How does a CD player work?
(Lansing State Journal, November 8, 1995)

When we hear sound, we are actually sensing air pressure changes.  A microphone uses this principle directly.

Microphones have a small membrane that responds to pressure variations by vibrating.  This motion generates a small electric current.

When recording a CD, the electric potential created by the microphone is measured about 44,000 times per second on a numerical scale ranging from 0 to 65,535; these numbers are stored digitally on tape as sequences of zeros and ones.  The zeros and ones, along with timing information, are printed as tiny depressions on a CD.

A small laser in a CD player reads these depressions.  As the CD rotates, the information is translated into a sequence of signals.  These signals are again interpreted was strings of zeros and ones, re-creating the original digital signal.  The decoded numbers are translated back into voltages, and the resulting electric signals are sent to a speaker.  The speaker's membrane moves the same way as the microphone membrane.

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