SCEN103 Class 32

Sound and its Digitization

Demonstration of Audio on the Web

• Storage of text information
  -- ASCII: American Standard Code for Information Interchange
  -- Demonstration of sample text (Using Xtree Gold and sample.txt)

• Demonstration of sound waves
  -- Oscilloscope output of electrical signal driving speaker
  -- Variation of frequency of audio generator
  -- Effects of enclosing speaker

• Digitization
  -- Effects of resolution (number of bits of digitization)
  -- Effects of sampling rate (number of digitizations per second)
  • Slide Show of sampling issues
  • Another Slide Show showing aliasing
  • Nyquist Sampling Criterion:
    -- a waveform must be sampled at a rate at least twice its highest frequency to avoid aliasing

• Playing time of an audio compact disk
  • Recall that storage capacity is 650 Mb (megabytes)
  • 2 bytes per sample of sound (16 bits)
  • 2 channels for stereo sound => 4 bytes per sample
  • Thus a CD can store 160 million audio samples.
  • Since the human hear only hears up to 22 kHz (22,000 cycles/sec), a sampling rate of 44 kHz is used for sampling audio (Nyquist sampling criterion!).
  • The time elapsed in making 160,000,000 samples at a rate of 44,000 samples per second is 3600 sec.
  • This is 1 hour, the nominal playing time for an audio CD.