An old-fashion telephone system is an example of the use of an analog signal. Sound - music, talking, etc. - is carried through the air as a pattern of
pressure waves. The microphone in a telephone senses how the air pressure varies from moment to moment and generates an output voltage which
varies with time in the same way. This pattern of voltage fluctuations is then carried by wires, amplifiers, etc., to a small loudspeaker in the other
telephone. On receiving the signal, the loudspeaker waggles with the same pattern, recreating the sound waves near the listener's ear. The pattern
of voltage changes transmitted from phone to phone is said to be an analog of the sound pressure variations. The voltage pattern is said
to be an analog signal.
Analog signals are relatively easy to create and carry from place to place. However, they suffer from the fact that every tiny detail of the pattern matters.
If the pattern is slightly altered by unwanted noise or distortion, the output will not be identical to the input. This is why
good analog hi-fi equipment is so hard to make (translation - it's expensive!).
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