Todays Lecture (Feb 8, 2000)

1
Todays Lecture (Feb 8, 2000)

• Summary of Last Lecture
• ASCII code Example
• The UPC Bar Code
• Human Factors
• Principle of Operation
• Coding Scheme
• Examples
• Sensor Operation

2
Summary of Last Lecture

• Computers and Communication systems represent
  information with bits
• A bit is a quantity which can take on two values
  only 1 and 0
• N bits can represent up to 2 to the power N
  symbols
• Mapping symbols to bits is encoding
• Extracting symbols from bits is decoding
• Keyboard Characters are mapped to bits via the
  ASCII code

3
Simple Code Example

• The following code represents four colors using
  2 bits

Code
Symbols Red Orange Yellow Blue
Bits 00 01 10 11
4
ASCII Code

• Symbols are all characters on the keyboard
• 8 bits represent each symbol
• The code is defined in the textbook
• To encode, write down the 8 bits for each
  character, repeat for all characters
• To decode, consider 8 bits at a time, extract
  character from table, and repeat for next 8 bits
  until done

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ASCII Code Example
ASCII Code
Symbols A B _at_
Bits 01000001 01000100 01000000 00100011
6
UPC Code

• Bar code represents all items sold in a store
• Human factors
• Code can be scanned forward or backward
• Should be scanned fast and not prone to errors
• Two numbers uniquely identify product

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UPC Example
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Fundamental Concept

• Each bit is represented by a bar of thickness T
• 1 is represented by a black bar
• 0 is represented by a white bar

1
1
0
1
0
T
9
Code Map

• Two codes are used depending which side the digit
  appears
• Number of 1s in codeword is odd when on left
  side
• Number of 1s in codeword is even when on right
  side

Symbols 0 1 2 3 4 5 6 7 8 9
Left Bits 0001101 0011001 0010011 0111101 0100011
0110001 0101111 0111011 0110111 0001011
Right Bits 1110010 1100110 1101100 1000010 101110
0 1001110 1010000 1000100 1001000 1110100
UPC Code
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Sensor Operation

• Laser reflects off white bars and doesnt reflect
  off black bars
• reflection 0
• no reflection 1
• Guard bits are used to synchronize and determine
  bar thickness
• Direction of scan is determined by decoding bits
  and counting number of 1s (black bars)