contents of the lecture

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Contents of the Lecture

1. Introduction2. Methods for I/O Operations3. Buses4. Liquid Crystal Displays5. Other Types of Displays6. Graphics Adapters7. Optical Discs

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2Input/Output Systems and Peripheral Devices (04-1)

4. Liquid Crystal Displays

Liquid CrystalsTN Technology Addressing Methods Backlighting Characteristics VA Technology IPS Technology

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Liquid Crystals (1)

Liquid crystals: discovered in 1888Changing 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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Liquid Crystals (2)

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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Liquid Crystals (3)

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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Liquid Crystals (4)

Phases of thermotropic liquid crystalsHigh 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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Liquid Crystals (5)

Nematic phase (N)Nema – thread; nemato – threadlike (Greek) Threadlike moleculesNo positional order Approximately parallel orientation order directorCan be easily aligned by an electric field


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Liquid Crystals (6)

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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Liquid Crystals (7)

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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TN Technology

TN Technology Principle of Operation Color Displays Structural Details STN Technology DSTN Technology FSTN Technology



Principle of Operation (1)

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 moleculesThe polarizing direction is changed by the twisting of molecules




Single pixel: TN liquid crystals introduced between two transparent electrodes

The electrodes are provided with alignment layers to control molecule alignment groovesThe grooves on the two electrodes are perpendicular to each otherThis results in a 90 twist of the longitudinal axes of molecules on the two electrodes

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Principle of Operation (4)Two polarizing filtersTwo glass plates Two transparent electrodes TN liquid crystal layerThe light is polarized by the first filter The polarizing direction is twisted with 90The light will also pass through the second filter

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When an electrical voltage is applied, the molecules realignThe 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


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Color Displays

Intermediate levels of brightness are required

Changing the voltage applied to the cells The white backlight contains all the wavelengthsThe 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




Structural Details

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TN Technology




STN Technology (1)

STN – Super-Twisted NematicThe 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 addressingSTN technology: the direction of the polarized light is rotated with an angle of 180 .. 270The diagram indicating the light transmission becomes more abrupt

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STN Technology (2)

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STN Technology (3)

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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STN Technology (4)

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


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DSTN Technology (1)

DSTN – Double Super-Twisted NematicSolved 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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DSTN Technology (2)

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 constantDisadvantages:

A more intense backlight is required Higher cost Higher thickness and weight

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TN Technology


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FSTN Technology

FSTN – Film Super-Twisted NematicColor 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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Addressing Methods

Addressing MethodsDirect AddressingMultiplexed Addressing

Passive Matrix DisplaysActive Matrix Displays

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Direct Addressing

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



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Multiplexed Addressing (1)

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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Multiplexed Addressing (2)

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 contrastTN displays have been improved through various methods

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Addressing Methods



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Passive Matrix Displays (1)

Use a set of multiplexed transparent electrodes The liquid crystal layer is placed between the electrodesThe electrodes are composed of indium tin oxide (ITO)

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A pixel – addressed when a voltage is applied across itThe 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 timeInertia of the pixels after the voltage is removedThe 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



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Active Matrix Displays (1)

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 impuritiesA 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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Backlighting (1)

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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Backlighting (2)

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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Backlighting (3)

Higher contrast and brightness can be obtainedThe 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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Backlighting (4)

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 obtainedChanging the backlight intensity can be achieved by pulse-width modulation of the current

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Backlighting (5)

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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Summary (1)

Liquid crystals have properties of the liquid matter and of the crystalline solid matterTypes of liquid crystals: thermotropic, lyotropic, metallotropicThermotropic liquid crystals present several phases depending on temperatureTN 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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Summary (2)

Improvements of the TN technology: STN, DSTN, FSTNThere are two addressing methods of the display elements: direct and multiplexedDisplays 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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Concepts, Knowledge (1)

Types of liquid crystalsPhases of thermotropic liquid crystalsNematic phaseSmectic phaseCholesteric phasePrinciple of operation of TN liquid crystal displaysSTN technologyDSTN technology


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Concepts, Knowledge (2)

FSTN technologyMultiplexed addressingPrinciple of passive matrix displaysPrinciple of active matrix displaysMethods for generating the backlightBacklighting: fluorescent lampsBacklighting: rows of white LEDsBacklighting: array of white LEDsBacklighting: array of RGB LEDs


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Questions

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?


contents of the lecture

Documents

types of liquid crystals

inputoutput systems

peripheral devices

liquid crystal displays5

smectic phase sm molecules

cholesteric phase n

layers positional order

concentration of molecules