Managing Digital Sound

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Managing Digital Sound

• Lecture Notes for Chapter 13 in the text
• Exploring the Digital Domain
• An Introduction to Computing with
• Multimedia and Networking

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Digital Sampling of Sound

• Digital sound is sound that has been converted
  to, or created in, a discrete form (namely a set
  of numeric values)
• Natural sound is a continuous phenomena and is
  converted to digital form by sampling techniques
• Properties of sound waves
• amplitude (measure of loudness)
• frequency (measure of pitch)

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Continuous Sound Wave
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Sampling Sound

• Spatial sampling
• sample rate
• resolution
• Analog-to-digital converters (ADCs)
• Digital-to-analog converters (DACs)

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Sampling of a Sound Wave Illustrated
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Sampling of a Sound Wave Illustrated (contd)
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Sampling of a Sound Wave Illustrated (contd)
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Resolution and Dynamic Range

• Resolution depends on how much memory is devoted
  to storing individual sample amplitudes
• 8-bit sound
• 16-bit sound
• Dynamic range and clipping

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Resolution Illustrated
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Storing Digital Sound

• Sample rate and resolution -- tradeoffs
• Voice and speech 8-bit resolution and 5-10 KHz
  sample rate
• CD-quality music 16-bit or higher resolution and
  44 KHz
• Audio file compression
• Sound on the Web
• downloading and playing sound files
• streaming audio

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Synthesizing Music

• Simple waveforms and oscillators
• Subtractive synthesis
• Additive synthesis
• FM synthesis
• Phase distortion synthesis
• ADSR (attack, decay, sustain, release) envelope
• Integrated synthesis

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Oscillators
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ADSR Envelope
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MIDI Instruments and Devices

• MIDI is a standard interface between electronic
  musical instruments and synthesizers
• Devices which are MIDI-compatible can communicate
  with each other
• Advantages to MIDI
• files are encoded and are much smaller than
  digitized sound files
• files can be easily edited and mixed for multiple
  tracks

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Speech Synthesis

• Speech synthesis involves creating speech from
  written text
• Analysis of written text focuses on breaking the
  text into phonemes
• English employs approximately 50 basic phonemes
• Rules allow a speech synthesis program to
  evaluate alternate pronunciations appropriate for
  the context
• Such rule-based phoneme analysis produces
  excellent speech synthesis results

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Speech Recognition

• Speech recognition attempts to interpret
  digitized speech for meaning
• The task is complicated by the differences among
  speakers and even the different ways a given
  speaker might pronounce the same word depending
  on mood, context, etc.
• Some success has been achieved by
  tailoring/training a program to recognize a
  particular speaker
• Some reasonably successful voice activation
  systems have been produced where vocabulary is
  limited to samll number of words
• Speech recognition remains a very challenging
  problem

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Speech RecognitionIllustrating the Difficulties
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Editing Digitized Sound

• Digital sound editing is part of a larger field
  called digital signal processing
• Once sound is digitized, it is in discrete
  numerical format
• Numerical transformations on this data can be
  used to
• change the sounds pitch
• change the sounds amplitude
• add echoes and other special effects

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Summary

• Digital sound is produced by sampling sound waves
  over time
• A digital sound file consists of sampled
  amplitudes at a number of discrete times within a
  given time interval
• The number of samples per second is called the
  sample rate
• The number of bits devoted to storing individual
  sampled amplitudes is called the resolution of
  the digitized sound 8-bit, 16-bit and higher
  resolutions are used depending on the kind of
  sound being digitized
• Fidelity will be largely determined by the sample
  rate and resolution