|
|
The video system (of
which the monitor is a part) is one of the most important components
in the PC. It affects directly your pleasure of working, and
actually also your health. At the same time, the video system shows
the biggest variation between different PC's.
Introduction
All computers are connected to some type of display. That is called
a monitor . Monitors are available in many different types and sizes
(generally 12 to 21 inches diagonal screen size). The monitor is a
part of the computer video system. To understand how to obtain a
good screen image, we need to look at the complete video system. It
includes three elements:
• The graphics card (also called the video card or video adapter).
It is an expansion card, which generates electric signals to the
monitor.
• The monitor itself, which is connected by a cable to the video
card.
• A device driver which Operating system uses to control the video
card, to make it send the correct signals to the monitor.
These three elements must be fitted and matched to achieve quality
images. Even the finest and most expensive monitor will only render
mediocre images if it is connected through a low quality video card.
All video cards depend on the right driver and proper settings to
function properly otherwise the card will not perform well.
Fast development
The video system has developed as explosively as the rest of the PC
during the last 10 years. These improvements have occurred in
different areas:
• The monitors – both the tubes and the electronics continue to
improve. They render better and better images - sharper, with better
resolution and better colors.
• New monitor types are arriving on the market. The flat TFT screens
are in the stores. In few years, they will replace CRT all over the
world.
• The video cards are getting faster. They can deliver better
images, which the new monitors are capable of producing. The user
gets more tuning options. New RAM types and the AGP bus will
increase speed. Video presentation and 3D games are other areas of
development, which will change the video card standards.
The video system is a sub system in the PC, with its own
technological development. At the same time, monitors and video
cards are areas, where manufacturers and dealers often cut corners.
As an ordinary user, you can improve your screen images
significantly with careful planning. That holds true when you buy
your PC - you must select your video system carefully. It also holds
true for existing video systems, where better drivers and software
optimizing can help produce the optimal screen image.
Concepts and terminology
When we talk about screens, there are currently three different
types to choose from:
• CRT – (Cathode Ray Tube) the common type screens. They are found
in different technologies, such as Invar and Trinitron.
• LCD – (Liquid Crystal Display) flat and soft displays. TFT – is
the most expensive display of this type. They are found in better
grade laptops, and are available also for use with desk top PC's.
The TFT screen is also called a "soft" screen, since the images
appear softer than from Cathode Ray Tubes.
Common principles
The principles in these screen types are quite different, but the
screen image design rests on the same concepts:
• Pixels: The screen image is made of pixels (tiny dots), which are
arranged in rows across the screen. A screen image consists of
between 480,000 and 1,920,000 pixels.
• Refresh rate: The screen image is "refreshed" many times per
second. Refresh rates are measured in Hertz (HZ), which means "times
per second".
• Color depth: Each pixel can display a number of different colors.
The number of colors, which can be displayed, is called color depth.
Color depth is measured in bits.
• Video RAM: All video cards have some RAM. How much depends on the
desired color depth. Video cards usually have 1, 2 or 4 MB RAM for
normal usage.
These concepts are central to the understanding of the video system.
Since the CRT screens are still by far the most common, they will
form the basis for this review.
Pixels – the basic element in the screen image
When you look at a screen image, it actually consists of thousands
of tiny dots. If you look close you can spot them. Each of these
dots is called a pixel. That is a contraction of the term Picture
Elements. In an ordinary screen, each pixel consists of three
colors: Red, green and blue. Thus, there are actually three "sub
dots" in each pixel. But they are so small that they "melt" together
as one dot. The individual pixel or dot then consists of three mini
dots, also called trio dot. Some screens do not have round dots, but
they work the same way. With the three basic colors, each of which
can be assigned with varying intensity, you can create many
different colors.
The cathode ray tube
A traditional picture tube is like a big glass bottle. There are
three electron guns in the narrow end. They fire towards the large
flat surface facing the user. The inside of the glass surface we
look at is coated with tiny phosphorous dots. They are arranged in
groups of three – a red, a green and a blue phosphorous dot.
Together they make a pixel. These dots light up, when hit by
elections from the electron gun. Each of the mini dots is hit by its
own electron gun. The more powerful the beam is, the brighter they
get. The electron beams are guided by electromagnets, which bend the
beams, so they hit the exact desired phosphorus dot. The electron
beams sweep across the screen very fast. Each of the three electron
guns must scan its intended color mini dots continually, from left
to right, line by line from top to bottom, typically about 70 to 85
times per second. The beam intensity can be adjusted for every mini
dot, to adjust the color. A typical screen image could consist of
480.000 pixels. That is called a 800 x 600 image. There are 800 dots
in each horizontal line, and there are 600 lines from top to bottom
of the screen. That adds up to 480,000 pixels.
Greater resolutions
The greater the number of pixels in the screen image, the better the
resolution. And the greater the resolution, the sharper the image
appears. The lowest resolution seen in modern PC's is found in text
based DOS screen images, which are 640 x 480 pixels. That is called
a VGA image. VGA was the standard, until Windows came on the market.
Back in the
eighties, there were even lower standards, like CGA, which I will
not even describe. As the PC's got more powerful, around 1990 a
demand developed for better screen resolutions. Windows is a graphic
environment, and it works fine in all screen resolutions. The same
programs work as well in 640 x 480 as in higher resolutions. Many
DOS games also demanded better screen quality. Anyway, VGA was the
last "real standard" working on any PC. Screen resolution was since
improved relative to VGA, and the term SVGA (Super VGA) came into
use. Later came XGA and other names, which each described different
resolutions.
Actually, the terms SVGA and XGA are not used much anymore. Instead
we are looking at resolution, image frequency and color depth. But,
let us stay with the resolution. It ties in with screen size, the
bigger the screen the bigger the possible resolution.
Screen and resolution need to be matched
The greater the resolution the more detail you can view on the
screen. On a Windows 95 desktop, you can see 2-3 as many icons in a
1280 x 1024 resolution as in a 800 x 600. The individual monitor can
be set to different resolutions. However, not all resolutions are
suitable. On a small screen the icons get too small, if you choose
too high a resolution. Therefore, resolution and screen size must be
matched! You can not judge a monitor just by its resolution. Equally
important is refresh rate and color depth. They will be described
later. But first:
Screen size
Monitor screen sizes are measured in inches, just like TV sets. The
most common sizes are 14", 15", 17", 19" and 21" screens. The
measurement is the diagonal size of the screen
However, the nominal size of the common CRT screens does not give a
true description of the visible size. The nominal size is the
internal diagonal of the the picture tube. However, the visible
diagonal is smaller! The visible diagonal of a CRT screen is always
about 10% smaller. Therefore, the visible image on a 17"
CRT screen and a 15.5" LCD screen is about the same.
Colors
Colors are a must! Good PC’s must be able to display many colors on
the screen.
Variable light intensity
The colors are created by varying the light intensity of the three
basic colors (red, green and blue).
Traditionally we work with the following degrees of color depth:
• 256 colors (8 bit color)
• 65,536 colors (16 bit color, also called 65K or HiColor)
• 16 million colors (24 bit color, also called True Color)
• 4 billion colors (32 bit color, also True Color)
For ordinary users, 256 bit colors is sufficient to render high
quality reproduction of photos. However, 256 bit colors have limited
usage, the colors can get rather coarse. 24 and 32 bit colors should
be the choice for graphics artists and professional photographers.
But how are these color variations created on the screen?
Three colors, each in their own depth
Each pixel is composed of three mini dots. The three mini dots are
so close together, as to be viewed as one pixel. But three mini dots
can be illuminated each with their own intensity. This allows us to
produce many different colors.
16 or 24 bit colors
We must be able to vary the three basic colors in a number of steps.
Typically. we use 8 bits for each color. 8 bits provide 256 possible
variations – from 0, giving a light intensity of zero – and up to
step 255, giving maximum intensity of that color. That will provide
the following possible color variations:
• Red in 256 steps
• Blue in 256 steps
• Green in 256 steps
All together there are 3 colors, each of which can be controlled in
256 steps. To identify each of the 256 steps, we need one byte of
data. However, we work with bits. Thus each color requires 8 bits of
data.
Since the PC works with bytes, in practice we use 8, 16, 24, or 32
bit colors. That produces a palette of colors, as we started to see.
In a 16 bit color image, each pixel can show any one of
approximately 16 million colors. It requires 16 bits of data to
describe the color choice of each pixel.
Refreshing the screen image
In traditional screens, the electron gun continually sends out very
precisely aimed beams of electrons, moving from pixel to pixel. The
beam actually flickers, as it sweeps the screen. Each dot on the
screen receives a quick flash of electrons, before the beam moves on
to the next dot. And the beam intensity is varied from dot to dot.
The phosphor coating on the screen has the peculiar ability to light
up, when hit by electrons. But the light quickly fades away. In
practice, The electron beam "visits" again, before there is any
visible fading of the light. It looks to us as a steady picture on
the screen, but actually it flickers every time the electron beam
hits the phosphor-coated dots.
The screen works overtime
Typically, each pixel is hit 60, 70, 75, or 80 times per second.
Thus, the electron gun must move extremely fast to make 18 million
or more hits per second. If the image is refreshed 75 times per
second, we talk about a refresh rate of 75 HZ. The video card issues
the refresh signals, thus controlling the refresh rate. Here we see
a screen with a resolution of 1280 x 1024 and a refresh rate of 75
HZ. That requires the electron gun to make 98 million pixel hits per
second! That screen works at a very hectic pace – which can
sometimes result in beam contamination.
Screen savers
Early monitors had low quality phosphor coatings. That could result
in a screen image to "burn-in" if left unattended. You could clearly
see that in work places, where the PC was used for only one program.
That program image remained clearly on the screen, after the PC was
shut down. That led to screen savers. This prevents the regular
image from burning in. Screens have improved a lot since then - the
screen image will not "burn in" in a modern screen. At the same
time, screen savers have developed into an art form of their own.
Use the screen savers. They can spice up day-to-day work.
|
|
