Posting-Frequency: monthly (second Monday)
COMP.SYS.IBM.PC.HARDWARE.VIDEO Frequently Asked Questions
New FAQ's coming soon! New additions to Chipsets List also!
This FAQ was compiled and written by Michael Scott with numerous
contributions by others, most notably Ralph Valentino who does a great
job of keeping up the main csiph FAQ, Sam Goldwasser who has developed
and now maintains the majority of diagnostic and repair FAQs for
sci.electronics and sci.electronics.repair, Bill Nott of Compaq
Computer Corporation and Dylan Rhodes of Hercules Computer Technology.
Acknowledgments are listed at end of this FAQ.
Posting to comp.sys.ibm.pc.hardware.video - please read!
For general information and rules on posting to the c.s.i.p.hardware
hierarchy, please refer to the main csiph FAQ, sections 1.2 - 1.6
Before posting to this very busy forum, PLEASE read _at least_ the
list of FAQ questions to ensure your question hasn't already been
answered here! If it has not been answered:
Be as specific as possible. If you are having video problems, please
include the following information:
Symptoms - What exactly are the symptoms?
Where do the symptoms exhibit themselves? i.e. only
in Windows, or with certain applications.
When did the problem start?
Did it ever work properly?
If so, what has changed since?
Under what circumstances are the symptoms seen?
Is the problem repeatable or intermittent?
What have you tried and with what results?
Hardware Configuration - CPU, RAM, bus type (ISA, VLB, PCI), video card
model, amount and type of video RAM, monitor
model if appropriate, video extension cables if
used, resolution/colour depth/refresh rate if
using SVGA or better resolutions.
Software Configuration - operating system and version, video card driver
and version, name and version of conflicting
Anything else unique about your system?
*** Email - make sure the email address in the From: field of your
posting is valid.
- as a courtesy to others, keep your .signature to 4 lines or
Remember, if you include all of the right information the first time,
you'll get an answer back faster, _and_ reduce unnecessary traffic on
the net! Remember to try official channels first - often the manufacturer
can answer common questions quickly.
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Refer to the GNU General Public License for full details.
Everyone is granted permission to copy, modify and redistribute this
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Where section authors are noted, the copyright is held by that author.
Where no author is noted, the copyright is held by the FAQ editor
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Unless otherwise specified, contributors are speaking for themselves
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If you'd like to contribute to the FAQ via comments, additional
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Latest and Greatest:
If you are concerned that this copy of the FAQ is out-of-date,
copies are archived at rtfm.mit.edu or its mirrors in the
/pub/usenet/comp.sys.ibm.pc.hardware.video directory. Alternatively,
you can browse the latest FAQ and download text or compressed versions at:
or get a compressed text-only version from:
For additional ways to retrieve the latest version of this FAQ, refer
to question 1.2 in the main comp.sys.ibm.pc.hardware FAQ, part1:
New sections have been added to the PC Video FAQ Web Site. These
contain information of a graphical or web-centric nature, and so
haven't been included in the text version of the FAQ. The additions
Identifying video card components
Video related sites on the web
Circuits for driving fixed frequency monitors
Table of Contents:
What does the csiph.video FAQ cover?
Are there other sources of info on video related subjects?
Can I use two video cards in the same system?
How can I hook more than one monitor to my video card?
Can I use my TV as a monitor?
Can I use my CGA/EGA/VGA monitor as a TV?
What kinds of monitors are available?
What types of flat-panel displays are available?
What do those monitor specifications mean?
What should I consider when buying a monitor?
What pixel addressabilities are best for my monitor?
What is a shadow mask?
Why does my monitor have 1/2/3 faint horizontal lines on it?
What's the difference between fixed frequency and multisynchronous
How can I get a fixed frequency (RGB) monitor to work on my PC?
What is a low emission monitor?
What does DPMS mean?
How can I maximize the life of my monitor?
Is it important to use a screen saver?
Should I be concerned about monitor emissions?
How do I calculate the minimum bandwidth required for a monitor?
How do I calculate how much VRAM/DRAM I need?
What is the difference between VRAM and DRAM?
(or, Should I buy a VRAM or DRAM based video card?)
What types of video RAM are available (or coming soon)?
What is the EEPROM, EPROM, PROM on my video card?
How does colour depth (bit planes) relate to the number of colours?
What are true color and high color?
Can I use a 64/128+ bit card in on an ISA/EISA/VLB/PCI bus?
Will my video speed up enormously with a VLB/PCI upgrade?
How can an 8/16/32/64/128+ bit video card work on my 16/32/64 bit
How does memory interleaving work to increase the speed of a video
Should I get 1 MB or 2 MB of video memory?
How does a video accelerator work, and will one help me?
What does a video codec do?
How does a 3D graphics accelerator work?
Which video card is best for DOS/Windows/X11/OS/2?
Is my card supported under Windows 95, OS/2, Linux-XFree86, etc?
Which video benchmark is the best?
Should I have video BIOS shadowing on?
Should I use a Universal VESA driver? (i.e. UNIVBE)
I have problems with my display card - how do I fix them?
Why are some of my Windows 3.1 icons black
(Extremely low memory, some icons may not be drawn)?
I have problems with my monitor - how can I fix it?
Are there known conflicts with my video card?
What are MDA, Hercules, CGA and EGA adapters?
What monitors will work with my MDA/Hercules/CGA/EGA card?
What is VGA, and how does it work?
What is the pinout for a standard VGA/PGA/EGA/CGA connector?
What are VGA/SVGA/UVGA/8514/a/XGA?
What is VESA SVGA?
What should I consider in buying a video capture card?
What type of camera do I need for video capture?
I want to add an MPEG card to my system. How does it work?
What is the feature connector on my video card for?
What is DCI?
How do I contact my video card/monitor vendor?
I need new drivers. Is there an Internet ftp/web site for my
Appendix A - Glossary
Appendix B - Popular Video Chipsets
Appendix C - Circuit for Converting from VGA to Fixed-Freq. RGB
In addition, you may be interested in the PC Video Chipset List, which
was originally compiled by Boogyman. It contains a list of common video
chipsets with a brief description, and a list of video card models and
the video coprocessors that they use.
Questions marked with an asterisk (*) will be answered in a future
release of this FAQ.
S) PC Video Frequently Asked Questions
Q) What does the csiph.video FAQ cover?
Issues related to pc compatible video systems are covered here. This
FAQ is primarily intended for hardware, but some software issues are
also considered. The hardware components that are dealt with include,
but are not necessarily limited to:
video capture cards
video playback add-in cards i.e. hardware MPEG decoder
Q) Are there other sources of info on video related subjects?
Some information is available on-line. Because some sites are less
stable than others, you may have to try a given site a couple of
times. For best results, try contacting on off-peak hours. If
you try a site at three or more times and can't connect, please
email the FAQ maintainer and that site will be removed from this
list. If you find a useful site that isn't listed here and seems
to be fairly stable, please send it in.
Last checked: 96/02/28
http://hawks.ha.md.us/hardware/monitor.html : Monitor info
http://www.devo.com/video : Fixed frequency PC video FAQ
http://www.cviog.uga.edu/monitors : Info on 480 monitors
Also contains links to other monitor resources
http://www.cviog.uga.edu/monitors/monitors/manufacturers.html : List
of phone numbers and WWW sites for 60+ monitor companies
http://www.cs.columbia.edu/~bm/3dcards/3d-cards2.html : FAQ for
3D graphics accelerators
http://www.dfw.net/~sdw/index.html : System Optimization Information
http://www.garlic.com/sid/ : The Society of Information Display
http://www.hercules.com/knowbase/ : Bug report and fixes for Hercules
http://www.hercules.com/monitors : WWW Monitor Database by Hercules
http://www.noradcorp.com : NoRad Corporation - info on EMF's, standards
http://www.paranoia.com/~filipg/HTML/FAQ/BODY/Repair.html : Part of
sci.electronics FAQ which contains monitor repair info
http://www.vesa.org/ : Video Electronics Standards Association
includes various VESA standards documents in Adobe Acrobat files
but is only accessible to VESA members! Non-members can
order standards - a price list is available.
http://www.ziff.com/~cshopper : Has a variety of articles from
back issues of Computer Shopper related to PC's including
video cards and monitors
Also see the references at the start of Appendix A - Glossary.
Q) Can I use two video cards in the same system?
[From: firstname.lastname@example.org (Ron Bean)]
The PCI bus has made this easier than it used to be, because it allows
multiple VGA cards to co-exist in one machine. You need a special driver
to let Windows see them as a single display, and since the drivers come
from the video card's manufacturer, that means that all the cards must be
identical (the Matrox Millenium has been mentioned as one, but there may
be others). All of the cards except one must have VGA emulation turned off
(the system needs one VGA card to boot, but more than one would cause
There are also video cards that have more than one VGA chipset and come
with special drivers that make them behave like a single VGA card, but
they may be expensive and hard to find. Brands that have been mentioned
include STB, Colorgraphics, and Appian Graphics. Check card models listed
in the Chipsets List (distributed with this FAQ) for multiple (usually 2
or 4) monitor support.
If you're running X windows, there is a program called x2x which allows
the keyboard and mouse from one X display to control another X display.
In the past, the only way to use multiple monitors was to use one
Hercules-type monochrome card and one color card (CGA, EGA, or VGA),
because the color cards all used the same address space. Most DOS software
will only use one at a time (you can switch between them with the 'mode'
command), but debuggers and CAD programs often support this type of
dual-display system. You may have to change a setting on your VGA card to
make it run in 8-bit mode in order for this to work. Note also that many
cheap clone monochrome cards include CGA emulation, and there may be no
way to disable it.
Windows 3.x can also be set up this way. Include the line
DualDisplay=TRUE (or ON) in your SYSTEM.INI file, in the 386enh section.
If you open a DOS shell window and type MODE MONO, the shell will appear
on the monochrome monitor (I don't know if this still works in Win95).
If you just want to display the same image on several monitors, there
are (expensive) signal splitters that will do this (try vendors that
specialize in things like cables and switchboxes). Signal splitters for
EGA/CGA are somewhat cheaper. See "How can I hook more than one monitor
to my video card?"
Q) How can I hook more than one monitor to my video card?
[ From: Sam Goldwasser (email@example.com) with a bit from
Michael Scott (firstname.lastname@example.org) and Bill Nott
The following discussion assumes that you want to display the same video
signal on a number of monitors. If instead you want use 2 or more
monitors to increase your screen real estate, refer to the section
"Can I use two video cards in the same system?".
The best way to do this is to purchase a commercial VGA signal splitter
or video distribution amplifier.. These are not cheap, but they will
provide the best results. A video splitter designed for VGA or SVGA
will include the proper high bandwidth video amplifiers as well as the
proper cable termination and shielding.
Someone may suggest that you just cut and splice a couple of VGA cables
together, but this won't provide good results. Major problems relate to
cable termination and interference.
In order for the video to be sharp and clear without ghosting or ringing,
the video cable must be treated as a transmission line. What this means
from a practical point of view is that it must use high quality coaxial
cable, multiple monitors must be daisychained and not star connected, and
the proper terminating resistors must be put only at the very end.
Another problem is that video signals operate at high frequencies, and as
a result they can cause interference with neighbouring electronic devices,
and even the monitor itself. In fact, the video cable can, when designed
improperly, act like a nice big antenna. To minimize the interference
emanating from the cable, considerations like conductor material, length,
shielding, connectors and chokes are taken into account. Chokes are those
(usually cylindrical) objects that are located at the ends of many video
The result of a good cable design is an impedance matched circuit, which
causes a minimum amount of interference, and provides a clean crisp signal
to the monitor.
If you know enough about electronics, and the monitors and video card in
question, then go ahead and design and build a splitter. If you don't,
you may cause additional problems. Basic rules for a cable-only solution:
1. Use high quality 75 ohm coax - RG59 is a generic part number but
many variations are available.
2. Multiple monitors must be daisychained and not split in a star
3. Only the last monitor should have its 75 ohm terminating resistors
in place. They should be removed from all other monitors or if they
have switches, set for HiZ.
4. Pay attention to the grounds - signal returns. Keep the stubs - the
connections to intermediate monitors - as short as possible.
This will work quite well for workstation monitors - those with BNC coax
connectors. Most PC monitors with the 15 pin VGA connectors do not have
any means of disconnecting the terminating resistors without actually
doing some desoldering - which you really should not attempt unless
you are familiar with the safety issues involved in working inside a
If you decide to build an active video splitter which uses video
amplifiers, be aware that the video and sync voltage levels are different
in a PC: The video is typically 0.7 V p-p, and the sync's are typically
TTL level (5 V p-p), so the splitter or amplifier must be able to handle
both levels. Finally, pay attention to the video bandwidth capability of
the splitter/amp if you care about preserving image detail information.
As noted a better solution is to buy an active video splitter. This will
include the proper high bandwidth video amplifiers and termination.
Q) Can I use my TV as a monitor?
Monitor prices are constantly dropping, but not as fast as many people
would like. A nice 17+ inch monitor is still $800 or more. It would
be nice if a cheap 20 or 27 inch television could be used instead.
Well, there are products available which will convert a VGA signal to
one that is compatible with your television set. Below, I will outline
the limitations that this type of setup has compared to a proper
computer monitor. This may or may not be a viable option for you
depending on what types of applications you use most frequently.
To understand what is and isn't possible, we need to know the
differences between a computer's video signal and the one expected
by your television (usually NTSC (North America) or PAL (Europe)).
Typically, PC's display in one of 320x200, 320x240, 640x480, 800x600,
1024x768, 1280x1024 or 1600x1200. The lowest three pixel
addressabilities are supported by VGA, and are the most common video
modes used for VGA (often called DOS) games. The higher resolutions
are typically used when in windowing environments like Windows 3.1,
OS/2, Win95 or X-Windows. In addition, computers refresh the screen
at varying rates, ranging from 50 to 75 or more Hertz (cycles/s).
Most newer monitors support non-interlaced video at all or all but
the highest pixel addressabilities. The digital signal is converted
into an analog one that the computer monitor understands. If the
signal is within the capabilities of the monitor, it will be displayed
as a screen image.
Televisions are also analog devices, like computer monitors, but they
are designed to accept a broadcast television signal like NTSC or PAL.
The NTSC standard supports a maximum of 525 lines, while PAL supports
up to 625. The screen refresh rates are fixed for NTSC and Pal as
60 and 50 Hz respectively. In addition, both standards are interlaced.
It is important to note that since computer monitors and televisions
are both analog devices, the number of colours is not a factor here.
To compare this to computer video modes, we have to do a little bit
of hand-waving, but basically, the best North American televisions can't
display more than about 500 lines. This roughly translates to a maximum
of 500 vertical pixels. In addition, the video amplifiers used in
televisions are fairly low bandwidth devices, and can't handle high
horizontal resolutions like 1024 or 1280 pixels. What this boils down
to is that standard televisions can display a video signal derived from
up to a 640x480 pixel mode.
To summarize, a VGA to TV converter can be used to translate the VGA's
RGB signal to a TV compatible signal for resolutions up to 640x480.
This means that the vast majority of VGA/DOS games will display
reasonably well on a television since most use 320x200 or 320x240
video modes. Converters that claim to handle higher resolutions have
scan converters in them that reduce the effective resolution to that
of NTSC or PAL television. i.e. You simply can't display better than
640x480 on a TV.
Some PC to TV converters are listed in the Chipsets section of this FAQ.
Q) Can I use my CGA/EGA/VGA monitor as a TV?
CGA and EGA monitors are digital, rather than analog like televisions
and more modern monitors, usually making them incompatible with TV.
Television signals contain all colour information along with syncs on
one conductor. In addition, there are two types of television signals
- the RF that comes in from cable or an antenna, and composite. The
line-in/out on a VCR is a composite signal, and doesn't contain all of
the different channel information that an RF cable signal does.
The original CGA monitors accept a composite signal, but it is TTL,
which uses a different voltage from composite. Some CGA (and
perhaps EGA?) monitors have composite-in jacks and circuitry inside
them to display a composite signal. If you have one of these, then
you can feed it a composite video signal from a VCR, laser disc
player or other composite video source.
Since the VGA/SVGA monitor was introduced, computers have used an RGB
video signal, with separate horizontal and vertical syncs. This means
that five separate wires are used to carry the video signal from the
computer to the monitor. In order to display a TV signal on a VGA
monitor, signals for all five wires have to be derived from one, the
so-called composite TV signal. This involves some electronic circuitry,
so it can't be accomplished simply by attaching all of the wires
Because of the demands of higher pixel addressabilities and refresh
rates, VGA and newer monitors run at horizontal refresh rates of
30 kHz or higher, which is double that of composite video (15.7 KHz).
Basically, these newer monitors are unable to sync to a low enough
frequency to display broadcast (NTSC or PAL) video. The end result
is that it is not feasible to use a VGA or better monitor to display
a television signal. The only real alternative is to purchase a
TV card for your computer which allows you to display a television
signal on your monitor. Personally, I'd rather spend the money on
a small TV rather than look at a four inch window on my already
cramped computer monitor.
Q) What kinds of monitors are available?
Since there is a large variety of different types available, only some
of the more common are listed here, along with their most common
applications. In fact, it's difficult to define exactly what a 'kind
of monitor' means. There are grayscale and colour, analog and digital,
flat and not. I'll try to give some general answers.
Monochrome, Grayscale and Colour
This one's easy. Monochrome monitors can display two colours, usually
black and one of white, green or amber. Grayscale monitors display
only intensities between white and black. Colour monitors display
combinations of red, green and blue, each in an independent intensity.
Even though each colour is displayed only in one frequency (the
frequency of light that a particular type of phosphor emits when excited)
the combination of the three colours in different intensities fools the
eye such that it perceives a full range of colours.
Analog and Digital
[From: Michael Scott (email@example.com) and Sam Goldwasser
Today, digital monitors are much less common than analog though in the
days of CGA and EGA the situation was reversed. Digital does _not_
mean that the monitor has digital controls. Rather, it indicates that
the monitor accepts a digital input signal. Examples of digital
monitors include early monochrome, the IBM EGA and CGA. Digital
monitors are limited by their internal hardware as to the number of
colours that they can display. Most digital monitors use TTL signals
(Transistor Transistor Logic). Note that some sales persons will call
a new analog monitor 'digital', in reference to the controls. Strictly
speaking they are wrong - see "Analog vs. Digital Controls" below.
Analog colour monitors can display an unlimited range of colours, since
they accept an analog video signal. This means that the horizontal and
vertical syncs, and actual video signals (usually red, green and blue)
are analog. The total number of colours that a given computer system
with an analog colour monitor can display is limited by the video card,
not the monitor. It is rare for video cards to use digital-to-analog
converters capable of generating more than 256 intensities per colour,
so it is rare for systems to be able to display more than 256*256*256
equals 16.7 million colours. Analog monitors can have digital controls
on the front panel, and have digital circuitry inside. The vast majority
of monitors currently in use are analog, as they are more flexible than
the digital variety and typically lower cost.
Most graphics cards put out an analog _or_ digital signal but not both.
Similarly, most monitors accept and analog _or_ digital signal. It is
feasible, however, to convert a digital video signal to analog and
vice versa, though building such a device requires considerable
Shadow Masks and Aperture Grilles
By far the most common type of monitor uses a shadow mask, which is a
fine metal grid which enables the electron beams for red, green and blue
to only impact their proper phosphor dots. One alternative to this design
is the aperture grille, which uses fine vertical wires for the same
purpose. Sony first used this aperture grille in their Trinitron line.
Which one is better is not clear cut and is largely a matter of personal
preference. Note that one complaint of Trinitron users is the presence of
1 or 2 very fine, almost invisible, horizontal stabilizing wires apparently
needed to keep the fine aperture grill wires from moving out of place.
You need to decide whether these will prove an unacceptable distraction.
Trinitrons are usually considered to be brighter and sharper - but this
is not always the case.
Analog vs. Digital Controls
[From: Michael Scott (firstname.lastname@example.org)]
An analog monitor can have either analog (dials or knobs) or digital
(buttons, sometimes with a dial) controls for brightness, contrast,
screen size and position, pincushioning and trapezoidal shape, among
others. Also, digital controls tend to be associated with a monitor's
ability to store factory and user calibrations for image size and
centering when operated at common video modes. This is desirable for a
user who may be switching between DOS and windows applications often, so
they don't have to be bothered with readjusting these controls after each
change. Analog controls have the benefit of being infinitely adjustable,
while digital controls are limited to a number of discrete steps for each
Flat Panel vs. Conventional Tubes
Cathode ray tubes (CRT's) are the most common, inexpensive and best
performing displays available for most users. Variations of CRT's
exist including older designs with double curvature, some with only
curvature in the horizontal plane (like Sony Trinitrons) and others
which are called flat screen.
Flat panel displays are usually used in laptops because of their small
size, but are expensive to manufacture and don't provide the high
refresh rates and bright colours that conventional CRT technology
provides. Flat panel displays range from monochrome LCD (Liquid Crystal
Display) to dual scan colour to active matrix colour. Because of the
difficulty of manufacturing these displays, and the fact that currently
their primary application is in laptops where the maximum display size
is usually less than eleven inches, high resolution flat panel displays
are rare and expensive. In future, it's very likely that flat panel
displays will replace conventional CRT technology for many home and
business computer users.
Q) What types of flat-panel displays are available?
[From: Michael Scott (email@example.com) and some from Bill Nott
Flat-Panel Display (FPD) technology is evolving rapidly, so I will only
touch on the most common current types of displays. There are other
types of displays still in use, though the most common ones are based
on LCD (Liquid Crystal Display) or PDP (Plasma Display Panels)
technology. Now, FPD's are expensive due to the difficulty in
manufacturing (typically ~65% yield - ~4 in 10 are discarded) and
relatively small number of units sold. As manufacturing techniques
improve and volume increases, prices will drop. In fact, in 1995,
yields are up, volumes are up, _and_ factory capacity has expanded to
the point where prices are dropping significantly this year. It appears
there will be an oversupply of panels this year. However, the prices are
still not down to the point where they can compete with CRT monitors in
[From: Michael Scott (firstname.lastname@example.org)]
The vast majority of FPD's are addressed in a matrix fashion, such that
a given pixel is activated by powering the corresponding row and
column. This means that an individual LCD element is required for each
display pixel, unlike a CRT which may have several dot triads for each
LCD displays consist of a layer of liquid crystal, sandwiched between
two polarizing plates. The polarizers are aligned perpendicular to
each other, so that light incident on the first polarizer will be
completely blocked by the second one. The liquid crystal is a
conducting matrix with cyanobiphenyls (long rod-like molecules) that
are polar and will align themselves with an electric current. The
neat feature of these molecules is that they will shift incoming light
out of phase when at rest. Light exiting the first polarizer passes
through the liquid crystal matrix and is rotated out of phase by
90 degrees, then it passes through the second polarizer. Thus,
unpowered LCD pixels appear bright. When an electric current is
passed through the crystal matrix, the cyanobiphenyls align themselves
parallel to the direction of light, and thus don't shift the light out
of phase, the light is blocked by the second polarizer and the LCD
So, basic LCD technology can generate bright or dark pixels, like a
monochrome (not grayscale!) monitor. In order for the eye to see
shades of gray, the LC activation time is modulated. i.e. a pixel
that is activated 50% of the time will appear as 50% gray. The
number of shades that can be generated without visible flicker is
limited by the response time of a LC element - typically 16 shades,
although some display manufacturers claim 64 or more shades.
Most colour LCD's use red, green and blue sub-pixels, similar to the
way that CRT's use coloured dots of phosphor. The concept is the same;
that when viewed from a distance, the human eye will perceive the
three sub-pixels as a single colour. Obviously, this requires three
times as many discrete elements as would a monochrome display of
the same resolution. A second method of implementing colour uses a
subtractive CYM (Cyan Yellow Magenta) system where white light is
generated at the back plane. The light then passes through each of
three LC layers, each one blocking one of the three colours. By
activating the LC layers in different combinations, a variety of
colours can be produced.
Common to all LCD displays is the requirement for either high ambient
light levels, or bright backlighting since liquid crystals don't
generate light - they can only block it. Typically, LCD's allow 5-25%
of incoming light (i.e. from the backlight source) to pass through.
The result of this is that LCD technology requires a significant
amount of energy, and this is an important consideration in light-
weight laptop design.
Specific type of LCD's
Passive Matrix (twisted-nematic) LCD's
PM LCD's come in several types including; supertwisted nematic,
double supertwisted nematic and triple supertwisted nematic. The
original PM LCD's had a very limited viewing angle and poor contrast.
Super and double supertwisted nematic designs provide an increased
viewing angle and better contrast. The triple supertwisted design
implements the subtractive CYM colour model mentioned above. PM
designs are addressed in matrix fashion, so a VGA PM display would
require 640 transistors horizontally and 480 vertically. Rows of
pixels are activated sequentially by activating the row transistors
while the appropriate column transistors are activated. This means
that a given row is activated for only a short time during a screen
refresh, resulting in poor contrast. Some implementations of PM
technology break the screen into two parts, top and bottom, and
refresh them independently, resulting in better contrast. These are
called Dual Scan PM LCD's. In addition, PM displays suffer from
very slow response times (40-200 ms) which is inadequate for many
applications. Aside from their performance shortcomings, PM
displays are inexpensive - their relatively low number of discrete
components reduces manufacturing complexity and increases yields.
Note that while dual scan displays are better than the original PM
LCD's, they still don't have the high refresh rates and brightness
of active matrix LCD's.
Active Matrix LCD's
Instead of using one switch (transistor) for each row and column, AM
LCD's dedicate one switch for each pixel. This results in a more
complex display which requires a larger number of discrete components,
and therefore costs more to manufacture. An AM display is basically a
large integrated circuit (IC). The benefits are significant over the
PM design. Pixels can be activated more frequently, giving better
contrast and control over modulation. AM technology can produce higher
resolution displays that can generate more, and brighter colours. The
main types of AM LCD's are; TFT (Thin-Film Transistors), MIM (Metal-
Insulator-Metal) and PALC (Plasma Addressed Liquid Crystal).
FE LCD's use a special type of LC which holds its polarization after
being charged. This reduces the required refresh rate and flicker.
Also, FE LCD's have a fast response time of 100ns. Although they are
very difficult to manufacture, and therefore expensive, FE LCD's may
provide AM quality at PM prices in future.
Plasma Display Panels
PDP's have been under development for many years, and provide rugged
display technology. A layer of gas is sandwiched between two glass
plates. Row electrodes run across one plate, while column electrodes
run up and down the other. By activating a given row and column, the
gas at the intersection is ionized, giving off light. The type of gas
determines the colour of the display. Because it has excellent
brightness and contrast and can easily be scaled to larger sizes, PDP's
are an attractive technology. However, their high cost and lack of
grayscale or colour have limited applications of PDP's. However,
advancements in colouring technology have allowed some manufacturers to
produce large full-colour PDP's. In future, large colour PDP's will be
more common in workstation and HDTV applications.
Q) What do those monitor specifications mean?
Refer to Appendix A - Glossary for definitions of terms not included
in this section.
Like so many other areas in high-technology, a bewildering array of
models are available, and along with them comes a list of
specifications. There are a few that will help you understand more
about the differences between specific models.
[Thanks to Bill Nott for straightening me out on bandwidth and dot
Bandwidth: This is a measure of the total amount of data that the
monitor can handle in one second, and is measured in megahertz (MHz).
The bandwidth of a monitor is limited by the design of the video
amplifiers. It is generally desirable to match the bandwidth of the
monitor with the dot clock of the video controller to take full
advantage of both devices. see dot clock. see 'How do I calculate
the minimum bandwidth required for a monitor?'
Dot Clock: This is the clock frequency (in MHz) used by the video
controller chip, sometimes termed pixel rate. Many newer graphics
processors have variable dot clocks, but usually only the highest is
quoted in specifications. It is a measure of the maximum amount of
throughput that a video controller can sustain. A higher dot clock
generally means that higher screen addressabilties, colour depths and
vertical refresh rates are possible. If you want to know the
_approximate_ maximum dot clock for your video card and it isn't
specified, you can calculate an approximate value (which tends to
overestimate) as outlined in "How do I calculate the minimum
bandwidth required for a monitor?"
Horizontal Scan Rate (HSR): This is a measure of how many scanlines of
pixel data the monitor can display in one second. The electron gun has
to scan horizontally across the screen and then return back to the
beginning of the next line ready to scan again. It is controlled by
the horizontal sync signal which is generated by the video card, but is
limited by the monitor. If too much data (i.e. too high a horizontal pixel
addressability) is sent to the monitor, it exceeds its ability to modulate
the electron gun, and the signal will be displayed incorrectly and/or
the monitor may be damaged. VGA and SVGA monitors must have a minimum
HSR of 31.5 kHz to be able to display the corresponding horizontal
resolutions. Now we begin to see how the vertical refresh rate and
the horizontal scan rate are related.
Refresh Rate (also Vertical Refresh Rate or Vertical Scan Rate): This
measures the maximum number of frames that can be displayed on the
monitor per second at a given pixel addressability (resolution). It is
controlled by the vertical sync signal coming from the video card. The
vertical sync tells the monitor to position the electron gun(s) at the
upper left corner of the screen, ready to paint another frame. The
maximum rate for a given monitor is dependent on the frequency
capability of the vertical deflection circuit and the pixel
addressability, since higher addressabilities require a higher
horizontal scan rate. For example, a monitor which can provide 72Hz
refresh rate at 800x600 may only be capable of 60Hz refresh at 1024x768.
In order to be considered a VGA or SVGA monitor, the unit must provide a
minimum vertical refresh rate of 60Hz. In general, higher is better, but
there is no point in paying more for a video card and monitor which
are capable of higher refresh rates if you won't notice a difference.
60 Hz is adequate for most people, but others are bothered by flicker
and prefer 72 Hz or faster to reduce eye strain. The minimum acceptable
refresh rate for you may also depend on the screen resolution and monitor
size. In general, higher addressabilities require higher refresh rates
to prevent flicker from becoming noticeable.
A monitor's maximum vertical refresh rate is limited by how fast it can
direct the electron beam over all of the picture elements on the monitor.
This involves moving the electron beam in the same manner as you would
read the words in a book, left to right, top to bottom. It is limited
by the maximum HSR, which determines the maximum horizontal pixel
addressability the monitor can display and the number of scanlines (i.e.
vertical addressability). For example, to display a screen with an
addressability of 640 pixels horizontally and 480 vertically, a monitor
with a HSR of 31.5kHz would take 480/31.5k = 15.2 ms to scan the entire
screen once. In one second, this monitor could be refreshed
1000ms/15.2ms = 65.6 times. However, the vertical sync - movement of
the electron gun to the upper left corner of the screen - requires some
time, so the resulting vertical refresh rate is only 60 Hz.
Built into the HSR and vertical refresh rate are the horizontal and
vertical blanking intervals, respectively. During horizontal blanking,
the electron beam is moved back across the screen from the right end of
one scan line to the beginning of the next scan line on the left of the
screen. This occurs once for each scan line displayed. The vertical
blanking interval occurs after the last scan line is displayed, and the
electron beam is directed back to the upper left corner of the screen
to begin displaying the next screen image.
Interlacing: Interlacing is a holdover from television standards which
use it as a way of putting more information on the screen than would
otherwise be possible. Original television technology could handle
thirty full frames of video per second. However, a 30 Hz refresh rate
results in highly annoying flicker, so the video signal is divided
into two fields for each frame. This is accomplished by displaying
first the odd scanlines (i.e. 1,3,5, etc.) for 1/60 of a second, and
then displaying the even scanlines for the next 1/60 of a second.
Your brain can integrate the two fields, and the result is a higher
effective resolution and lower flicker. Ideally however, you want to
display a frame of video information at full resolution - i.e. have
one horizontal scanline for each horizontal line of pixels and display
it at a high enough refresh rate that flickering is not an issue.
Fortunately, modern monitor technology is capable of non-interlaced (NI)
display at high vertical refresh rates. Many non-interlaced monitors
can only work in non-interlaced mode up to a maximum pixel addressability,
above which they revert to interlaced mode. For this reason, it is
important that you ensure that the monitor you buy is capable of
non-interlaced display at the maximum addressability and vertical refresh
rate that you want to use. Typically, interlaced computer monitors
refresh at about 87Hz, or 43.5 full frames per second. Interlaced
displays can result in annoying flicker, especially noticeable with
thin horizontal lines because the scanline is alternating between the
line and background colours. It's very noticeable if you look at the
top or bottom edge of a window on an interlaced monitor.
Dot Pitch: Images on a computer monitor are made up of glowing blobs
of phosphor. On colour monitors, the smallest discrete picture element
consists of three phosphor blobs, one each of red, green and blue.
These elements are called dot triads. On most monitors the blobs are
arranged in rows and columns, often with every other row staggered:
R G B R G B R - Red
B R G B R G G - Green
R G B R G B B - Blue
B R G B R G
So, in the above example, a shape like the following might be a
The dot pitch is measured as the shortest diagonal distance between
the centers of any two neighbouring dot triads. This is the same as
the shortest diagonal distance between any two phosphor blobs of the
same colour. As dot pitch decreases, smaller objects can be resolved.
Resolution: First, the correct term that _should_ be used in place
of resolution for most computer video discussion is pixel addressability.
This is because in actuality, when we talk about 'resolution' being
say, 640x480, we are referring to how many pixels can be addressed
in the video frame buffer. Resolution should actually be defined
as the smallest sized object that can be displayed on a given
monitor, and so is really more closely related to dot pitch. So,
two definitions are given here. The first is technically more
correct, while the second is the more common interpretation (though
The technically correct answer:
[From: Bill Nott (BNott@bangate.compaq.com)]
Resolution: The ability of a monitor to show fine detail, related mostly
to the size of the electron beam within the CRT, but also to how well
the focus is adjusted, and whether the video bandwidth is high enough.
Note that the dot pitch of a CRT is generally an indication of the
tube's resolution ability, but only because the manufacturers try to
maintain a spot size enough larger than the dot pitch to prevent Moire'
patterning from appearing.
The more mainstream usage:
This refers to the maximum number of pixels which can be displayed on
the monitor at one time, and is expressed as (number of horizontal
pixels) by (number of vertical pixels) i.e. 1024x768. While a higher
maximum resolution is, in general, a good thing, keep in mind that as
the resolution gets higher, the pixel size gets smaller. The resolution
capability of a monitor puts practical limits on the maximum pixel
addressability a user may want to use. You may notice that most
addressabilities are in the ratio of 4:3. This is also a holdover from
television technology which uses the same 4:3 aspect ratio. As a result,
monitor size can be quoted with one diagonal measure, since the
horizontal and vertical sizes can be calculated from the 4:3 ratio. In
future, HDTV (High Definition Television) will use 16:9 (the same aspect
ratio as used in movie theatres) and this may spill over into computer
The following are recommendations:
Monitor Size 14" 15" 17" 20"
640x480 A A B B
800x600 C A A B
1024x768 D C A A
1280x1024 D D C A
Legend: A - Optimal
B - Grainy, pixels become visible
C - Usable, but objects become small and fine detail becomes
D - Not Recommended, objects are difficult to see and fine
detail can not be perceived
These are only recommendations. Personally, I can only afford a 14" NI
monitor, and I run it at 1024x768. Objects are small, but my vision is
[From: Sam Goldwasser (email@example.com)]
Keep in mind that there is also a very wide variation in the quality of
the images between manufacturers and between models. Many factors
contribute to this variation including video amplifier bandwidth,
sharpness of the electron beam (focus), dot pitch of the CRT shadowmask
(or line pitch of a Trinitron's aperture grill), stability of the power
supplies, bandwidth of the video card, quality of the cables, etc.
[From: Bill Nott (BNott@bangate.compaq.com)]
Note: Many monitors are able to operate (synchronize, and present an
image) at pixel addressabilities beyond their resolution capabilities.
When operated in this way, fine detail (single pixels) within the image
may not be perceptible by the user.
[From: Bill Nott (BNott@bangate.compaq.com) and Michael Scott
Size: Monitor sizes are typically quoted in inches, and this is
measured across the diagonal length of the monitor i.e. the longest
possible measurement. Industry practice has been to list the size of
the picture tube as the size of the monitor, but this has lead to some
problems. For example, a tube may measure 17" across the diagonal, but
due to glass thickness and that the tube is encased in the monitor
housing, the viewable area is only 15.5". So, just because two monitors
are advertised as being the same size doesn't mean that they have the
same viewable area.
Part of the source of this inconsistency is that the monitor _tube_
manufacturers do not specify image performance such as focus and
convergence up to the extreme edge of the phosphor, so the image size is
adjusted to that which the tube supplier specifies. (Many monitors today
provide the possibility of adjusting the image size larger than this,
but may neglect to tell the user to expect image quality degradation
beyond the calibrated image size.)
Some users may have allowed themselves to think (or wish) that the
size designation should refer to the image size, but this has never been
true. Regardless, within the US, the Federal Trade Commission (the body
which brought standardization to the TV industry with use of the "V"
terminology) is working to produce a standard for computer monitors.
Some vendors actually quote viewable area in addition to the tube size,
but this is not provided by all vendors yet. Until then, caveat emptor -
take a measuring tape with you when you go shopping.
Q) What should I consider when buying a monitor?
[From: Michael Scott (firstname.lastname@example.org) with contributions from
Andy Laberge (email@example.com) & Bill Nott (BNott@bangate.compaq.com)]
Your monitor may be the most expensive option of a new computer system,
and is the part that you will be looking at most of the time, so it
pays to get the right one for your purposes. You will have to decide
what size is appropriate for your work - in general bigger is better,
but do you really want to shell out $3000 for a huge 21" monitor that
weighs 80 lbs and covers most of your desk? The most common monitor
sizes are 14", 15", 17" and 21". See "What pixel addressabilities are
best for my monitor?" Make sure that your monitor can display the
highest screen addressability that you want to be able to use, and that
the refresh rate at that addressability is reasonable (generally >=60 Hz).
Note that VESA and European standards groups are moving towards 75 and
85 Hz recommendations, respectively. You should expect to pay more
for a monitor capable of higher refresh rates because they use faster
video amplifiers and deflection circuits. You also have to know
whether the monitor is interlaced or not at the higher addressabilities.
In addition, decide what features you would like in your monitor including:
pincushioning and/ or trapezoidal controls, individual RGB gain and
cut-off controls, remote control, programmable memory for presets,
warranty & service, etc. Once you have decided what you want, and have
narrowed the field to a few choices, you should go somewhere that you
can compare the possibilities beside each other.
Typically, CRT manufacturers today do not specify the image performance
such as focus, convergence, and geometry, out to the edges of the tube.
As a result, you have to evaluate these parameters for yourself. Also,
users typically do not want their images overscanned as in TV displays,
especially when using GUI's. If monitors overscanned, parts of the image
near the edges of the screen may not be visible. Thus, the useable image
size of a monitor will be smaller than the maximum useable phosphor area
which may be specified for the FTC. VESA has already established and
published a standard for useable image size in a computer monitor.
Comparing and Testing Monitors
First make sure that the monitor(s) has warmed up for at least ten
minutes. The heat escaping from the rear of the monitor should not be
much more than that generated by a colour television. Some monitors are
now coming with fans installed for positive ventilation. Next, adjust
the brightness so that the illuminated part of the screen has the same
brightness as the unilluminated border. Increase contrast to a reasonable
level (fairly high) and reduce screen glare as much as possible. Now
you're ready to check the following:
Focus: It is important that the electron gun be focused in the center
of the screen and near the corners. The corner areas are typically
problematic. Look at bright text on a dark background in the center,
and in the corners of the screen. Letters should be quite legible, and
pixels shouldn't bleed into each other at the screen edges. Bill Nott
suggests looking at lower case e's and m's to see if they're readable
Convergence: Look closely at white lines on a black background. If
the lines are white along the edges, convergence is good. If, however,
a band of another colour is visible along the line, then colour
reproduction of small objects such as characters or lines may be poor.
Even if color banding is present, the monitor may still be within the
manufacturer's specification. If you can see distinct differently
colored lines, chances are the monitor does not meet the specification,
but color fringing, while possibly considered objectionable, is likely
to be present in almost every monitor built.
Pincushioning: Hold something straight (like the edge of a piece of
paper) up to the edge of the screen image while viewing the display
straight on, from a typical viewing distance. If the image edges bow
away from the straight edge, the monitor is exhibiting
pincushioning or barreling. Barreling occurs when too much
pincushion correction is applied, such that the display bulges outward.
Some monitors provide a pincushioning adjustment, but if one is
unavailable and pincushioning is severe, significant geometric
distortion is likely. Check the pincushioning for different screen
addressabilities/refresh rates, as it may vary.
Geometric Distortion: Move an object of consistent size ( a window
works well) around the screen and measure its height and width with
a ruler. Significant variations in the size at different locations
indicate geometric distortions that may not be correctable.
Colour Purity: Display pure red, green and blue and for each look for
colour inconsistencies in the display that may indicate poor colour
[From: Sam Goldwasser (firstname.lastname@example.org)]
White Purity: Display a totally white screen. The brightness should be
reasonable uniform and there should be no objectionably obvious coloured
or tinted splotches.
Color Bleeding: Display bright primary colored object - red, green, and
blue. There should be no colored trails off to the right of the bright
Moire: This will depend on resolution and size. There should be no
objectionable contour lines visible in the background or smooth areas
of the image.
[From: Andy Laberge (email@example.com) and Michael Scott
Overall Impression: Is the image clear, bright and sharp? Remember
that you will be looking at the monitor for hours at a time, and that
a minor flicker may become irritating over time. When possible, look
at the specific monitor you want to buy, as each monitor has undergone
a calibration procedure and some may be better than others - even of
the same model. This is one big advantage that local stores have
over mail order companies - you can look at the monitor before paying.
Failure Rate: Inquire about failure and repair rates for each model.
Sometimes retailers stop carrying products because of high returns.
How long has the manufacturer been in business? Do they have a good
reputation for reliability and performance? Will the retailer deal
with any warranty claims, or do you have to go directly to the
manufacturer? Will the manufacturer supply parts and schematics for
your monitor in future? You may not be doing the work yourself, but
a monitor repair technician may need these sometime after the
warranty period expires.
Q) What pixel addressabilities are best for my monitor?
There is no right answer to this question, because it is subjective.
However, my recommendations are:
Aspect 14" 15" 17" 20"
640x480 4:3 O O G G
800x600 4:3 A O O G
1024x768 4:3 NR A O O
1280x960 4:3 NR NR A O
1280x1024 5:4 NR NR A O
1600x1200 4:3 -- -- NR A
1600x1280 5:4 -- -- NR A
O - Optimal
G - Pixels are large enough to appear grainy
A - Acceptable
NR - Not Recommended - unless you like looking through a magnifying glass
Keep in mind that the aspect ratio can be important. Standard televisions
and computer monitors are designed to work with a 4:3 aspect ratio. If
you use a 5:4 aspect ratio on a monitor with a 4:3, your screen image will
be compressed vertically, making circles appear as ellipses. The error
associated with using a pixel addressability aspect ratio of 5:4 with a
monitor os 4:3 is about 6%.
END of comp.sys.ibm.pc.hardware.video FAQ - Part 1/4
Michael J. Scott R.R.I., U of Western Ontario
firstname.lastname@example.org 'Need a good valve job?'
PC Video Hardware FAQ: http://www.heartlab.rri.uwo.ca/videofaq.html
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