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From: unicorn@pnet01.UUCP
Newsgroups: rec.aviation,sci.electronics,rec.audio
Subject: Re: Noise-cancelling microphone
Message-ID: <1131@crash.CTS.COM>
Date: Mon, 25-May-87 18:16:40 EDT
Article-I.D.: crash.1131
Posted: Mon May 25 18:16:40 1987
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Xref: utgpu rec.aviation:1381 sci.electronics:661 rec.audio:1470

How quickly we decide something technological cannot be done,
because it used to be impractical.  And in these rapid-moving
times too !  

I observed the "noise source canceller" in operation on a
"News from Science" blurb on CNN or somesuch a couple of
months ago.  Naturally, you need a whole buncha power, and
the key limitation seems to a requirement for a very high
periodic component in the sampled signal.  Fortunately,
much industrial noise is like this.

As for spacing, sound travels 1116 feet/sec.  Got a tape 
measure ?