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1. Introduction
2. Methods for I/O Operations
3. Buses
4. Liquid Crystal Displays
5. Other Types of Displays
6. Graphics Adapters
7. Optical Discs
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Liquid Crystals
TN Technology
Addressing Methods
Backlighting
Characteristics
VA Technology
IPS Technology
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Liquid crystals: discovered in 1888
Changing the state of a material known as
cholesteryl benzoate from solid into liquid
Substances that exhibit anisotropy of
properties  variable depending on the
direction of measurement
Equilibrium state – mesomorphic
State between solid crystalline and liquid
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Light passing through liquid crystals
follows the alignment of the molecules
Applying an electric or magnetic field
changes the molecular alignment of liquid
crystals
Three types of liquid crystals:
Thermotropic
Lyotropic
Metallotropic
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Thermotropic liquid crystals
Transition into several phases with temperature
changes
Lyotropic liquid crystals
Present phase transitions determined primarily
by the concentration of molecules in a solvent
Metallotropic liquid crystals
Composed of organic and inorganic molecules
Phase transitions also depend on the organic /
inorganic composition ratio
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Phases of thermotropic liquid crystals
High temperature: liquid (isotropic) phase
Low temperature: solid (crystalline) phase
Nematic phase
Smectic phase
Cholesteric phase
Types of ordering for the phases:
Positional order of molecules
Orientation order of molecules
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Nematic phase (N)
Nema – thread; nemato –
threadlike (Greek)
Threadlike molecules
No positional order
Approximately parallel
orientation order 
director
Can be easily aligned by an
electric field
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Smectic phase (Sm)
Molecules maintain the
orientation order
They align in layers
Positional order along
one direction
SmA (left)
SmC (right)
Other Sm phases exist
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Cholesteric phase (N*)
Typical for cholesterol
esters  cholesteric
Structure similar to a stack
of 2D nematic layers
The director in each layer is
twisted with respect to
adjacent layers
Twisted nematic (TN)
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Liquid Crystals
TN Technology
Addressing Methods
Backlighting
Characteristics
VA Technology
IPS Technology
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TN Technology
Principle of Operation
Color Displays
Structural Details
STN Technology
DSTN Technology
FSTN Technology
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Liquid crystal displays are passive
Use a light source (backlight) or a mirror
(to reflect ambient light)
The operation is based on the properties
of polarized light
The light waves are oriented in parallel with
a specific direction
Can be obtained with a polarizing filter
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The polarized light passes through a TN
liquid crystal layer
The light follows the alignment of molecules
The polarizing direction is changed by the
twisting of molecules
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Single pixel: TN liquid crystals introduced
between two transparent electrodes
The electrodes are provided with alignment
layers to control molecule alignment 
grooves
The grooves on the two electrodes are
perpendicular to each other
This results in a 90 twist of the longitudinal
axes of molecules on the two electrodes
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Two polarizing filters
Two glass plates
Two transparent electrodes
TN liquid crystal layer
The light is polarized by the
first filter
The polarizing direction is
twisted with 90
The light will also pass
through the second filter
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When an electrical voltage is
applied, the molecules realign
The direction of longitudinal
axes tends to align in parallel
to the field
The light is not twisted → is
blocked by the second filter
By controlling the voltage,
different levels of gray can be
obtained
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Displays for which the light is blocked in
the areas with no voltage applied
The polarizing directions are parallel
The optical effect is more dependent on the
thickness of display when no voltage is
applied
The eye is more sensitive to variations of
brightness in the dark state  spotted image
This variant may also increase power
consumption
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The response of a TN cell to an applied voltage
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Percent transmission of light
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TN Technology
Principle of Operation
Color Displays
Structural Details
STN Technology
DSTN Technology
FSTN Technology
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Intermediate levels of brightness are
required
Changing the voltage applied to the cells
The white backlight contains all the
wavelengths
The color components are obtained
through filtering of the white light
Each pixel is composed of three subpixels
for the primary RGB colors
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TN Technology
Principle of Operation
Color Displays
Structural Details
STN Technology
DSTN Technology
FSTN Technology
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TN Technology
Principle of Operation
Color Displays
Structural Details
STN Technology
DSTN Technology
FSTN Technology
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STN – Super-Twisted Nematic
The difference between the voltages for
which a cell is on / off must be very small
The TN technology is impractical for large
sizes with conventional addressing
STN technology: the direction of the
polarized light is rotated with an angle of
180 .. 270
The diagram indicating the light
transmission becomes more abrupt
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Advantages of STN technology compared
to the TN technology:
Higher contrast
Wider viewing angle
Simpler control for the percent
transmission of light through the liquid
crystal cells
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Disadvantages of STN technology:
Slower response time compared to the TN
technology
Lower brightness level
Higher manufacturing costs
Early STN displays presented an undesirable
coloration  shifted transmission spectrum
In the on state: yellow
In the off state: bluish
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TN Technology
Principle of Operation
Color Displays
Structural Details
STN Technology
DSTN Technology
FSTN Technology
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DSTN – Double Super-Twisted Nematic
Solved the coloration problem of the STN
technology by adding a new STN layer
For the second layer, the twisting direction
of the polarized light is opposite to that of
the first layer
In the off state, the phase shift due to the
first layer is compensated by the second
layer  black pixel
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The on state of the pixel is not affected by
the second STN layer  white pixel
Both layers consist of the same type of
liquid crystal  the characteristics are
constant
Disadvantages:
A more intense backlight is required
Higher cost
Higher thickness and weight
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TN Technology
Principle of Operation
Color Displays
Structural Details
STN Technology
DSTN Technology
FSTN Technology
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FSTN – Film Super-Twisted Nematic
Color compensation is achieved with a thin
polymer film instead of the glass layer
Advantages compared to DSTN
technology:
Lower cost
Lower thickness and weight
Lower-power backlight
Disadvantage:
Reduced contrast
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Liquid Crystals
TN Technology
Addressing Methods
Backlighting
Characteristics
VA Technology
IPS Technology
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Addressing Methods
Direct Addressing
Multiplexed Addressing
Passive Matrix Displays
Active Matrix Displays
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Used for displays with a
small number of display
elements
Each element (segment or
pixel) can be addressed or
driven separately
A voltage should be
applied to each element
to change orientation of
the crystals
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Addressing Methods
Direct Addressing
Multiplexed Addressing
Passive Matrix Displays
Active Matrix Displays
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Used for displays with
a large number of
pixels
Each pixel sits at the
intersection of a row
electrode and a
column electrode
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Advantage:
Reduced complexity of the circuits
For a matrix of 100x100 pixels, 200 drivers
are needed (compared to 10,000 with direct
addressing)
Disadvantage:
Reduced contrast
TN displays have been improved through
various methods
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Addressing Methods
Direct Addressing
Multiplexed Addressing
Passive Matrix Displays
Active Matrix Displays
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Use a set of multiplexed
transparent electrodes
The liquid crystal layer
is placed between the
electrodes
The electrodes are
composed of indium tin
oxide (ITO)
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A pixel – addressed
when a voltage is
applied across it
The pixel becomes
opaque when it is
addressed
When the voltage is
removed, the pixel
deactivates slowly
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The display controller scans across the
matrix of pixels
Delay since the voltage is applied to a pixel
until it is turned on  response time
Inertia of the pixels after the voltage is
removed
The time to scan the entire matrix must be
shorter than the time needed for the
pixels to deactivate (turn-off time)
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Disadvantages:
Crosstalk – interference between pixels 
the occurrence of shadows for bright objects
The viewing angle is limited
The response time is relatively slow  the
current image is still maintained on the
screen after a new image is displayed
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Addressing Methods
Direct Addressing
Multiplexed Addressing
Passive Matrix Displays
Active Matrix Displays
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The front glass plate of
the display is coated with
a continuous electrode
The rear glass plate is
coated with electrodes
divided into pixels
Each pixel is connected in
series with a thin film
transistor (TFT)
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A pixel of the active
matrix display
Active elements: field
effect transistors (FET)
Based on amorphous
silicon (a-Si)
Based on polysilicon
(p-Si)
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An image is created by scanning the
matrix:
A row of pixels is selected by applying
voltage to the row electrode connected to
the transistor gates on that row
Voltages corresponding to the image are
applied to the column electrodes connected
to the transistor sources
The operations are repeated for each row
Refresh rate of the screen: 50 or 60 Hz
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Advantages (compared to passive matrix
displays):
Faster response time
Higher contrast
Higher brightness level
Wider viewing angle
Disadvantages:
More intense backlight is required
Higher cost
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Defective pixels
For high resolutions, a large number of
transistors are needed
SXGA resolution: 1280x1024x3  3.93 million
transistors
Defective transistors due to impurities
A lit pixel (permanently turned on)
A black pixel (permanently turned off)
Manufacturers set limits for an acceptable
number of defective pixels
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Liquid Crystals
TN Technology
Addressing Methods
Backlighting
Characteristics
VA Technology
IPS Technology
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There are several types of backlighting
With fluorescent lamps
CCFL – Cold Cathode Fluorescent Lamp
Placed at the edges of the display
For uniform distribution of light: a diffuser
panel and polarizers
For portable devices, the voltage needs to be
converted to a higher voltage
It is not possible to build very thin displays
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With rows of white LEDs
EL-WLED – Edge-Lit White LED
Placed at the edges of the display
Diffuser panel and polarizers
TV sets with this type of backlighting (LED
TV) use an active-matrix LCD technology
Very thin displays can be built (< 1 cm)
Compared to CCFL backlighting: lower power
consumption (30..40 %); longer lifetime
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Higher contrast and brightness can be
obtained
The initial high costs of the displays have
been reduced
The color range (gamut) is slightly narrower
compared to that of CCFL lit displays
It is more difficult to maintain the uniformity
of brightness in the long term  brighter or
darker areas
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With an array of white LEDs
Array of white LEDs uniformly distributed
behind the display panel
Used for TV sets
It is possible to set the backlight intensity
differently in different areas (local dimming)
A much higher dynamic contrast can be
obtained
Changing the backlight intensity can be
achieved by pulse-width modulation of the
current
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With an array of RGB LEDs
Triads of LEDs: R, G, and B
An extended color range can be achieved
Pure and saturated colors
High cost
Method used for professional graphics
editing LCD displays
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Liquid crystals have properties of the liquid
matter and of the crystalline solid matter
Types of liquid crystals: thermotropic,
lyotropic, metallotropic
Thermotropic liquid crystals present several
phases depending on temperature
TN has been the first technology used for
liquid crystal displays
It is based on the properties of polarized light,
which follows the alignment of molecules
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Improvements of the TN technology: STN,
DSTN, FSTN
There are two addressing methods of the
display elements: direct and multiplexed
Displays with multiplexed addressing may use
a passive matrix or an active matrix
Active matrix displays have important advantages
compared to passive matrix displays
Generating the backlight with an array of RGB
LEDs is the most advantageous method
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Types of liquid crystals
Phases of thermotropic liquid crystals
Nematic phase
Smectic phase
Cholesteric phase
Principle of operation of TN liquid crystal
displays
STN technology
DSTN technology
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FSTN technology
Multiplexed addressing
Principle of passive matrix displays
Principle of active matrix displays
Methods for generating the backlight
Backlighting: fluorescent lamps
Backlighting: rows of white LEDs
Backlighting: array of white LEDs
Backlighting: array of RGB LEDs
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1. What are the main phases of
thermotropic liquid crystals?
2. How does the polarized light pass through
a TN liquid crystal layer?
3. What is the difference between TN and
STN technologies?
4. What are the disadvantages of passive
matrix displays?
5. What are the advantages of LED
backlighting?
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