Non-Linearities in Systems
Absolutely perfect linearity does not exist in any real system. There are many different types of non-linearity, and they exist in varying degrees in all mechanical systems, although many actual systems approach linear behavior, especially with small input levels. If a system is not perfectly linear, it will produce frequencies in its output that do not exist in its input. An example of this is a stereo amplifier or tape recorder that produces harmonics of its input signal. This is called "harmonic distortion", and it degrades the quality of the music being reproduced. Harmonic distortion almost always gets much worse at high signal levels. An example of this is a small radio that sounds relatively "clean" at low volume levels, but sounds harsh and distorted at high volume levels.
Many systems are very nearly linear in response to small inputs, but become non-linear at higher levels of excitation. Sometimes a definite threshold exists in which input levels only a little above the threshold result in gross non-linearity. An example of this is the "clipping" of an amplifier when its input signal level exceeds the voltage or current swing capacity of its power supply. This is analogous to a mechanical system where a part is free to move until it hits a stop, such as a loose bearing housing that can move a little before being stopped by the mounting bolts.
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