Input and Output Devices

Input and Output Devices

Input devices

Enable user to enter commands and data.

Output devices

Enable computer to communicate information to user.

•          Display and Sound
•          Monitors
•          Video Cards
•          Projectors
•          Sound Systems

Monitors

Most common output device.

Connected to video card.

Categorized by color output.

Monochrome o One color with black background.

Grayscale

o Varying degrees of gray

·         Color

o Display 16 to 16 million colors 

Cathode Ray Tube (CRT)

Most common type of monitor

Electrons fired from the back

Electrons excite phosphor to glow

Phosphor is arranged in dots called pixels (picture elements)

o Unique address of each pixel

Dot mask ensures proper pixel is lit

o Dot mask - sheet of metal perforated with holes    

CRT color

Phosphor dots arranged in triads

Red, green, and blue dots

Three colors blend to make colors

Varying the intensity creates new colors 

  

CRT vs. LCD (liquid-crystal display)

Very large

o Appr. 16 inch deep vs. a few inches deep

Very heavy

o Over 30 kg vs below 5 kg Use a lot of electricity     

Liquid-crystal display (LCD)

q Special liquid-crystal used for image display

q Liquid-crystal is transparent normally 

q Becomes opaque when charged with electricity 

q May not be clearly visible in bright light 

q Have limited viewing-angle

 Types 

1.  Passive matrix LCD 
2.  Active matrix LCD

  

 Passive matrix LCD

Pixels arranged in a grid

 Pixels are activated indirectly

o Activation through ICs (transistors)

 o Row and column are activated

 Animation can be blurry

o E.g. mouse pointer moved quickly would leave trail

  

 Active matrix LCD

 Each pixel is activated directly

 Pixels have 4 thin film transistors (TFTs)

 o One each for red, green, blue

o One for opaqueness

 Transistors arranged in a thin film

 Animation is crisp and clean 

 Drawbacks to LCD

•   More expensive than CRT
•  Must sit directly in front of screen
•  Can be more fragile than CRT 

  

 Paper-white displays

 High contrast between fore and background

 Document designing

o E.g. newspaper and magazine composing

 Electro-luminescent displays (ELD)

•  Similar to LCD q Uses phosphor held between 2 films to produce light
•  Grid of wires send current through film

 Plasma monitor 

 Gas is excited to produce light 

 Intensity controlled by voltage applied at various points 

  

 Monitors impacts user effectiveness

Monitors should have

• Crisp text
• Clear graphics
• Adjustable controls
• Clear edges 
•  

 Size of monitor

 Measured in inches

 Measured diagonally

 Actual size

o Distance from corner to corner

   Viewable size

o Useable portion of the screen 

  

 Resolution

•  Number of pixels on the screen
•  Higher number creates sharper images
•  Higher number creates smaller images 

 Refresh rate

•  Number of time the screen is redrawn
•  Modern equipment sets this automatically
•  Improper settings can cause eyestrain 

 Dot pitch 

•  Distance between the same color dots 
•  Ranges between .15 mm and .40 mm 
•  Smaller creates a finer picture 
•  Should be less than .22 for good quality 

  

 Video Cards

• Device between the CPU and monitor 
• Better cards result in better output
• Removes burden of drawing from CPU
• Have their own processor and RAM
• Modern cards have up to 512 MB RAM
• Capable of rendering 3D images 

  

 Eyestrain 

 Fatigue of the eyes 

 Steps to avoid 

•  Choose a good monitor 
•  Place the monitor 2 – 3 feet away 
•  Center of screen below eye level