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From: mark@mips.UUCP
Newsgroups: rec.aviation,sci.electronics,rec.audio
Subject: Noise-cancelling microphone
Message-ID: <1027@mips.UUCP>
Date: Tue, 19-May-87 20:42:19 EDT
Article-I.D.: mips.1027
Posted: Tue May 19 20:42:19 1987
Date-Received: Thu, 21-May-87 06:54:13 EDT
Lines: 35
Xref: utgpu rec.aviation:1297 sci.electronics:633 rec.audio:1407

I recently read (in Aviation Week & Space Technology) an article about
a noise-cancellation system for military pilots' helmets.  Seems that
military cockpits are _very_ noisy, with helicopter blades whooshing,
fighter engines roaring, etc, so the pilot can barely hear his/her radio.

The new system puts a microphone in each earcup, and uses the resulting
signal to measure the "error" at the pilot's ear [error == signal at ear
minus signal from radio] and thus cancel much of the ambient noise
(about 30 dB as I recall).  So the pilot hears the radio but not the
loud powerplant.  Pretty cool.

If memory serves, the whole system is contained in a cigarette-pack sized
box that goes in the pilot's pocket (so the aircraft isn't modified, just
the helmet).  Noise cancellation is done with digital circuitry.

Now, a few questions:
 1.  Has anybody out there experimented with this sort of thing?  How
     well did it work?

 2.  Could a hobbyist-grade version be built on the cheap, using e.g.
     analog circuits for the signal processing?  Or is the major cost
     the modified earpieces with mikes?

 3.  Can the idea be used "backward" to make a microphone which cancels
     ambient cockpit noise and only picks up the pilot's voice?  For
     example, have a 2nd mike that faces _away_ from the pilot, and use
     this signal as an approximation of the "error" (ambient noise).
     Or is this already available per Scott Dorsey's recommendation of
     the "Telex noise-cancelling element"?

Regards,
-- 
-Mark Johnson	**DISCLAIMER: The opinions above are personal.**  Hi Max!
UUCP: 	{decvax,ucbvax,ihnp4}!decwrl!mips!mark   TEL: 408-720-1700 x208
USmail:	MIPS Computer Systems, 930 E. Arques, Sunnyvale, CA 94086