Continuous Media: Audio

• Sound Waves
• Analog vs. Digital

Sound Waves

Sound is produced by vibration, causing a disturbance in air pressure.

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 vs. pitch
• Amplitude
• Measures displacement of air pressure wave from its mean
• Measured in decibels: dB = 20 log₁₀(A/B)
• Amplitude vs. loudness

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

Online Sources of Digital Audio

This is just a starting point.

• http://webbay.com/freebies.htm
• http://www.50megs.com/mp3buttload/
• http://longs-trains.com/~music/
• http://windham.com/music.html
• http://www.bga.com/~fap7/synthesis/synlibry.htm