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Video Display Devices

Display Hardware

• Video display devices

Input devices

• Locator Devices

• Keyboard devices

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Display Technologies

Cathode Ray Tubes (CRTs)– Most common display device today– Evacuated glass bottle– Extremely high voltage– Heating element (filament)– Electrons pulled towards

anode focusing cylinder– Vertical and horizontal deflection plates– Beam strikes phosphor coating on front of tube

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Electron Gun

• Contains a filament that, when heated, emits a stream of electrons• Electrons are focused with an electromagnet into a sharp beam and directed to a specific point of the face of the picture tube• The front surface of the picture tube is coated with small phosphor dots• When the beam hits a phosphor dot it glows with a brightness proportional to the strength of the beam and how often it is excited by the beam•The picture is repeatedly repainted (refresh CRT)

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CRT Monitor

Electron Guns

Red Input

GreenInput

Blue Input

Deflection Yoke

Shadow Mask

Red, Blue, and Green

Phosphor Dots

CRT

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List the properties of CRT

1. Persistence

2. Resolution3. Addressability

4. Aspect ratio

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Persistence

How long small spots continue to emit light after the beam is moved. How long it takes for the emitted light from the screen to decay to one-tenth of its original intensity.– Lower persistence requires high refresh rate & it is

good for animation. – High persistence is useful for displaying highly

complex static pictures.– Graphics monitors are usually constructed with 10 to

60 microseconds.

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Resolution

• The maximum number of points that can be displayed without an overlap on a CRT is referred to as the resolution.

• The smaller the spot size, the higher the resolution.• The higher the resolution, the better is the graphics system• High quality resolution is 1280x1024

• The intensity distribution of spots on the screen have Gaussian shape.

• Adjacent points will appear distinct as long as their separation is greater than the diameter at which each spot has intensity of about 60% of that at the center of the spot.

Intensity distribution

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Addressability

• Addressability is the number of individual dots per inch (d.p.i.) that can be created. If the address of the current dot is (x, y) then the next dot will be (x + y), (x + y + 1)

• The picture on a screen consists of intensified points.

• The smallest addressable point on the screen is called pixel or picture element

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Aspect ratio

This number gives the ratio between vertical points and horizontal points necessary to produce equal length lines in both directions on the screen. Aspect ratio = ¾ means that a vertical line plotted with 3 points is equal in length to a horizontal line plotted with 4 points.

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Display Technologies: CRTs

1. Raster Scan Displays

2. Vector Displays

3. Liquid Crystal Displays (LCDs)

4. Plasma Panel

5. Organic LED Arrays

Raster Scan Displays

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Raster Scan Displays (1)

– Raster: A rectangular array of points or dots– Pixel: One dot or picture element of the raster. Its

intensity range for pixels depends on capability of the system

– Scan line: A row of pixels– Picture elements are stored in a memory called frame

buffer

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Raster Scan Displays (2)

– Frame must be “refreshed” to draw new images– As new pixels are struck by electron beam, others

are decaying– Electron beam must hit all pixels frequently to

eliminate flicker– Critical fusion frequency

• Typically 60 times/sec

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Raster Scan Displays (3)

– Intensity of pixels depends on the system for example black and white screens each point can be on or off thus it needs one bit of memory to represent each pixel.

– To paint color screen additional bits are needed. If three bits are used, then number of different colors are 2*2*2.

– A special memory is used to store the image with scan-out synchronous to the raster. We call this the frame buffer.

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Raster Scan Displays (6)

Raster CRT pros:– Allows solids, not just wire frames– Leverages low-cost CRT technology (i.e., TVs)– Bright! Display emits light

Cons:– Requires screen-size memory array– Discrete sampling (pixels)– Practical limit on size

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Frame Buffers

• A frame buffer may be thought of as computer memory organized as a two-dimensional array with each (x,y) addressable location corresponding to one pixel.

• Bit Planes or Bit Depth is the number of bits corresponding to each pixel.

• A typical frame buffer resolution might be

– 640 x 480 x 8

– 1280 x 1024 x 8

– 1280 x 1024 x 24

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Monochrome Display(Bit-map Display)

Electron Gun

1 bit 2 levels

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3-Bit Color Display

3

red

green

blue

COLOR: black red green blue yellow cyan magenta white

R G B

0 0 0

1 0 0

0 1 0

0 0 1

1 1 0

0 1 1

1 0 1

1 1 1

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True Color Display24 bit planes, 8 bits per color gun. 224 = 16,777,216

Green

Red

Blue

N

N

N

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Color Map Look-Up TablesExtends the number of colors that can be displayed by a given number of bit-planes.

01000

011

67

100110100001

0

67

255

10011010 0001

R G B

RED

GREEN

BLUE

Pixel displayedat x', y'

Pixel inbit mapat x', y'

0 x0

y

xmax

maxy

Bit map Look-up table Display

Video look-up table organization: each table entry is a 12 bit per entry.

A pixel with value 67 is displayed on the screen with the red electron gun at 9/15 (binary 1001) of maximum, green at 10/15, and the blue is 1/15.

2 Basic Techniques for Color Display

Beam-Penetration Method

Shadow-Mask Method

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Beam-Penetration Method• Used with random scan monitors

• The screen has two layers of phosphor: usually red and green

• The displayed color depends on how far the electron beam penetrates through the two layers.

• A beam of slow electrons excites only the outer of the red layer, a beam of fast electrons penetrates through the red layer and excites the inner green layer, and at intermediate beam speeds, combinations of the two colors are emitted to show other colors (yellow & orange)

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Shadow-Mask MethodColor CRTs have– Three electron guns– A metal shadow mask to differentiate the beams

• The Shadow mask in the previous image is known as the delta-delta shadow-mask.

• The 3 electron beams are deflected and focused as a group onto the shadow mask, which contains a series of holes aligned with the phosphor-dot patterns.

• The 3 beams pass through a hole in the shadow mask and activate a dot triangle, which appears as a small color spot on the screen.

• A second arrangement of the 3 electron guns is in-lineWhere the corresponding red-green-blue color dots on the

screen are aligned along one scan – line instead of a triangular pattern.

Controlling Colors in shadow mask

• Different colors can be obtained by varying the intensity levels of the three electron beams.

• Example: Simply turning off the red and green guns, we get only the color coming from the blue phosphor.

• Yellow = Green + Red• Magenta = Blue + Red• Cyan = Blue + Green

• White is produced when all the 3 guns possess equal amount of intensity.

Application:

• Color CRT’s in graphics systems are designed as RGB monitors, which employ the shadow mask technique.

• The color CRT takes the intensity level for each electron gun ( red, green and blue) directly from the computer system without any intermediate processing.

• An RGB color system with 24 bits of storage per pixel is generally referred to as full-color system or a true-color system. It allows 256 voltage settings for each electron gun and nearly 17 million colors.

Flat-Panel Displays

• Although CRT monitors are still in use and form 70% of the users, still other technologies are emerging in the market to replace the classic CRT model.

• So What does Flat-Panel mean?

It refers to a class of video devices

that have a:-

reduced volume,

reduced weight and

reduced power requirements

compared to a standard CRT

In existence since 1964’s,Flat panel displays are much thinner and flatter than traditional TVs

Very Light in weight

• Thinner than CRTs and can be hung on walls.• Also available as pocket notepads on which notes can

be scribbled and can be carried around easily.• Current usage:

– Small and big TV monitors– Calculators– Pocket video games– Laptop computers– Advertisement boards in elevators and showrooms.– Portable monitors.

Emissive and Non-Emissive

• Emissive displays (also called emitters) are devices that convert electrical energy into light.

• Example: – Plasma panels, – thin–film electroluminescent displays,– LEDs (Light emitting diodes)

Flat CRTs have also been tried to be devised where the electron beams are accelerated parallel to the screen and then deflected 900.

Sadly the above was not successful at all.

Non – Emissive Flat-Panel Displays

• Non-emissive displays or non-emitters use optical effects to convert sunlight or light from some other source into graphics patterns.

• Example:LCD (Liquid – crystal device)

Plasma Panels• It is an example of an emissive

flat panel display.• Also called as a gas-discharge display.• Can be wall mounted.

• Working:• A plasma display is comprised of two parallel sheets of

glass, which enclose a gas mixture usually composed of neon and xenon (some manufacturers also use helium in the mix) that is contained in millions of tiny cells sandwiched in between the glass.

Working of Plasma Panel• Electricity, sent through an array of electrodes that are in

close proximity to the cells, excites the gas, resulting in a discharge of ultraviolet light.

• The light then strikes a phosphor coating on the inside of the glass, which causes the emission of red, blue or green visible light. (Each cell, or pixel, actually consists of one red, one blue and one green sub-pixel). The three colors in each pixel combine according to the amount of electric pulses fed to each sub-pixel, (which varies according to the signals sent to the electrodes by the plasma display’s internal electronics), to create visible

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Working of Plasma Panel (pic)

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Plasma Panel

Plasma display panels– Similar in principle to

fluorescent light tubes– Small gas-filled capsules

are excited by electric field,emits UV light

– UV excites phosphor– Phosphor relaxes, emits

some other color

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Plasma Panel (2)

Plasma Display Panel Pros

– Large viewing angle

– Good for large-format displays

– Fairly bright

Cons

– Expensive

– Large pixels (~1 mm versus ~0.2 mm)

– Phosphors gradually deplete

– Less bright than CRTs, using more power to some extent in order to make up for the less illumination.

Backlight• A backlight is a form of illumination used in liquid crystal displays

(LCDs).

• As LCDs do not produce light themselves (unlike for example Cathode ray tube (CRT) displays), they need illumination (ambient light or a special light source) to produce a visible image.

• Backlights illuminate the LCD from the side or back of the display panel, unlike front lights, which are placed in front of the LCD.

• Backlights are used in small displays to increase readability in low light conditions, and in computer displays and LCD televisions to produce light in a manner similar to that of a CRT display.

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Liquid & A crystal?????

• How many states exist in science?

• What are these things called liquid crystals?

• The name "liquid crystal" sounds like a contradiction. We think of a crystal as a solid material like quartz, usually as hard as rock, and a liquid is obviously different. How could any material combine the two?

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Liquid Crystal Displays (LCDs)

– LCDs: organic molecules, naturally in crystalline state, that liquefy when excited by heat or E field

– Crystalline state twists polarized light 90º.

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Display Technology: LCDs

Transmissive & reflective LCDs:– LCDs act as light valves, not light emitters, and

thus rely on an external light source.– Laptop screen: backlit, transmissive display– Palm Pilot/Game Boy: reflective display

Working of LCD• An LCD works entirely differently compared to Plasma Panel. • An active matrix LCD’s light source is generated by small

fluorescent bulbs. The white light from these bulbs is diffused to create a uniform light source by shining it through a polarizer located in the back of the display, which allows light to go through in only one direction.

• Individual LCD cells in the panel are then turned “on” and “off” by applying a small electric charge to the thin film transistors (TFT), located in each sub-pixel.

• This charge causes the liquid crystals to twist, allowing white light to be passed through red, green and blue color filters and a front polarizer in front of the LCD cells. The image is formed according to which crystals twist to let light through or block it.

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Working of LCD (pic)

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Display Technology: DMD / DLPDigital Micromirror Devices (projectors) or Digital Light

Processing

Microelectromechanical (MEM) devices, fabricated with VLSI techniques

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Organic LED Arrays

• Organic Light-Emitting Diode (OLED) Arrays– The display of the future? Many think so.– OLEDs function like regular semiconductor LEDs– But they emit light

• Thin-film deposition of organic, light-emitting molecules through vapor sublimation in a vacuum.

• Dope emissive layers with fluorescent molecules to create color.

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Organic LED ArraysOLED pros:

– Transparent– Flexible– Light-emitting, and quite bright (daylight visible)– Large viewing angle– Fast (< 1 microsecond off-on-off)– Can be made large or small– Available for cell phones and car stereos

OLED cons:– Not very robust, display lifetime a key issue– Currently only passive matrix displays

• Passive matrix: Pixels are illuminated in scanline order, but the lack of phospherescence causes flicker

• Active matrix: A polysilicate layer provides thin film transistors at each pixel, allowing direct pixel access and constant illum.

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Display Processor

• Also called either a Graphics Controller or Display CoProcessor

• Specialized hardware to assist in scan converting output primitives into the frame buffer.

• Fundamental difference among display systems is how much the display processor does versus how much must be done by the graphics subroutine package executing on the general-purpose CPU.

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Video ControllerCycles through the frame buffer, one scan line at a time. Contents of the memory are

used the control the CRT's beam intensity or color.

X address

Y address

Pixel value(s)

Raster scan generator

Data

Horizontal and vertical deflection signals

Intensity or color

Linear address

Set or increment

Set or decrement

M e m o r y

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Hard-copy devices

ο Ink-jet printer

ο Laser printer

ο Film recorder

ο Electrostatic printer

ο Pen plotter

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Input Devices

• Locator Devices: – to indicate a position and/or orientation

– to select a displayed entity– Tablet, Mouse, Trackball, Joystick, Touch Panel, Light Pen

• Keyboard devices: – to input a character string

– Alphanumeric keyboard (coded - get single ASCII character, unencoded - get state of all keys - more flexible)

• Valuator Devices: – to input a single value in the space of real numbers

– Rotary dials (Bounded or Unbounded), Linear sliders

• Choice Devices: – to select from a set of possible actions or choices

– Function keys