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From: fish@ihu1g.UUCP (Bob Fishell)
Newsgroups: net.audio
Subject: Re: What does "distortion" sound like? - (nf)
Message-ID: <358@ihu1g.UUCP>
Date: Thu, 17-May-84 13:25:32 EDT
Article-I.D.: ihu1g.358
Posted: Thu May 17 13:25:32 1984
Date-Received: Fri, 18-May-84 02:39:10 EDT
References: <32900004@convex.UUCP>
Organization: AT&T Bell Labs, Naperville, IL
Lines: 30

(oo)
"Distortion" is a catch-all  term which is used to describe any deviation
in the shape of time-varying signal from input to output, except for
amplitude.  There are several subtypes, the most commonly referenced are
harmonic and intermodulation distortion.  Harmonic distortion is a
measure of how accurately a component can reproduce a pure sinusoid;
it gets its name from the fact that any waveform other than a pure
sinusoid produces harmonics, and can be expressed by a Fourier infinite
series that includes the fundamental frequency and all its harmonics.
IM distortion, on the other hand, is a measure of how well a system
can reproduce a complex waveform consisting of many frequencies mixed
together.

Figures for THD (Total harmonic distortion) and IM distortion are
usually given with components, but they only represent part of the
picture; amplifiers with identical THD and IMD figures can sound
different.  However, most people can't tell the difference between
0.1% THD and 0.001% THD.  When you can actually hear distortion,
there's usually something wrong, like overdriving and amplifier,
which produces clipping.  This is a strident, garbled noise that
is an indication of the signal amplitude trying to exceed the 
saturation voltage of the output devices.  This type of distortion
destroys tweeters.  Worn phono styli also produce audible distortion,
as do dirty or worn tape heads and failing internal devices.  These
are only a few of the most common sources of distortion; it can occur
from many other sources.
-- 

                               Bob Fishell
                               ihnp4!ihu1g!fish