Analog and Digital in the Physical World

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Analog and Digital in the Physical World

• How does information actually move around?

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Signal

• A signal is a varying physical quantity that
  carries information
• Analog physically resembles the information
  that is being represented
• Phonograph groove
• Digital discrete, usually binary

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Examples of Media carrying signals

• Electric voltage in telephone wire
• Intensity of visible light in fiber optic
• Voltage in coaxial cable (as used by cable TV)
• Electromagnetic radiation (wireless)
• Visible
• Radio frequency

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The Electromagnetic Spectrum

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Spectrum

• A spectrum is a range of frequencies
• Electromagnetic spectrum all frequencies of
  electromagnetic radiation - visible light, radio,
  infrared, etc.
• All travel at speed of light so
• wavelength speed of light/frequency
• Short wavelength ltgt high frequency
• Long wavelength ltgt low frequency
• Transmission through atmosphere varies by
  wavelength

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Carrier radio frequencies

• Several signals can coexist in the same medium at
  different frequencies
• E.g. AM Radio .53-1.7 MHz, FM 88-108 MHz
• E.g. TV broadcast channels use part of the radio
  frequency spectrum
• VHF channels 2-4 54-72MHz, ch 5-6 76-88MHz
• UHF band 300-3000 MHz (Ultra High
  Frequency)
• E.g. WiFi (802.11b) 2.40 - 2.45 GHz
• And visible light at much higher frequencies!

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Tuning or Filtering

• Receivers select one frequency from many so
  different frequencies act like independent
  channels
• Handling higher frequencies requires more
  sophisticated technologies, so higher frequency
  bands have been used more frequently

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Modulation

• How is information carried on the carrier?
• In radio, frequencies are assigned to stations by
  the FCC, but they are actually allocated a band
  or channel of frequencies
• AM bands 10 kHz
• e.g. AM1030 1025..1035 kHz
• FM bands 200 khz
• E.g. FM103.3 103.2MHz 103.4MHz
• FCC separates bands to avoid interference, but
  separation is more than modern electronic tuners
  require
• AM signal in varied Amplitude
• FM signal in varied Frequency

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Cable

• Cable TV and data on coaxial cable run from
  cable company to drops for each house
• Data speeds depend on traffic, and location of
  congestion depends on the network topology
• Different frequencies used for data and analog TV
• Widespread conversion to all-digital

Cable Co
drops
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DSL (Digital Subscriber Line)

• Uses same copper telephone wire as old analog
  voice
• DSL High frequency data (200-800 KHz) mixed with
  low frequency voice (0.3-3.4KHz) on same
  telephone wires
• Uploads and downloads use different frequency
  bands
• High frequencies fade faster so distances are
  limited
• Max distance from DSLAM box 18,000 ft.
• Phone companies now starting to install fiber to
  the home

Telco
DSLAM
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Wireless information transport

• Cordless telephone handsets
• Cell phones
• WiFi, WiMax, and other broadband
• Broadcast radio and TV
• Satellite radio and TV
• Satellite Internet
• Many other more proprietary uses at other
  frequencies

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Even Power Lines can Carry Data!
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Energy

• Energy is what you pay for use more, pay more
• Measured in BTUs in the US and Joules everywhere
  else
• 124,000 BTU/gallon of gasoline
• 1,026 BTU/cubic foot of natural gas
• So you can measure cost-effectiveness of an
  energy source by its /BTU

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Power

• Power Energy/unit time
• Electric power usually measured in watts
• 100 watt light bulb uses energy at a rate of 100
  watts 100 joules/sec about 1/7 horsepower
• Electric energy is sold in kwh kilowatt-hours
  (watts/1000) hours
• Strength of broadcast communication signal is its
  power

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Power Spectrum
Power spectrum shows distribution of power in
signal across frequencies
Example 1KHz and 1.2KHz signals of equal
power No signal at any other frequency
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Voice SpectrogramPower Spectrum over Time

• Color power (blue low, red high)

Frequency
Time
Ri ce Uni ver si ty
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Modulation

• There is only one sine wave at a given frequency,
  so how does the information get carried at a
  particular frequency?
• Modulation Encoding information on a signal
• Analog radio modulation technologies
• FM Frequency Modulation
• AM Amplitude Modulation

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Amplitude Modulation
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FM Frequency Modulation
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Problems with AM and FM

• Power varies with amplitude
• to be precise, power is the square root of the
  average of the square of the signal power, the
  root mean square or rms power
• As signal fades with distance, some parts of the
  signal drop out before others
• But AM can transmit over longer distances because
  AM frequencies bounce off ionosphere and diffract
  around hills and buildings but FM frequencies are
  absorbed, causing shadows
• This difference between AM and FM is due to the
  frequency bands allocated to them, not the
  modulation technique!

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HD Radio

• HD Hybrid Digital (not Hi Definition, which
  it need not be)
• Uses frequencies near stations carrier frequency
  for low power digital broadcast
• 100x weaker than analog carrier signal
• Modern electronics makes it possible to detect HD
  signals without distorting main analog signal