Continuous Media: Audio

• Sound Waves
• Analog vs. Digital

Sound Waves

Sound is produced by vibrations in a medium, usually air. Sound travels when the disturbance at the source is transmitted to neighboring molecules and propogated in the form of a wave.

The resulting pattern of oscillating pressure is called a waveform.

• Frequency (wavelength)
• Measured in hertz (Hz) or cycles per second
• Humans can hear 20 Hz - 20 kHz
• Frequency is perceived as pitch
• Amplitude
• Measures displacement of air pressure wave from its mean
• Measured in decibels: dB = 20 log₁₀(A/B)
• Amplitude vs. loudness
• Duration is the length of time the sound lasts

Timbre is the quality that gives a periodic waveform its unique sound, e.g. a trumpet vs. a clarinet

Find out more about how

• waveforms correspond to human speech
• sound waves combine

Analog Versus Digital

Analog signals are continuous and analogous to the original physical phenomenon.

Digital signals are measurements taken at discrete time/space intervals.

• Sampling rate
• Measured in hertz (Hz) or samples per second
• Nyquist rate = 2x the maximum frequency
• Aliasing results from too low sampling rate
• Quantization
• Number of bits used to represent each sample
• Quantization error caused by lack of precision
• Conversions
• Analog to Digital Converter (ADC) samples analog signal, filters components above the Nyquist frequency
• Digital to Analog Converter (DAC) produces signals from numbers, may oversample for better sound
• Examples
• Voice-quality audio is recorded at 8 kHz, 8 bits per sample
• CD-quality audio is recorded at 44.1 kHz, with 16-bit linear Pulse Code Moduloation (PCM) encoded value

Find out more about

• sound processing
• digital sound

Digital Audio Formats

• Sampled Sound Files
  • Size of (monaural) digital audio = sample rate * sample size * sample time
  • AIFF and WAV are uncompressed
  • MP3 is lossy, using psychoacoustic models to identify "less-important" frequencies
  • Advantages:
    • high quality
    • ease of creation
    • ease of editing
    • consistent playback quality
• Synthesized Sound
  • MIDI = musical instrument digital interface, composed of messages
  • Advantages:
    • editing control
    • small files

Find out more about

• audio file compression
• MIDI file structure
• more digital audio compression