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 www.aber.ac.uk & follow links:
 Learning & Teaching
 Past Exam Papers
 IMAPS
 Physics
 Examinations 2007
 Semester1
 PH19510

3

 First Communications from prehistory to early man
 Expressions (1,000,000 BC)
 Gestures & Body Language
 Early Spoken language (100,000 BC)
 First cave paintings (30,000 BC)

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 Analogue Information
 Any value between limits
 Most ‘real world’ quantities are analogue
 Temperature, pressure, light intensity etc
 Digital Information
 Only discrete values possible
 Numbers, Letters, other abstractions
 Binary

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 Noise is endemic in circuits
 Freedom from noise
 è process information
faithfully

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 Alphabets  26 characters
 Number systems  biunary 
hexagesimal
 Semaphore
 Hand signalling with flags – 26 characters
 Chappe Tower – 92 symbols, pairs = 8,464 codes
 Morse – 2 symbols, variable length codes
 Binary – On/Off

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 1874 Multiplexing printing telegraph system
 Up to 4 telegraph channels on single wire
 Time Division Multiplexing
 5 bit code

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 Sample analogue signal
 nbits
 2^{n} levels
 Quantisation Noise

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 Rate and number of levels dependent on quality
 fsample = 2 x fsignal
 Speech 8bit (256 levels) 8kHz
 CD quality 16bit (65,536 levels) 44kHz

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 Multiplexing
 Time division
 Frequency Division

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 First used on telegraph
 Interleave messages
 Synchronised clocks
 Digital Signals

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 Speech Signal
 Modulate 60Khz Carrier
 Put many signals along 1 wire.
 Separate in frequency space

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 1904 J.Fleming
 Heated filament
 Electrons liberated
 If Anode is +ve
 Electrons attracted
 Current Flows
 One way device
 Diode

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 1907 Lee DeForest
 Grid between Cathode & Anode
 ve voltage on grid repels electrons
 Control of anode current
 1911 Amplification

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 Emitter at ground
 +ve voltage on collector
 CollectorBase reverse biased
 è no current
 Apply +ve voltage on base
 Electrons pulled from emitter into base
 Collector base depletion region shrinks
 è many electrons flow from
Emitter to Collector
 Amplification

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 Certain minimum cost per component
 Complexity doubles every year
 Technology drives chip sizes down

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 Keeping information secret
 Steganography
 Cryptography
 Cryptanalysis
 Undo someone else’s cryptography

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 Algorithm è substitute
letters
 Key è cipher alphabet

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 AlKindi
 800 – 873 AD
 Analysis of text
 frequency of letters
 double letters (ee, oo, mm, tt …)
 adjacent letters
 single letter words
 common words

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 Patented 1921 by Arthur Scherbius
 Used in WWII
 Plugboard
 Rotors
 substitution
 changes every character

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 Pros
 Computer can mimic any machine (Turing)
 Ability to perform complicated encryption easily
 Working with binary numbers rather than letters, closer to mathematical
process
 Cons
 Cryptanalysis eased
 Try many keys quickly
 Computer data tends to have fixed form è known plaintext attacks

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 Alice wants to send a secret message to Bob
 Eve is eavesdropping

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 2 prime numbers p and q
 Public key N = p × q
 Easy to multiply number
 Difficult to factor
 Make N > 10^{308}

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 James Gleick
 Vintage
 ISBN
 £8.99
 http://www.around.com

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 Not randomness
 Chaos is
 deterministic – follows basic rule or equation
 extremely sensitive to initial conditions
 makes long term predictions useless

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 Mathematical map of all possibilities in a system
 Eg Simple Pendulum
 Plot x vs dx/dt
 Damped Pendulum
 Undamped Pendulum

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 Edward Lorentz
 From study of weather patterns
 Simulation of convection in 3D
 Simple as possible with nonlinear terms left in.
 Aperiodic – doesn’t repeat

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 "Clouds are not spheres, coastlines are not circles, bark is not
smooth, nor does lightning travel in straight lines"  B.B.
Mandelbrot
 Fractals are rough or fragmented geometric shapes that can be subdivided
into parts, each of which is exactly, or statistically a reducedsize
copy of the whole : selfsimilarity

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 Consider objects in 1,2,3 dimensions
 Reduce length of ruler by factor, r
 Quantity increases by
N = r^{D}
 Take logs:
 D is dimension

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34

 www.aber.ac.uk & follow links:
 Learning & Teaching
 Past Exam Papers
 IMAPS
 Physics
 Examinations 2007
 Semester1
 PH19510

35

 Communications
 Communications in prehistory
 Development of language, writing and counting
 Telegraph, telephones, television
 Codes to transmit information
 Modulation and multiplexing
 Encryption to hide information
 Chaos
 Simple chaotic system
 Fractals
