Silicon, Circuits, and the Digital Revolution

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Twenty-Ninth Class

Sound and Its Digitization

Storage of text information
ASCII: American Standard Code for Information Interchange
Demonstration of sample text (sample.txt)

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

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 disc
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 (actually 74 minutes).
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Last updated November 29, 2004.
© George Watson, Univ. of Delaware, 2000.