Visual display units

Question 1

Describe a pixel?

Answer 1

Picture element
The smallest thing that can be turned on or off to produce an image
A dot
Anything you see on a computer screen is a combination of these dots or pixels

Question 2

Describe resolution

Answer 2

Total number of pixels displayed

Resolution = # pixels across * # lines displayed

Question 3

Describe Screen size.

Answer 3

Measured diagonally across the front face
Measured in inches
Viewable areas may be less

Question 4

What is refresh rate?

Answer 4

How often the screen is refreshed or redrawn
Screen needs to be refreshed regularly as the phosphors stop glowing and the image to be displayed changes
Measured in Hertz (i.e.. Hz) per second

Question 5

What are video standards?

Answer 5

Defines the resolution and colors
for displays
The standard used, will be determined by the monitor and the video adapter card

Question 6

What are the video standards for high resolution displays?

Answer 6

Quad – is a mode with four times as many pixels (twice the width and twice the height)
Hex – is a mode with 16 times as many pixels (four times the width and four times the height)

Question 7

What are the three types of monitors?

Answer 7

Cathode Ray Tube (CRT)
Liquid Crystal Display (LCD)
Plasma

Question 8

What is dot pitch?

Answer 8

Spacing between each pixel

Typically 0.24mm

Question 9

Compare black and white monitors with color monitors.

Answer 9

Black and White monitor
Use white pixels
Made up of one phosphor
Colour monitor
Uses three primary colours: Red; Green; and Blue (RGB)
Made up of three phosphors, so close that the human eye sees the image as one single pixel

Question 10

What are the two types of pixel shapes?

Answer 10

Triad
Three electron guns arranged with overlapping circles in a triangle
Circular holes in shadow mask
Used in smaller screens (e.g. monitors)
Trinitron
Three electron guns arranged in one line
Three parallel slots
Oval slots in shadow mask
Used in large screens (e.g. televisions)

Question 11

What is an interlaced display?

Answer 11

Image on screen is created in two halves
First the odd numbered lines then the even numbered lines
Cheaper
Produce a flickering image and jerky video motion
Mostly found on older PC monitors

Question 12

What is a noninterlaced display?

Answer 12

Entire screen is created in one go
Less flicker
Smoother video motion
More commonly found

Question 13

How does a Cathode Ray Tube work?

Answer 13

Electron gun generates an electron beam
The yoke is an electromagnet. Varying the magnetism in the yoke causes the electron beam to focus in particular areas of the screen
The shadow mask is a metal screen with holes in it. The metal blocks the electron beam but the holes will let the electron beam through when it will strike a pixel, and only one pixel. Makes sure the image is sharp
The phosphor screen is made up of the phosphors that glow when hit by the electron beam. An image is formed by turning on some phosphors but not others.
The electron beam sweeps over the phosphor screen, making some phosphors glow, so fast that the user sees a stable image on the screen.
Other components such as the panel glass, funnel glass and the inner magnetic shield provide the structure of the CRT

Question 14

Describe the radiation inside a CRT?

Answer 14

Whenever an electric current passes through a conductor (such as a wire or a copper track on a circuit board) it gives off an electro magnetic field
The electron gun, electromagnet and coil inside a CRT monitor produce magnetic fields that radiate from the monitor
Most of this radiation is exposed from the rear of the monitor

Question 15

What are the pros and cons of a CRT monitor?

Answer 15

Pros
Greater colour fidelity
Superior contrast
Better viewing angle from off-axis

Cons
Large/take up space
Heavy
Emit X-ray band radiation (health hazard)
Refreshing can give headaches
High operating voltage

Question 16

Compare CRT to LCD monitors.

Answer 16

CRT Advantages
Has the best color quality
Faster response (No lag)
True black colors 
Lower cost

LCD Advantages
Lower power consumption
Lower heat is generated
Less weight
Less radiation
Smaller form factor

Question 17

What do LED and LCD stand for?

Answer 17

LCD Screens:- Liquid Crystal Display Screens

LED Screens:- Light Emitting Diode Screens

Question 18

What is the matrix in monitors?

Answer 18

A two dimensional array; that is, an array of rows and columns
The background area of colour display

Question 19

What are the two methods that are used to apply charges to liquid crystal cells?

Answer 19

Passive Matrix

Active Matrix

Question 20

Describe the charateristics of a Passive Matrix.

Answer 20

Thin Film Transistor (TFT) for each row and column
Old way LCD display
Grid of wires and at each intersection is a LCD element i.e a single pixel .
Cheaper
Use relatively few electrodes arranged along the edges of the liquid crystal layer and rely on timing to be sure the correct cells are charged
The charges in the cells fade quickly, causing a faded look
Slower response time

Question 21

Describe the characteristics of an active Matrix?

Answer 21

Individual TFT’s for each cell/sub pixel (i.e. RGB) and Capacitors to control a pixel
Not Modern but advance than Passive Display
Brighter
Wider viewing angle
Expensive
Provide a more precise and stronger charge, creating more vivid colours
Faster response time

Question 22

Describe how an LCD works.

Answer 22

Light emanating from a fluorescent panel behind a computers display panel spreads out in waves that vibrate in all directions
A polarising filter in front of the light panel lets through only the light waves that are vibrating more or less horizontally
In a layer of liquid crystal cells – there is one for each colour (RGB)
The light emerging from each liquid crystal cell passes through one of the three colour filters – RGB – that are arranged close to each other
The coloured beams of light pass through a second polarising filter that is aligned to let pass only those light waves that are vibrating more or less vertically
The light that passed through a liquid crystal to which a full electrical charge was applied is now oriented perfectly to pass through the second filter.
Any light that was not twisted at all when passed through the liquid crystals is now blocked completely

Question 23

What is the basic info for an LCD

Answer 23

The LCD screen is more energy efficient and can be disposed of more safely than a CRT.
Its low electrical power consumption enables it to be used in battery-powered electronic equipment.

Question 24

Describe the charateristics of an LED monitor?

Answer 24

Very thin display
Can be hang on the wall like a painting
High color quality then LCD
More energy efficient then LCD and Plasma
Larger sizes than Plasma

Question 25

How does an LED work?

Answer 25

LED screens are actually better described as LCD with LED back lighting.
Like an LCD screen LEDs have a front layer of LCD (crystals) that make up the colors and pixels
But unlike LCDs, LEDs use L.E.D. lights to illuminate the screen.
This means the screen turns on much faster (near instant)
The colors are crisper – the fluorescent lamps used by LCDs often added a yellow hue to the image displayed
Thinner, LEDs are much thinner the fluorescent lamps

Question 26

Compare LCD to LED

Answer 26

LED
Power consumption is less
Fast response time
Color accuracy is better
Can be extremely slim
20,000:1 dynamic contrast

LCD
Power consumption is more
slow response time
color accuracy is less
Can not be extremely slim
500,000:1 Dynamic contrast

Question 27

What are the Pros and cons of a Plasma screen

Answer 27

Pros
Thin display, less than 2 inches thick
Flat display
Can be hang on the wall like a painting
Wider viewing angle than LCD (160 degrees)
Larger sizes than LCD
Cons
More expensive than LCD
Plasma display is dimmer than LCD
Screen Burn / burn-in effect
Not good at high altitudes
Gets hot and shouldn’t be used in temperatures over 35°C

Question 28

How does a plasma screen work?

Answer 28

Two sets of electrodes
Address Electrodes are positioned vertically in the rear of the display
Display Electrodes are positioned horizontally in the front of the pixels
These electrodes run through layers of glass and magnesium oxide, which protect and insulate the electrodes from each other
Pixels are called cells, these are depressed in ridges called ribs
The ribs separate the cells
Trapped inside each cell is a mixture of xenon and neon gases
Illuminate tiny coloured fluorescent lights to form an image
Each pixel is made up of three fluorescent lights: a red light; a green light; and a blue light
Display varies the intensities of the different lights to produce a full range of colours
The central element in a fluorescent light is a plasma, a gas make up of free-flowing ions (electrically charge atoms) and electrons (negatively charged particles)
When an electrical current runs through a plasma, negatively charged particles are rushing toward the positively charged areas of the plasma, and the positively charged particles are rushing toward the negatively charged areas
In this mad rush, particles are constantly bumping into each other
These collisions excite the gas atoms in the plasma, causing the plasma screens to release light photons of energy