
you will use. For a 1280 by 1024 display a 75 Hz refresh rate requires a
horizontal sweep rate of about 80 kHz. The
maximum horizontal sweep rate is usually given in a monitor's user manual.
Of course, to use any resolution and refresh rate it must be supported by
the display controller's driver software.
The
bottom line is: Buy the best
monitor and controller you can afford. Use
true color mode and the highest resolution and refresh rate they will support.
You may be able to exceed the monitor manufacturer's recommended
resolution. Don't worry that you
are "wasting" resolution that you can't see - in truth you can
probably never actually resolve the stated resolution (i.e. when displaying a
pattern of alternating black and white lines of one pixel each, can you
distinguish the individual lines). As
long as increasing the resolution improves the appearance it is worthwhile.
[LCD-based flat panel displays are now a common alternative to traditional CRT displays. And the best of these displays are surprisingly good. In addition to good image quality, LCDs have the advantages of lower power consumption and less toxic waste. However, the average LCD today is still not as good in image quality as the average CRT and costs much more.]
Next
month I hope to discuss setting up and calibrating your display system for
optimum performance.
Copyright (C) 2004 Greg Marshall
No portion of this web site may be copied or reproduced in any manner without express written consent.