Download Displays - La Salle University

Monitors
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Monitors
• The monitor is the primary output device
for a PC.
• The additional circuitry needed for a
computer to interface with a monitor is on
the video/display card/adapter.
• There are two basic types of monitor
– CRTs (Cathode Ray Tube)
– Flat Panels or LCDs (Liquid Crystal Display)
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CRT
• A CRT consists of a large vacuum tube. The tube
is narrower in the rear where it houses an
“electron gun” and then widens in the front
display/screen area.
– A color monitor typically has three electron guns.
• Electrons are essentially boiled off of the electron
gun’s cathode.
• The electrons are then accelerated toward the
electron gun’s anode. They are also collimated.
• The yoke is a set of electromagnets that direct the
beam of electrons toward a particular point on the
screen.
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Cathode Ray Tube
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Monitor Electron Gun(s)
The device
shoots three
electron beams
toward the
screen.
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Mask
• A little before the screen is the mask. The mask
absorbs electrons that have been somewhat
misdirected. It keeps the “red” electron gun from
exciting a blue or green spot.
• Some variations on the idea go by the names
–
–
–
–
Aperture grill
Shadow mask
Slotted mask
Enhanced dot pitch
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Phosphors
• The electron beam is directed toward the screen
which is covered on the inside with phosphors.
• A phosphor absorbs the energy from the electron
beam. Then to release that energy and return to its
normal state, it emits photons (light “particles”).
• In addition to computer screens, phosphors are
used with fluorescent lights and various glow-inthe-dark items. Also some detergents contain
phosphors so that clothes will glow under a black
light.
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Phosphors (Cont.)
• The important characteristics of phosphors are:
– What gets them excited.
– What color they emit.
– How long they emit (their persistence).
• Color CRTs will use a set of three phosphors
corresponding to the colors red, blue and green.
• The persistence is important in monitors – too
short and it flickers; while too long and it smears.
• Examples of phosphors include Zinc Sulfide and
Strontium Aluminate.
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Dot Pitch
• The screen is covered with interlaced arrays of
red, green and blue phosphors.
• A screen characteristic is the dot pitch or
phosphor pitch, which is the distance between the
centers of two like colored phosphors.
• Computer monitor dot pitches can range from
around .18 mm to .39 mm (millimeters).
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Triad
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Monitor Specs
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Monitor Specs (Cont.)
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Pixels and Resolution
• A pixel which is short for picture element, is the smallest
logical unit in a display.
• The smallest possible pixel would consist of a triad – three
phosphors, one of each color – but typically pixels consist
of more triads.
• A screen is broken into a two-dimensional array of pixels.
• The resolution of the display is given by the number of
horizontal elements times the number of vertical elements.
• Because a pixel is just a logical unit, the same display can
support a number of various resolutions.
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Resolution Setting
Start/Settings/
Control Panel/
Display/Settings
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Aspect Ratio
• Monitors will support a set of standard
resolutions such as 640  480, 800  600,
1024  768, …
• These resolutions have the property that if the
horizontal number is divided by 4 and then
multiplied by 3, one gets the vertical number.
• The display is said to have a 4:3 aspect ratio.
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Monitor Specs
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Monitor Size
• Given the fixed 4:3 aspect ratio, the size of
a monitor can then be given by its diagonal
length of the screen.
• The only thing to watch here is the
distinction between the display size and the
visible display size.
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Monitor Specs
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Painting an image
• The display image is “painted” starting with the electron
guns pointing to the upper left hand corner.
• The guns sweep across horizontally. The intensity of their
fire (and thus the intensity of the glowing phosphor)
depends on the signal they receive.
• The guns return to the left and move one row down
without firing.
• With this the guns have completed one raster line.
• The rate at which they can do this is called the horizontal
refresh rate (HRR).
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Raster Line/Horizontal Refresh Rate
One horizontal line is
known as a raster
line.
The number of
horizontal lines per
second is known as
the horizontal refresh
rate. It is given in
Hertz or kilohertz.
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Vertical Refresh Rate
• The guns sweep through line after line until the
lowest line is drawn.
• Then the guns return to the upper left hand corner.
• The number of times the guns return to the upper
left hand per second is known as the vertical
refresh rate (VRR) or just refresh rate –
generally reported in Hertz.
• Too high a VRR can damage the monitor, too low
a VRR can cause flickering and user eye strain.
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Refresh Frequency Setting
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Bandwidth
• A monitor’s bandwidth is the maximum
number of times the electron gun can be
turned on and off per second.
• The gun intensity does not have to change
on a triad level but on a pixel level.
• Because the resolution can change, but the
bandwidth cannot, the refresh rate can be
resolution dependent.
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Refresh Rates and Resolutions
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Color Depth
• Color depth is the number of bits used to specify
the color for each pixel.
• The number of possible colors will be 2 raised to
the color depth. This will be related to the number
of intensity level of the electron guns.
• For example “true color” uses 32 bits and thus has
232 = 4,294,967,296 possible colors.
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Color depth setting
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RAMDAC
• The CRT uses an analog signal, so part of the video card
is a RAMDAC.
• RAMDAC stands for Random Access Memory Digitalto-Analog Converter.
• The RAM part uses SRAM to hold the color table that is
used to convert a logical color into three separate digital
signals. (It’s in memory because one can change the
color depth.)
• Those three digital signals are sent to three DACs (digitalto-analog converters).
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CRT Connector
The three colors account
for six of the pins in the
connector
1: red to monitor
6: red return from monitor
2: green to monitor
7: green from monitor
3: blue to monitor
8: blue from monitor
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LCD: Pros and Cons
• Pros:
–
–
–
–
–
–
–
Thinner
Lighter
Less power
No flicker
No radiation
Brighter?
Less problem with
reflection
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• Cons
– More expensive (but
they have come down
in price a lot recently)
– More limited viewing
angle
– Fixed resolution
– Poor with fast
changing video
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Light
• Varying electric fields produce magnetic
fields.
• Varying magnetic fields produce electric
fields.
• The combined result is a propagating
electromagnetic disturbance that we like to
call light.
• I.e., light is an electromagnetic wave
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Directions
• Light is made up of electric fields and
magnetic fields.
• Electric fields and magnetic fields points in
particular directions.
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Radiation
• With the light that comes from a heated
object, such as a light bulb filament or the
sun, there is no preferred direction for the
electric field.
• The electric field is in a random direction
that changes from instant to instant.
• (Laser light is different in this respect.)
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Interacting with a material
• When light impinges on a material, the
electric and magnetic fields of the light
interact with the charged electrons and
protons making up the material.
• This can result in reflection or absorption or
transmission.
• What if the material has a different
behaviors in different directions?
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Huh?
• Consider a set of metal bars, they could conduct
electricity along the bars but not perpendicular to
them.
• Then the part of the light with the electric field in
one direction would respond differently from the
part of the light with the electric field in some
other direction.
• Some light may be absorbed and some light may
be transmitted.
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Polarizing
• There’s no “maybe” about it. These
directionally prejudiced materials exist.
• A polarizing film or filter is used to make
sunglasses. Light is reduced by transmitting
only light with a particular direction.
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Effect of a Polarizer
Before: random red
direction can be thought of
as part vertical (blue) and
part horizontal (green).
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After: after polarizing the
only “component” left is
aligned with the polarizer
(in this case horizontal).
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Cross polarizers
• An electric field in a generic direction (i.e. light
having a generic polarization) can be thought of as
being made up some part vertical and some part
horizontal.
• A vertical filter would take out the vertical part, a
horizontal filter would take out the horizontal part.
• If we had a vertical filter followed by a horizontal
filter (so-called cross polarizers), there would be
nothing left.
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An intermediate
• If there’s an intermediate polarizer that is neither
horizontal nor vertical some light gets through.
• The vertical leads to vertical pointing electric
fields.
• But vertical can be thought of as made of part
parallel to the intermediate direction and part
perpendicular to it. The parallel part gets through.
• That part can be thought of as part vertical and
part horizontal and the horizontal part gets
through.
• Really.
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A funny intermediate
• This is where the liquid crystals come in.
• The liquid crystal is going to serve as this
intermediate between two crossed
polarizers.
• It can help twist the polarization of light so
that it is not blocked out by the crossed
polarizers or in other conditions it does not
twist the light and the light is blocked out.
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Liquid Crystal
• In crystals molecules tend to stay in one
place and to be lined up.
• In liquids molecules tend to flow around
and to lie in random directions.
• Liquid crystals are an intermediate phase
associated with certain elongated molecules.
Liquid crystals tend to flow but also tend to
be aligned.
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Here’s the twist
• The aligned, elongated molecules mean that these
materials have a selected direction – like the metal
bars, they can serve as a polarizer.
• The liquid crystal is arranged so that direction of
the molecules slowly twist from vertically aligned
to horizontally aligned.
• This twisted liquid crystal slowly guides the
polarization of the light from vertical or horizontal
so that some light gets through the cross
polarizers.
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Going straight
• Applying an electric field to the twisted
liquid crystal can cause it to straighten up.
• Without the twist, the polarization of the
light stays vertical and is blocked out when
it gets to the horizontal polarizer.
• Varying the field, varies twist, varies the
intensity if light that passes through.
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Source of light
• One approach to LCDs is to use the ambient light
from the room. The outside light passes through
the liquid crystals and polarizers, is reflected by a
mirror and returns unless the cross polarizing
effect takes place.
– Note many digital watches are also LCD
• A more dependable approach is to use
backlighting. A light source is placed behind the
polarizers and liquid crystal.
• The brightness of a display is measured in units
called nits.
– Low end: 100 nits – high end: 300 or more nits
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Nit
• “A nit is a non-SI unit of luminance equal
to one candel per square meter (cd/m²). It
is often used to quote the brightness of
computer displays, which typically have
luminances of 200 to 300 nits.”
– http://encyclopedia.thefreedictionary.com/Nit+(
unit)
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Color
• To achieve color, simply add a color filter
that only allows red light to pass through to
one region. Nearby add a blue filter and a
green filter.
• Then we have the LCD equivalent of the
phosphor triad.
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Panel type
• An electric field must be delivered to a particular
location on the display.
• Cheaper flat panels use a passive matrix a set of
vertical wires and another set of horizontal wires.
This approach tends to be slow and leads to some
overlap or blurring.
• Better flat panels use an active matrix or thin
film transistor (TFT) approach. Each dot is
controlled by its own transistor or set of
transistors. This leads to tighter, faster control of
the individual dots.
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Contrast Ratio
• Contrast ratio measures the difference in
light intensity between the brightest white
and the darkest black.
• The higher the ratio the better quality
image.
• Meyers said CRTs had better contrast.
Thompson & Thompson said LCDs had
better contrast. ???
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Viewing Angle
• Early laptop displays suffered from a
limitation in the angle at which the image
could be seen.
• Looking straight on the image was fine, but
from a angle, the image was dim and
distorted and even disappeared.
• Things have improved but this is still a
limitation of an LCD compared to a CRT.
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LCDs are digital
• CRTs worked with analog signals. LCDs work
with digital signals.
• In order to allow some video cards to be used,
some LCDs take the analog signal and convert it
back to digital.
• A Digital Video Interface (DVI) is new
connection standard developed to eliminate the
intermediate switch to analog when it is not
necessary.
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References
• PC Hardware in a Nutshell, Thomson and
Thomson
• All in One A+ Certification, Michael Myers
• http://computer.howstuffworks.com/monitor
.htm/printable
• http://plc.cwru.edu/tutorial/enhanced/files/lc
d/tn/tn.HTM
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