Klarinet Archive - Posting 000608.txt from 2001/02

From: Tony@-----.uk (Tony Pay)
Subj: Re: [kl] The straight path of the audio engineer
Date: Fri, 16 Feb 2001 15:55:07 -0500

On Fri, 16 Feb 2001 09:36:30 -0800 (PST), Bilwright@-----.net said:

> 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.

I'm sorry Bill, but I don't think that this is at all the image that
"The Conversation" dramatises.

I was trying to make clear the notion that sine waves of FREQUENCIES P
and Q in an amplifier input may give rise to difference tones of
FREQUENCIES P+Q and P-Q if the amplifier is a defective, nonlinear one.

That's nothing to do with the letters 'P' and 'Q' being interpreted as
independent recordings from different microphones, and then being
combined in order to make clearer what was said. That's quite another
idea.

And when you write:

> 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).

...it confuses the issue even more. In what I said, the combination
tones of FREQUENCIES P+Q and P-Q arise from the nonlinearity of the
amplifier, which was just a normal amplifier, trying to do a normal job.

There's no reason to suppose that a defective, nonlinear amplifier would
help at all in the unravelling of an ambiguous signal -- in fact, quite
the contrary -- so this has nothing to do with whatever black box
Hackman used, or whatever (P.Q) might mean, for different RECORDINGS P
and Q.

If you want to talk about the film, please use another notation,
in another thread.

We have to try to keep matters straight. It's bad enough as it is.

Tony
--
_________ Tony Pay
|ony:-) 79 Southmoor Rd Tony@-----.uk
| |ay Oxford OX2 6RE GMN artist: http://www.gmn.com
tel/fax 01865 553339

... I idiot-proof my programs, but then along comes a bigger idiot.

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