Cheet Sheets 1995 February/CHEET38.ZIP/INSTALL.DAT/1DUS.TXT

Text File | 1995-02-07 | 7.9 KB | 182 lines

~Report on VDU's

This report written by Wayne Roberts in 1989, and edited for
Cheet Sheets for a brief look into VDU's.  Some of the
information maybe a little dated but I'm sure the text given
here will be of use as a general guide to VDU's.


~The VDU and how it works.

A Visual Display Unit (commonly known as a monitor) is the
standard output device for most computer systems.  The computer
system deals with all the information generated and outputs the
necessary data to the VDU screen.  Some screen images can only
produce a display of alpha numeric characters and on other
screens, every dot can be controlled by the system.  Each screen
has a variety of different modes in which it can operate.

There are two main types of VDU, Colour and Monochrome.  As well
as a Cathode Ray Tube (CRT), monitors use an electron gun and
phosphur gun to produce the image on the screen.

(a) In monochrome monitors, negatively charged electrons are
emitted from the back of the CRT towards the front of the CRT by
an electron gun.  The front of the CRT is coated with a phosphur
substance that gives off light energy, when struck by the
electron gun.  The light given off can be green, amber or black
and white, depending on the phosphur used.

(b) Colour monitors, however, have triple circuitry and more
parts, hence giving them a higher price.  The phosphur screen is
not a continuous coating, but a shadow-mask perforated with
holes.  Each hole has the mask corresponding to a specific
primary colour (Red, Green and Blue).


~What is Multisync ?

Basically, multisync means that the monitor can be used with any
graphic standards card, such as the Hercules, EGA, CGA, VGA and
MCGA, to provide various resolutions and frequencies, as opposed
to the single frequency of machines such as Hitachi's Multi 560
model.  Multisync is an obvious primary feature of monitors
which allows you to run all various standard with appropriate
cards.



~The refresh rate and reflection of the screen.

The refresh rate, along side the reflection from the screen, are
considered to be the most important features of a VDU, but the
ergonomics of the VDU's are often forgotten when being bought.
If personnel are to sit at terminals most hours of the day they
need monitors that don't make their eyes sore and are fast at
displaying things on the screen. The screen needs to be
refreshed about every 25 to 30 seconds to eliminate annoying
screen flickers.



~Types of screen modes.

Each monitor is compatible with a variety of different screen
modes which decipher what sort of output the monitor produces.
Many of these displays were introduced to the market by IBM
computers.  Below is a short list of some of the more common
screen displays :-

(1) Hercules - This is a text only display.  Introduced in 1981.
Its resolution is normally 720 by 350 pixels.  This display has
no graphic facilities.

(2) MDA - This stands for Monochrome Display Adapter.
Introduced in 1981.  It is a text only display, with a
resolution of 80 characters across, by 25 lines down.

(3) CGA - This stands for Colour Graphics Array.  Introduced in
1981 by IBM.  Its resolution is normally 320 by 200 pixels with
eight colours, and 640 by 200 pixels with four colours, selected
from a palette of eight.  IBM no longer make CGA compatible
cards.

(4) EGA - This stands for Enhanced Graphics Adapter.  Introduced
in 1984 by IBM.  Its resolution is normally 640 by 350 pixels,
with 16 colours, selected from a palette of 64 colours.

(5) VGA - This stands for Video Graphics Array.  Introduced in
1987 by IBM.  Its resolution is normally 640 by 800 pixels, with
16 simultaneous colours, from a 256 colour selection.


(6) PGC & PGA - This stands for Professional Graphic Controller
and Professional Graphic Array.  These were introduced in 1984
by IBM. Its resolution is normally 640 by 480 pixels, and 256
colours can be displayed simultaneously from a palette of
256,000 colours.

(7) MCGA - This stands for Multi Colour Graphics Array.
Introduced in 1987 by IBM.  Its resolution is 320 by 200 pixels,
with 256 simultaneous colours, from a palette of 256,000
colours.

(8) DGIS - This is display mode which is compatible with earlier
industry graphics standards and an alternative to VGA (on
Rasterex monitors for example).  Its resolution is 720 by 512
pixels with 256 colours or 720 by 540 pixels with 16 colours.

(9) TTL - This stands for Transistor to Transistor Logic.


~Resolution of the screen.

The resolution of the screen is very important when it comes to
doing detailed graphical work.  The screen is made up of pixels
(little dots), the more the pixels the higher the resolution.
The higher the resolution the better quality image you will see
and the screen modes of the terminal and VDU determines the
resolution of the screen (as described in 2.04 and in the table
below).  The resolution of the screen is measured by the number
of pixels across the screen in a single line multiplied by the
total number of lines on the screen.

However, since the physical dimensions of PC monitors can vary,
a more accurate way of measuring the resolution of the screen is
by its dot pitch.  The dot pitch is the distance in millimetres
between the centre of two adjacent pixels. Screen displays with
a smaller dot pitch will produce sharper screen images.


~Colour capabilities.

This is one of the more essential elements of a colour display
when it comes to highly accurate colour graphic designs as many
colours often need to be displayed on the screen at one time.
So RGB (Red Green Blue), EGA, CGA monitors are often restricted
to a very low amount of colours. Whereas with VGA you can have a
large amount of colours on the screen at one time.

Here is a table which states the colour capabilities of some of
the more common graphic modes.



~Price of the monitors.

The price of the monitors is not extremely important when
considering a suitable monitor, as monitors aren't judged on a
"price is excellence" basis.  Some companies sell their VDU's at
extremely cheap prices and the quality of the monitor is better
than one which is say three time in cost.  The cost of
monochrome monitors is very low usually about £100 +. Whereas
colour monitors can be priced at anything from £400 and graphic
terminals price from an outstanding £10,000.  (These prices have
decreased since the writing of this article a third of the
quoted price is about right).


~Use of the monitors or graphic workstations.

There are a wide variety of uses put to monitors and the
standard of the VDU reflects mainly on its final use.
Monochrome monitors are, by far, the cheapest and are used more
effectively with word processing or spreadsheet applications all
day.  However, with DeskTop publishing, CAD/CAM or other
graphically oriented applications, the use of EGA and VGA
monitors and graphic cards are essential.

With the more detailed graphically oriented applications, such
as 3D graphical designs, need a more sophisticated type of
monitor.  These monitors come as workstations that are specially
used for graphic design. The price of these workstations are
quite expensive (£10,000 +), but they can produce outstanding
images, many of which look real to life. There are many
companies that manufacture graphic workstations, some of the
leading companies include Apollo, Sun, DEC and Silicon Graphics.


~Since the writing of this article, many new types of graphics
~have been introduced, including the most popular SVGA !!!
~Use this only as a general guide....