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Klarinet Archive - Posting 000587.txt from 2001/02

From: (Tony Pay)
Subj: [kl] The straight path of the audio engineer
Date: Fri, 16 Feb 2001 09:02:37 -0500

It struck me that all this confusion about linearity and nonlinearity
would seem quite strange to an audio engineer. And because our musical
world is considerably impacted by the work of audio engineers, the
subject may not be quite so arcane or 'theoretical' as all that. In
fact, what I'm about to say is probably well known to many people on
this list who have hi-fi systems of their own.

In the transmission of a performance from performers to listeners via an
audio recording, the sound of the instruments or voices involved
typically passes through air to the microphone; then via a
recording system and a replay system to a loudspeaker; and finally
through air to the ears of the listener.

An audio engineer's primary aim in designing these four components --
microphone, recording system, replay system and loudspeaker -- is to
make the part of the passage that extends from the microphone to the
loudspeaker as transparent as possible, so that it is as though the
sound had passed through just another chunk of air, rather than the

(I say the 'primary aim' because sometimes realism is not what is being
aimed at: the idea is to 'enhance' reality. Nevertheless realism is
what most of us think of as the desired end result.)

To this end, the electronics are designed to be as linear as possible.

Anyone who has bought a hi-fi amplifier, just to choose one bit of the
system, will be aware that there are various degrees of technical
excellence you can obtain by spending more or less money. Though I am
not an expert in these matters, I do know that there are two specs that
contribute to technical excellence, and thus realism of reproduction.

One of these is called total harmonic distortion (THD) and the other is
called intermodulation distortion (IMD). You want these two figures as
low as possible, as their names suggest.

Well, surprise surprise, THD is a measure of how single pure sine tones
have harmonics added to them by nonlinearities in the amplifier (input
frequency P has added frequencies 2P, 3P, etc in the output); and IMD is
a measure of how two pure sine tones have sum and difference tones added
to them by nonlinearities in the amplifier (input frequencies P and Q
have added frequencies P+Q, P-Q in the output).

So an audio engineer's mouth would probably drop open in disbelief if
you suggested that *air itself* had such properties.

_________ Tony Pay
|ony:-) 79 Southmoor Rd
| |ay Oxford OX2 6RE GMN artist:
tel/fax 01865 553339

... We all have failures. The question is, what do we do with them?

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