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Klarinet Archive - Posting 000594.txt from 2001/02

From: (William Wright)
Subj: Re: [kl] The straight path of the audio engineer
Date: Fri, 16 Feb 2001 12:36:30 -0500

<><> Tony Pay wrote:
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).

This is precisely the image that "The Conversation" dramatizes,
and this is why the film caught my attention --- although I didn't fully
recognize why at the time. In the film:

Gene Hackman records a conversation with three microphones under
difficult circumstances --- microphones in suit cases and coat pockets,
suspicious fugitives whispering and looking over their shoulders as they
walk circles in a crowded park, another directional microphone on nearby
rooftop, and so forth.
When Hackman returns to his workshop, he mounts the tapes on three
synchronized tape decks and he adjusts the timing + amplitude of the
tape decks back and forth until he gets an audible conversation.
So this is a matter of adjusting the 'sum' (P+Q) by manipulating
phase and amplitude.
But of course any surveillance agent could do this, and Gene
Hackman is the world's best. Certain key words in the conversation are
still impossible to understand. So what does Hackman do? He pulls a
special 'black box' off the shelf and plugs it into the audio system.
Of course, nobody else in the world knows how to build this black box.
By twiddling some more knobs, he brings out the last few words (and
reaches the wrong conclusion about the conversation, but that's part of
what makes it a good film). Since Hackman is not adjusting the speeds
or amplitudes or starting points of the three tapes any longer, we can
infer that the black box is doing something else besides simple
summation and amplification. It is doing non-linear things (P.Q).
The film is really an effective dramatization of the principle.


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