A computer display is a marvelous thing. An unassuming dark gray surface
can suddenly transform into an artist's canvas, an engineer's gauges, a
writer's page or your very own window to both the real world and a huge
range of artificial worlds!
Because we use them daily, many of us have a lot of questions about our
displays and may not even realize it. What does "aspect ratio" mean? What is
dot pitch? How much power does a display use? What is the difference between
CRT and LCD? What does "refresh rate" mean?
In this edition of
we will answer all of these questions and many more! By the end of the
article, you will be able to understand your current display and also make
better decisions when purchasing your next one!
Often referred to as a monitor when packaged in a separate case, the
display is the most-used output device on a computer. The display provides
instant feedback by showing you text and graphic images as you work or play.
Most desktop displays use a
cathode ray tube
(CRT), while portable computing devices such as
crystal display (LCD), light-emitting diode (LED),
or other image projection technology. Because of their slimmer design and
smaller energy consumption, monitors using LCD technologies are beginning to
replace the venerable CRT on many desktops.
When purchasing a display, you have a number of decisions to make. These
decisions affect how well your display will perform for you, how much it
will cost and how much information you will be able to view with it. Your
- Display technology - Currently, the choices are mainly between
CRT and LCD technologies.
- Cable technology - VGA and DVI are the two most common.
- Viewable area (usually measured diagonally)
- Aspect ratio and orientation (landscape or portrait)
- Maximum resolution
- Dot pitch
- Refresh rate
- Color depth
- Amount of power consumption
In the following sections we will talk about each of these areas so that
you can completely understand how your monitor works!
Displays have come a long way since the blinking green monitors in
text-based computer systems of the 1970s. Just look at the advances made by
IBM over the course of a decade:
- In 1981, IBM introduced the Color Graphics Adapter (CGA), which was
capable of rendering four colors, and had a maximum resolution of 320
pixels horizontally by 200 pixels vertically.
- IBM introduced the Enhanced Graphics Adapter (EGA) display in 1984.
EGA allowed up to 16 different colors and increased the resolution to
640x350 pixels, improving the appearance of the display and making it
easier to read text.
- In 1987, IBM introduced the Video Graphics Array (VGA) display system.
Most computers today support the VGA standard and many VGA monitors are
still in use.
- IBM introduced the Extended Graphics Array (XGA) display in 1990,
offering 800x600 pixel resolution in true color (16.8 million colors) and
1,024x768 resolution in 65,536 colors.
If you have been around computers for more than a
decade, then you probably remember when NEC announced the MultiSync
monitor. Up to that point, most monitors only understood one frequency,
which meant that the monitor operated at a single fixed resolution and
refresh rate. You had to match your monitor with a graphics adapter that
provided that exact signal or it wouldn't work.
The introduction of NEC MultiSync technology started a trend towards
multi-scanning monitors. This technology allows a monitor to understand
any frequency sent to it within a certain bandwidth. The benefit of a
multi-scanning monitor is that you can change resolutions and refresh
rates without having to purchase and install a new graphics adapter or
monitor each time. Because of the obvious advantage of this approach,
nearly every monitor you buy today is a multi-scanning monitor.
Most displays sold today support the Ultra Extended Graphics Array (UXGA)
standard. UXGA can support a palette of up to 16.8 million colors and
resolutions of up to 1600x1200 pixels, depending on the video memory of the
in your computer. The maximum resolution normally depends on the number of
colors displayed. For example, your card might require that you choose
between 16.8 million colors at 800x600, or 65,536 colors at 1600x1200.
A typical UXGA adapter takes the digital data sent by application
programs, stores it in video random access memory (VRAM)
or some equivalent, and uses a digital-to-analog converter (DAC) to convert
it to analog data for the display scanning mechanism. Once it is in analog
form, the information is sent to the monitor through a VGA cable. See the
|1: Red out
||6: Red return
||11: Monitor ID 0 in
|2: Green out
||7: Green return
||12: Monitor ID 1 in
or data from display
|3: Blue out
||8: Blue return
||13: Horizontal Sync
||10: Sync return
||15: Monitor ID 3 in
or data clock
You can see that a VGA connector like this has three separate lines for
the red, green and blue color signals, and two lines for horizontal and
vertical sync signals. In a normal
television, all of
these signals are combined into a single composite video signal. The
separation of the signals is one reason why a computer monitor can have so
many more pixels than a TV set.
Since today's VGA adapters do not fully support the use of digital
monitors, a new standard, Digital Video Interface (DVI) has been
designed for this purpose. Because VGA technology requires that the signal
be converted from digital to analog for transmission to the monitor, a
certain amount of degradation occurs. DVI keeps data in digital form from
the computer to the monitor, virtually eliminating signal loss. The DVI
specification is based on Silicon Image's Transition Minimized Differential
Signaling (TMDS) and provides a high-speed digital interface. TMDS takes the
signal from the graphics adapter, determines the resolution and refresh rate
that the monitor is using and spreads the signal out over the available
bandwidth to optimize the data transfer from computer to monitor. DVI is
technology-independent. Essentially, this means that DVI is going to perform
properly with any display and graphics card that is DVI compliant. If you
buy a DVI monitor, make sure that you have a video adapter card that can
connect to it.
Two measures describe the size of your display: the aspect ratio and
the screen size. Most computer displays, like most televisions, have
an aspect ratio of 4:3 right now. This means that the ratio of the width of
the display screen to the height is 4 to 3. The other aspect ratio in common
use is 16:9. Used in cinematic film, 16:9 was not adopted when the
television was developed due to the difficulty of creating a CRT that could
accommodate the format. CRT manufacturing processes have greatly improved,
and the aspect ratio has never been a problem with the manufacture of
alternative display technologies such as LCD. In fact, with widescreen
increasing in popularity, most manufacturers now offer 16:9 displays or plan
to in the near future.
The display includes a projection surface, commonly referred to as the
screen. Screen sizes are normally measured in inches from one corner to
the corner diagonally across from it. This diagonal measuring system
actually came about because the early television manufacturers wanted to
make the screen size of their TVs sound more impressive. Because the listed
size is measured from the inside beveled edges of the display casing, make
sure you ask what the viewable screen size is. This will usually be somewhat
less than the stated screen size.
Popular screen sizes are 15, 17, 19 and 21 inches. Notebook screen sizes
are usually somewhat smaller, typically ranging from 12 to 15 inches.
Obviously, the size of the display will directly affect resolution. The same
pixel resolution will be sharper on a smaller monitor and fuzzier on a
larger monitor because the same number of pixels is being spread out over a
larger number of inches. An image on a 21-inch monitor with a 640x480
resolution will not appear nearly as sharp as it would on a 15-inch display
Resolution refers to the number of individual dots of color, known as
pixels, contained on a display. Resolution is typically expressed by
identifying the number of pixels on the horizontal axis (rows) and the
number on the vertical axis (columns), such as 640x480. The monitor's
viewable area (discussed in the previous section), refresh rate and dot
pitch all directly affect the maximum resolution a monitor can display.
Briefly, the dot
pitch is the measure of how much space there is between a display's
pixels. When considering dot pitch, remember that smaller is better. Packing
the pixels closer together is fundamental to achieving higher resolutions.
A display normally can support resolutions that match the physical dot
(pixel) size as well as several lesser resolutions. For example, a display
with a physical grid of 1280 rows by 1024 columns can obviously support a
maximum resolution of 1280x1024 pixels. It usually also supports lower
resolutions such as 1024x768, 800x600, and 640x480.
Question of the Day for details on dot pitch.
In monitors based on CRT technology, the refresh rate is the number
of times that the image on the display is drawn each second. If your CRT
monitor has a refresh rate of 72 Hertz (Hz), then it cycles through all the
pixels from top to bottom 72 times a second. Refresh rates are very
important because they control flicker, and you want the refresh rate as
high as possible. Too few cycles per second and you will notice a
flickering, which can lead to headaches and eye strain.
Televisions have a lower refresh rate than most computer monitors. To
help adjust for the lower rate, they use a method called interlacing.
This means that the
in the television's CRT will scan through all the odd rows from top to
bottom, then start again with the even rows. The
phosphors hold the
light long enough that your eyes are tricked into thinking that all the
lines are being drawn together.
Because your monitor's refresh rate depends on the number of rows it has
to scan, it limits the maximum possible resolution. A lot of monitors
support multiple refresh rates, usually dependent on the resolution you have
chosen. Keep in mind that there is a tradeoff between flicker and
resolution, and then pick what works best for you.
The combination of the display modes supported by your graphics adapter and
the color capability of your monitor determine how many colors can be
displayed. For example, a display that can operate in SuperVGA (SVGA) mode
can display up to 16,777,216 (usually rounded to 16.8 million) colors
because it can process a 24-bit-long description of a pixel. The number of
bits used to
describe a pixel is known as its bit depth.
With a 24-bit bit depth, 8 bits are dedicated to each of the three
additive primary colors -- red, green and blue. This bit depth is also
called true color because it can produce the 10,000,000 colors
discernible to the human
eye, while a 16-bit display is only capable of producing 65,536 colors.
Displays jumped from 16-bit color to 24-bit color because working in 8-bit
increments makes things a whole lot easier for developers and programmers.
Simply put, color bit depth refers to the number of bits used to describe
the color of a single pixel. The bit depth determines the number of colors
that can be displayed at one time. Take a look at the following chart to see
the number of colors different bit depths can produce:
Number of Colors
(High Color, XGA)
(True Color, SVGA)
(True Color + Alpha Channel)
You will notice that the last entry in the chart is for 32 bits. This is
a special graphics mode used by digital video,
to achieve certain effects. Essentially, 24 bits are used for color and the
other 8 bits are used as a separate layer for representing levels of
translucency in an object or image.
Nearly every monitor sold today can handle 24-bit color using a standard
VGA connector, as discussed previously.
Power consumption varies greatly with different technologies. CRTs are
somewhat power-hungry, at about 110 watts for a typical display, especially
when compared to LCDs, which average between 30 and 40 watts.
In a typical home
computer setup with a CRT-based display, the monitor accounts for over
80 percent of the electricity used! Because most users don't interact with
the computer much of the time it is on, the U.S. government initiated the
Energy Star program in 1992. Energy Star-compliant equipment monitors
user activity and suspends non-critical processes, such as maintaining a
visual display, until you move the
mouse or tap the
According to the EPA, if you use a computer system that is Energy Star
compliant, it could save you approximately $400 a year on your electric
bill! Similarly, because of the difference in power usage, an
LCD monitor might
cost more upfront but end up saving you money in the long run.
CRT technology is still the most prevalent system in desktop displays.
Because standard CRT technology requires a certain distance between the beam
projection device and the screen, monitors employing this type of display
technology tend to be very bulky. Other technologies make it possible to
have much thinner displays, commonly known as flat-panel displays. Liquid
crystal display (LCD) technology works by blocking light rather than
creating it, while light-emitting diode (LED) and gas plasma work by
lighting up display screen positions based on the voltages at different grid
intersections. LCDs require far less energy than LED and gas plasma
technologies and are currently the primary technology for notebook and other
mobile computers. As flat-panel displays continue to grow in screen size and
improve in resolution and affordability, expect them to gradually replace