Topic-3- Display Devices And Interfacing

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CATHOD RAY TUBE (CRT)

A CRT is an electronic tube designed to display electrical data.

The basic CRT consists of four major components.

1. Electron Gun

2. Focussing & Accelerating Anodes

3. Horizontal & Vertical Deflection Plates

4. Evacuated Glass Envelope

1. Electron Gun

It is used for producing a strain of electrons

2. Focussing & Accelerating Anodes

These are used for producing a narrow and sharply focus beam of electrons.

3. Horizontal and Vertical Deflection Plates

These are used for controlling the path of the beam.

4. Evacuated Glass Envelope

With a phosphorecent screen which produces bright spot when struct by a high velocity electron

beam.

Working of CRT

Course Code – CM-4-G Subject – CHM (17428) Subject Teacher –Prof. Y. S. Modhe

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Heater element is energized by alternating current to obtain high emission of electron

from cathode. Control grid is bised negative with respect to cathode it controls the

density of electron beam to focus the electron beam on the screen focusing anode is used.

the focusing anode operate at a potential of twelve hundred (1200 V) and accelerating

anode at 2000 V to accelerate the electron beam.

Two pairs of deflection plates provided in the CRT these deflection plates are mounted at

right angle to each other to provide electron beam deflection along vertical and horizontal

axis of the screen.

The screen consists of a glass which is coated by some florescent material lie zinc

silicate. Which is semi transparent phosphor substance.

When high velocity electron beam structs the phosphorescent screen the light emits from

it.

The property of phosphor to emit light when its atoms are excited is called fluorescence.

Applications of CRT

In cathode ray oscilloscope

As a display device in radar

In televisions

In computer Monitors

Characteristics of Monitor – Any 4 points (1 mark each ) 1. Resolution describes the number

of potential pixels the monitor is capable of displaying. Resolution = Total Horizontal Pixels x

Total vertical pixels

2. Frame rate : It is is the number of Frames per Second. Higher the frame rate , less the flicker

problem

3. Video Bandwidth : It is the highest input frequency a monitor can handle and helps in

determining the resolution capabilities of the monitor .The video bandwidth is measured in MHz.

Higher the video bandwidth , better the image quality. Bandwidth = Hor. Pixel X Ver. Pixel X

Frame rate

4. Scanning frequency – Horizontal and vertical

Horizontal : The frequency at which the horizontal lines form an image is called Horizontal

Scanning Frequency( KHz)

Vertical :It is the frequency at which screen gets refreshed is the Vertical scanning

frequency(Hz). 5. Dot pitch : It is the measurement of the distance between dots on a CRT. It is

independent of the size of the tube or the displayed image

6. Pixel: An individual dot on the screen. Each pixel can be of different color thus producing the

images

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Differentiate between CRT and LCD displays (any four points 1M each)

Feature CRT LCD

Viewing Angle 180 degrees, almost all angles 150 degrees, Not all angles

Weight Heavy Lighter

Resolution Better compared to LCD Low resolution

Size Much larger than LCD Thinner

Glare Since this has glass screen, it

has glare

No glare

Power On an average a 17” monitor

uses 80 watts

For 19” LCD takes 17- 31 Watts

Viewable screen The viewable area is always

about 0.9 - 1.1 inch smaller

than the size specified on

paper.

A 15 inch LCD always has a same

size of viewable area as the

specifications say.

Radiation CRTs emit electromagnetic

radiation. Much of it is

filtered by the lead heavy

glass front

LCDs emit a very small amount

of radiation compared to CRTs.

State any two differences between interlaced and non-interlaced monitor

Interlaced Non - interlaced

1. Scans every other line of the image in one

pass & the remaining lines in other pass

Scans all lines in single pass

2. Difficult on the eyes

easy on the eyes

3. Flicker is more

Flicker is less

4. Effective image refresh rate is half the

vertical scanning rate

Entire image is refresh at vertical Scanning

rate

Topic-3- Display Devices And Interfacing

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Write any four advantages of LCD display over CRT display. (any 4 points, 2M)

1. LCD monitors consume less power. An average 19-inch LCD uses 45 watts of electricity,

while a 19- inch CRT uses 100 watts.

2. LCD monitors are smaller, thinner and weigh half as much as CRTs.

3. An LCD monitor's tilt, swivel, height and orientation from horizontal to vertical can all be

adjusted easily.

4. LCD monitors don't produce the flicker that CRTs do, generating less eye strain

LCD Block Diagram

Working

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The main principle behind liquid crystal molecules is that when an electric current is applied to

them, they tend to untwist. This causes a change in the light angle passing through them.

This causes a change in the angle of the top polarizing filter with respect to it. So little light is

allowed to pass through that particular area of LCD. Thus that area becomes darker comparing to

others.

For making an LCD screen, a reflective mirror has to be setup in the back. An electrode plane

made of indium-tin oxide is kept on top and a glass with a polarizing film is also added on the

bottom side.

The entire area of the LCD has to be covered by a common electrode and above it should be the

liquid crystal substance. Next comes another piece of glass with an electrode in the shape of the

rectangle on the bottom and, on top, another polarizing film.

It must be noted that both of them are kept at right angles. When there is no current, the light

passes through the front of the LCD it will be reflected by the mirror and bounced back.

As the electrode is connected to a temporary battery the current from it will cause the liquid

crystals between the common-plane electrode and the electrode shaped like a rectangle to

untwist.

Thus the light is blocked from passing through. Thus that particular rectangular area appears

blank

Compare Passive matrix and Active matrix LCD display. (Any four points 1M each)

Active Matrix Passive Matrix

Contrast Good(100+) Poor(10-20)

Viewing Scale Wide Limited

Gray Scale 256 16

Response time Fast(<50ms) Slow(100-200ms)

Multiplex ratio >1000 480

Manufacturability Complex Simple

Cost High Moderate

Used in Colour monitor Monochrome

monitor

xplain passive and active matrix LCD with diagram. (02-marks for diagram, 01-mark for

passive LCD explanation, 01-mark for active LCD)

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Passive matrix LCDs use a simple grid to supply the charge to a particular pixel on the display.

The liquid crystal material is sandwiched between the two glass subtrates and a polarizing film is

added to the outer side of each substrate. To turn on a pixel, the integrated circuit sends a charge

down the correct column of one substrate and a ground activated on the correct row of the other.

The row and column intersect at the designated pixel, and that delivers the voltage to untwist the

liquid crystals at that pixel.

To address a pixel the column containing the pixel is sent a charge, the corresponding row is

connected to ground. When sufficient voltage is placed across the pixel, the liquid crystal

molecules align parallel to the electric field.

In passive matrix LCDs (PMLCDs)there are no switching devices, and each pixel is addressed

for more than one frame time.

Active matrix LCD In active matrix LCDs, a switching device and a storage capacitor are

integrated at the each cross point of the electrodes.

The active addressing removes the multiplexing limitations by incorporating an active switching

element.

In contrast to passive matrix LCDs, active matrix LCDs have no inherent limitation in the

number of scan lines, and they present fewer cross talk issues.

Touch Screen Display

Describe the construction and working of resistive touch screen display.(Construction- 2M;

working – 2M)

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Construction: Note: Any other diagram showing the different layers can be considered. A

resistive touchscreen panel comprises several layers, the most important of which are two thin,

transparent electrically-resistive layers separated by a thin space. These layers face each other

with a thin gap between. The top screen (the screen that is touched) has a coating on the

underside surface of the screen. Just beneath it is a similar resistive layer on top of its substrate.

One layer has conductive connections along its sides, the other along top and bottom.

Working: A voltage is applied to one layer, and sensed by the other. When an object, such as a

fingertip or stylus tip, presses down onto the outer surface, the two layers touch to become

connected at that point: The panel then behaves as a pair of voltage dividers, one axis at a time.

By rapidly switching between each layer, the position of a pressure on the screen can be read.

Describe the construction and working of plasma display. (Diagram 2 marks, Working 2

marks)

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Plasma is a slate of gas made up of free flowing ions (+ve) and electrons. Under normal

conditions a gas is made up of uncharged particles.

In plasma display xenon and neon atoms are used.

When an electric current is passed through plasma, the electrons rush towards the positive

electrode and ions rush towards the negative electrode.

During this rush they collide with each other.

These collisions excite the gas atoms in the plasma, causing them to release photons of

energy.

These are ultraviolet photons invisible to human eye.

The released ultraviolet photons interact with phosphor material on the inside wall of the

cell and phosphors give off colored light.

Each phosphor has three separate cells, a red, a blue and a green phosphor.

These colors blend together to create the overall color of the cell.

The xenon and neon gas in plasma contain hundreds of thousands of tiny cells positioned

between two plates of glass.

Long electrodes are sandwiched between the glass plates on both the sides of the cells.

The address electrodes are at the rear glass plate and the discharge electrodes are

transparent and mounted along the front glass plate.

Both sets of electrodes extend across the entire screen.

To ionize the gas in a particular cell, the electrodes that intersect at that cell are charged.

When an electric current flows through the gas in the cell, the gas atoms are stimulated

sad they release ultraviolet photons.

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By varying the pulses of current flowing through the different cells intensity of each sub-

pixel color can be varied to create hundreds of different combinations of red, green and

blue.

With neat diagram, describe video accelerator card.(Diagram-2M; description- 2M)

The core of the accelerator is the graphics chip (or Video chipset).

The graphics chip connects directly with the PC expansion bus. Graphics command and

data are transmitted into pixel data and stored in Video memory offers a second data bus

that is routed directly to the Video board’s RAM DAC (Random Access Memory Video

to Analog Converter). The graphics chip directs RAM DAC operation and ensures that

VRAM data is available.

The RAM DAC then translates Video data into red, green and horizontal and vertical

synchronization signals output signals generated by the monitor.

This architecture may appear simple, but this is due to high level of integration provided

by the chipsets being used.