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Audio

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Voice Ipod internet Ipod audio. Audio Technology. Podcasting: A ... DVDs store 6 distinct channels. Left, right, center, low frequency, left-rear, right-rear ... – PowerPoint PPT presentation

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Title: Audio


1
Audio
  • CS 110
  • Fall 2005

2
Audio Technology
  • Podcasting A modern problem
  • Voice ? Ipod ? internet ? Ipod ? audio

3
Audio Technology
  • Podcasting A modern problem
  • Analog-to-Digital TCP / IP
    Digital-to-Analog

4
Analog Signals
  • Analog Data is represented by continuously
    variable physical quantities
  • Microphone senses changes in pressure from sound
    waves

5
Analog Signals
  • Cassettes preserve the analog nature of the data

magnetism
6
Infinite Data
  • Analog data has a value for any moment in time
  • Infinite number of moments of time
  • A computers RAM is finite!
  • Convert from analog to digital

7
Analog to Digital
  • Sample the continuous data at specific moments in
    time (discretize)
  • Record the microphone pressure at each of these
    moments

8
Discretizing
  • How many samples are required?
  • Humans cannot hear above 20,000 Hz
  • A tuning fork that oscillates 20,000 times per
    second
  • To avoid errors (aliasing), CDs record 44,000
    samples per second (44 kHz)

9
Quantizing
  • What is precision of recorded samples?
  • Infinite variety of microphone pressures are
    possible
  • CDs limit precision to 5 decimal places
  • 2 bytes are required for this precision

10
Verify CD Specifications
  • Some quick math
  • 44,000 samples per second
  • 2 bytes per sample
  • 60 seconds per minute
  • CD max capacity ? 650 MB
    ? 74 minutes for 2 channels (left
    right)

multiply
5,280,000 bytes/min. ? 5 MB/min.
11
Beyond CD Formats
  • You want more?
  • DVDs have more storage capacity
  • 4.5 Gbyte
  • DVDs store 6 distinct channels
  • Left, right, center, low frequency, left-rear,
    right-rear
  • 96kHz (samples), 24-bit (quantization)

12
Beyond CD formats
  • You want smaller files?
  • Fewer samples too choppy (in time)
  • Smaller precision too choppy (in volume)
  • Compression!
  • Lossless - zip files, Apple lossless (ALAC)
  • Lossy - MP3, WAV, Real Audio

13
Lossless Compression
  • Example 14 digits 3 3 3 3 3 3 7 7 7 7 1 2 5
    5 tally repeated numbers
    8 digits 6-3, 4-7, 1, 2, 2-5

14
Lossy Compression
  • MP3
  • CD requires 1,408,000 bits/sMP3 enables 128,000
    bits/s (or more)
  • Whats lost?
  • Specific sounds humans dont hear well
  • Psychoascoutic experiments humans dont hear
    equally well across all frequencies

15
Analog to Digital Converter(ADC)
  • Electronic devices contain ADCs
  • Ipod, cell phone, computer sound card
  • Analog microphone fluctuations are quickly
    converted to digital data
  • In 1988, an entire computer would be required to
    perform this task

16
Utilizing Internet
  • Sharing digital audio
  • IP address of data source
  • IP address of data target (recipient)
  • Data to be transmitted
  • TCP/IP accomplishes the transmission of the data

17
Digital to Analog Converter(DAC)
  • Electronic devices contain DACs
  • Ipod, cell phone, computer sound card
  • Digital data is reconstituted as a continuously
    varying voltage signal that causes speaker
    diaphragm to move

18
Related History
  • Phonograph
  • Invented by Edison
  • Sound vibrations recorded as bumps on sheet of
    tinfoil or wax
  • A format war
  • Edison records were cylinders
  • Victrola and Columbia used discs
  • Columbia selected better songs (and won the
    format war)

19
Telephones
  • Max Matthews father of digital audio"
  • at ATT doing speech research for telephony
  • Point-to-point hardwire connection
  • Limited to human voice
  • 180-3200 Hz
  • Modern phones digitize at 8 kHz

20
Synthesized Audio
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