What

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Whats a Code?

1. A system of signals used to represent letters and
   numbers in transmitting messages.
2. A system of symbols, letters, or words given
   certain arbitrary meanings, used for transmitting
   messages requiring secrecy and brevity.

-- American Heritage Dictionary
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Who are these people?
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Sending messages using Clicks and Clacks
Western Electric Morse Code Telegraph Sounder
Telegraph Key
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How does this work?

• When a current flows through the induction coil,
  the resulting magnetic field attracts an armature
  that is held up against a metal arm. When the
  current is switched off, the armature is drops to
  its resting position, resulting in a "click".
  When the current returns, the armature is raised
  back to the upper arm resulting in a "clack."

http//www.answers.com
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• What can you do with Clicks and Clacks?
• Morse Code
• A fast click-clack is a dot.
• A slow click-clack is a dash.
• Morse code is comprised on only two symbols.
• Morse code is a binary code (two symbols).

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Famous Morse Codes

• SOS An easy to remember distress call, similar
  to 911 today.
• SOS . . . _ _ _ . . .
• V for Victory Beethovens 5th Symphony
• Dah Dah Dah Daaahhhmmmm . . . _

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Samuel Morse Artist/Inventor 1830s
Note not all letters have the same length code
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Not all codes are the same length
Number of Dots and/or Dashes (length of code) Number of Possible Codes
1 21 2
2 22 4
3 23 8
4 24 16
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Reasons for using codes of varying lengths

• Most commonly occurring characters are given
  short codes
• This is a form of file compression

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Spies can break simple codes that replace each
letter with another symbol just by counting the
frequency of the symbols.
Frequency Table for the English Language
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Zip File Compression

• Eliminate Redundancy
• Replace common words with short codes.
• A table is created indicating short codes for
  common words.

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More Binary Coding Schemes

• Braille uses raised or flat dots with six dot
  locations.
• 26 or 64 unique codes are possible.

Braille
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Translating Braille
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What about the humans?
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We have ten fingers

• Therefore we tend to use ten symbols for
  constructing numbers
• 0 1 2 3 4 5 6 7 8 9

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Our numbering System is Arabic

• Indo-Arabic of Indian Origin, introduced into
  the west around 1000 A.D.

1. Positional - number depends on digits position.
2. No Special symbol for the number ten.
3. Need for ZERO

Hindu Numerals from 100 A. D.
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How many apples are

• 3,674.5 (in base-10)
• This is
• 3 X 103 6 X 102 7 X 101 4 X 100 5 X 10-1
• 3000 X 600 70 4 5/10

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How many apples are

• 3,524.5 if you happen
• to be a bird with six digits?
• (base-6)
• This is
• 3 X 63 5 X 62 2 X 61 4 X 60
• 5 X 6-1
• 3216 X 536 12 4 5/6
• 648 180 12 4 5/6 844.83

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The Mayans used a base-20 numbering system
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Computers have tiny electrical switches
(transistors) that are controlled by two voltages
0000 0 0001 1 0010 2 0011 3 0100 4 0101 5 0110 6 0
111 7 1000 8 1001 9 1010 A 1011 B 1100 C 1101 D 11
10 E 1111 F

• 28 256
• 216 65,536
• 232 4,294,967,296

A 32-bit microprocessor can access 4 billion
memory addresses or 4GBytes
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Be a secret agent!

• Using tic-tacs of multiple colors, devise a
  coding scheme for coding a sentence of about
  15-20 characters.
• A sentence may have numbers and letters.
• Determine how you will transport this code.
• Keep codes as small as possible.
• Can you memorize your codes?

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Competition

• Half of your team will code the message.
• One person will transport the code to another
  room.
• The other half of the team will decode the first
  message and code a response.
• The first team to decode the 2nd message has to
  eat all the tic-tacs!

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How to construct numbers

• The Roman Way
• I - one
• V - five
• X ten
• L - fifty
• C - 100 (cento in Italian)
• D 500 (diecicento)
• M 1000 (mille)

• 1945 MCMXLV
• or MCMXXXXV ?
• or MCMVL ?
• 1907 MCMVII
• 1909 MCMIX
• Cumbersome to add, confusing to subtract.
• Very hard to multiply and divide.