Digital Audio

1
Digital Audio
2
SoundMultimedia

• Sound (and its derivatives speech, music, etc.,
  generally referred to, if audible to humans, as
  audio) has a significant part to play in
  multimedia applications
• From interacting through a multi-modal user
  interface (e.g., Surfing the Web by voice) and
  text-to-speech systems (Apple Speech
  Technologies), to software agents capable of
  expressing themselves in natural language (e.g.,
  VirtualFriend) Internet-based radio and TV (e.g,
  RealAudio, and Internet Radio and TV sites)
  video-conferencing (e.g., Cu-SeeMe) and Internet
  telephony (e.g., VocalTec Communications)
  generating computer music, sounds for games, and
  computer-controlled musical instruments (e.g.,
  MIDI) to personalised elevator music and
  refrigerators that hum along to your mood, audio
  is essential.

3
THE NATURE OF SOUND

• acoustics the study of the behaviour of sound
  in enclosed spaces
• 3 conditions necessary for sound to exist
• A source a vibrating physical body or object.
• A medium any physical substance capable of
  carrying the vibrations of a sound source from
  one place to another
• A receiver any physical object, which is in
  contact with the sound medium and is capable of
  vibrating in the same way as the sound source.

4
Sound properties perception

• A vibrating source pushes particles in air back
  and forth, creating areas of compression and
  rarefactions.
• When a molecule moves, it collides with the next
  one and make it moves too and may eventually
  reach the ears of a listener.
• When we hear sound, air vibrates against our
  ear-drums causing them to vibrate too. These
  vibrations are detected and analyzed by our
  brains.

5
Sound Receivers

• The perception of sound involves a great more
  deal than its reception
• Any physical object capable of vibrating like a
  sound source can be described as a sound receiver
• Ear to convert sound vibrations into their
  electrical equivalent and then encode the
  information in the form of a stream of pulses
  suitable for transmission to the brain through
  nerve fibres

6
Sound Receivers

• Microphone represents the ears of modern sound
  recording and for that reason alone it is almost
  certainly the most important single item of
  equipment in sound production technology
• Designed to convert sound energy into electrical
  energy, the microphone is called an
  electro-acoustic transducer

7
Robert Boyles Experiment
8
Properties of Sound

• Frequency
• Amplitude
• The shape of waveform

9
Frequency

• As the wave travels, the disturbance of particles
  is in the direction of the wave travel. The
  particles vibrate about their normal positions.
  Each complete vibration of a particle is called a
  cycle (i.e. from its starting position, to a
  maximum distance in one direction, back through
  the starting position, then to a maximum
  displacement in the opposite direction and back
  to the starting place). . The frequency of a wave
  is measured as the number of complete
  back-and-forth vibrations of a particle of the
  medium per unit of time. If a particle of air
  undergoes 1000 longitudinal vibrations in 2
  seconds, then the frequency of the wave would be
  500 vibrations per second. A commonly used unit
  for frequency is the Hertz (abbreviated Hz),
  where 1 Hertz 1 vibration/second.

10
Frequency
11
Frequency

• The frequency of a sound is the number of periods
  in a second and is measured in hertz (Hz). 1000
  Hz 1 kiloHertz (kHz). Audible (to humans)
  frequency occurs in the 20Hz to 20kHz range
  (normally in range 1 to 5 KHz). Babies can hear
  up to 20KHz!!!. Other frequency ranges are
• Infra-sound 0 - 20Hz
• Ultrasound 20kHz - 1GHz
• Hypersound 1GHz - 10THz

12
Amplitude The shape of waveform

• The shape of the waveform that repeats at regular
  intervals is called a period, and sounds musical
  (e.g., a bird singing). A waveform that is not
  periodic sounds like noise (drums, coughing,
  sneezing, etc.)
• The amplitude of a sound is a property
  subjectively heard as loudness or volume
• Measuring unit used to deviate the pressure wave
  from its mean value (idle state) unit Decibel
  (dB)
• Spoken language 60dB
• Heavy Road Traffic 80 dB
• Rock band 120dB

13
Digitizing Audio

• An Analog-to-Digital Convervter (ADC) measures
  the amplitude of pressure waves at regular time
  intervals (called samples) to generate a digital
  representation of the sound. The reverse
  conversion, to play digital sound through an
  analog device (such as speakers) is performed by
  a Digital-to-Analog Converter (DAC).

14
Digitizing Audio Sampling

• The number of samples taken per second is called
  the sampling rate. CD quality sound is sampled at
  44,100 Hz, which means that it is sampled 44,100
  times per second. This appears to be well above
  the frequency range of the human ear. However,
  the Nyquist sampling theorem states that "For
  lossless digitization, the sampling rate should
  be at least twice the maximum frequency
  responses."
• The human ear can hear sound in the range 20Hz to
  20KHz, and the bandwidth (19980Hz) is slightly
  less than half the CD standard sampling rate.
  Following the Nyquist theorem, this means that CD
  quality sound can represent frequencies only up
  to 22,050Hz, which is much closer to that of
  human hearing.

15
Digitizing Audio Sampling

• Natural sound is a continuous phenomena and is
  converted to digital form by sampling techniques

16
Digitizing Audio Sampling
17
Digitizing Audio Sampling Rate?

• No of samples per cycle too low
• Monotonous sound
• Nyquist sampling theorem states that "For
  lossless digitization, the sampling rate should
  be at least twice the maximum frequency
  responses."

18
Digitizing Audio Quantizing

• Resolution
• The accuracy with which we present each sampled
  amplitude
• 8 bit 255 varieties of sound
• Higher bit, better quality
• CD quality 16 bit, recorded using 20 or 24 bit
  quality

19
Digitizing Audio Quantizing
20
Digitizing Audio Dynamic Range

• Dynamic range
• The total range of amplitude values that can be
  stored
• Amplitudes outside the range will be given the
  highest or lowest values in the range (whichever
  applies) the sound wave will be effectively
  chopped off at the limits of the range. This
  effect is called clipping.

21
Size of Digital Audio

• Length of sound (seconds)
• Sampling rate (kHz/s)
• Resolution bit
• Whether using stereo or mono
• Mono is suitable for narration, single sound
  without background sound

22
Example

• Calculate the size of this digital audio file
• 1 minute, 44.1 kHz, 16 bit of resolution, using
  stereo.
• Storage size 60 seconds 44100 Hz 16 bits
  2 (stereo)
• 84672000 bits
• 1058400 bytes
• 10335.937 KB
• 10.3359 MB

23
Quality of Digital Sound

• Quality of Source file
• Editing process will decrease the quality of
  original sound. Better record using high quality
• Capture and playback device
• More expensive, better functionality offer
• Depends on cost of device

24
Quality of Digital Sound

• Properties used when capturing a sound
  frequency, sampling rate, channel no
  (mono/stereo)
• Trade-off with size of file and processing time