SWE 423: Multimedia Systems

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SWE 423 Multimedia Systems

• Chapter 3 Audio Technology (1)

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Audio

• Audio is a wave resulting from air pressure
  disturbance that reaches our eardrum generating
  the sound we hear.
• Humans can hear frequencies in the range
  20-20,000 Hz.
• Acoustics is the branch of physics that studies
  sound

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Characteristics of Audio

• Audio has normal wave properties
• Reflection
• Refraction
• Diffraction
• A sound wave has several different properties
• Amplitude (loudness/intensity)
• Frequency (pitch)
• Envelope (waveform)

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Audio Amplitude

• Audio amplitude is often expressed in decibels
  (dB)
• Sound pressure levels (loudness or volume) are
  measured in a logarithmic scale (deciBel, dB)
  used to describe a ratio
• Suppose we have two loudspeakers, the first
  playing a sound with power P1, and another
  playing a louder version of the same sound with
  power P2, but everything else (how far away,
  frequency) is kept the same.
• The difference in decibels between the two is
  defined to be
• 10 log10 (P2/P1) dB

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Audio Amplitude

• In microphones, audio is captured as analog
  signals (continuous amplitude and time) that
  respond proportionally to the sound pressure, p.
• The power in a sound wave, all else equal, goes
  as the square of the pressure.
• Expressed in dynes/cm2.
• The difference in sound pressure level between
  two sounds with p1 and p2 is therefore 20 log10
  (p2/p1) dB
• The acoustic amplitude of sound is measured in
  reference to p1 pref 0.0002 dynes/cm2.
• The human ear is insensitive to sound pressure
  levels below pref.

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Audio Amplitude
Intensity Typical Examples
0 dB Threshold of hearing
20 dB Rustling of paper
25 dB Recording studio (ambient level)
40 dB Resident (ambient level)
50 dB Office (ambient level)
60 - 70 dB Typical conversation
80 dB Heavy road traffic
90 dB Home audio listening level
120 - 130 dB Threshold of pain
140 dB Rock singer screaming into microphone
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Audio Frequency

• Audio frequency is the number of high-to-low
  pressure cycles that occurs per second.
• In music, frequency is referred to as pitch.
• Different living organisms have different
  abilities to hear high frequency sounds
• Dogs up to 50KHz
• Cats up to 60 KHz
• Bats up to 120 KHz
• Dolphins up to 160KHz
• Humans
• Called the audible band.
• The exact audible band differs from one to
  another and deteriorates with age.

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Audio Frequency

• The frequency range of sounds can be divided into
• Infra sound 0 Hz 20 Hz
• Audible sound 20 Hz 20 KHz
• Ultrasound 20 KHz 1 GHz
• Hypersound 1 GHz 10 GHz
• Sound waves propagate at a speed of around 344
  m/s in humid air at room temperature (20 ??C)
• Hence, audio wave lengths typically vary from 17
  m (corresponding to 20Hz) to 1.7 cm
  (corresponding to 20KHz).
• Sound can be divided into periodic (e.g.
  whistling wind, bird songs, sound from music) and
  nonperiodic (e.g. speech, sneezes and rushing
  water).

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Audio Frequency

• Most sounds are combinations of different
  frequencies and wave shapes. Hence, the spectrum
  of a typical audio signal contains one or more
  fundamental frequency, their harmonics, and
  possibly a few cross-modulation products.
• Fundamental frequency
• Harmonics
• The harmonics and their amplitude determine the
  tone quality or timbre.

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Audio Envelope

• When sound is generated, it does not last
  forever. The rise and fall of the intensity of
  the sound is known as the envelope.
• A typical envelope consists of four sections
  attack, decay, sustain and release.

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Audio Envelope

• Attack The intensity of a note increases from
  silence to a high level
• Decay The intensity decreases to a middle level.
• Sustain The middle level is sustained for a
  short period of time
• Release The intensity drops from the sustain
  level to zero.

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Audio Envelope

• Different instruments have different envelope
  shapes
• Violin notes have slower attacks but a longer
  sustain period.
• Guitar notes have quick attacks and a slower
  release