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From: eichin@athena.mit.edu (Mark W. Eichin)
Newsgroups: sci.electronics
Subject: Re: Jamming supermarket music systems (was: Jamming Walkmans)
Message-ID: <1909@bloom-beacon.MIT.EDU>
Date: Sun, 29-Nov-87 07:50:16 EST
Article-I.D.: bloom-be.1909
Posted: Sun Nov 29 07:50:16 1987
Date-Received: Tue, 1-Dec-87 05:55:04 EST
References: <4149@utai.UUCP> <1160@uhccux.UUCP> <3810@bellcore.bellcore.com>
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Reply-To: eichin@athena.mit.edu (Mark W. Eichin)
Organization: Many Intoxicated Topologists
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The subcarrier systems (under the tradename StoreCast) are a 38KHz
subcarrier off of an FM broadcast station (19KHz is for the L-R
signal.) Thus, anything that can interfere with normal FM transmission
should do the job here; note that it would be equally illegal.[1] A
StoreCast receiver can be built with only slight modifications to any
`well-built' FM stereo unit, I even purchased a kit somewhere that had
an extra amplifier stage that you could tap into your FM unit to
receive such broadcasts, with the reminder that you cannot use the
gadget in a commercial manner.

If you were to build such a jammer, wouldn't you need so much power
that it would freak the electronic cash registers, if you got too
close? FCC Class A & B regard emissions, not absorption; the last
REALLY shielded machine I saw was the IBM 9000[2] `lab workstation'
which was this 68K based un!x box that could be safely run next to an
induction furnace... Actually, TEMPEST requirements deal with that, I
suspect, if you can find someone to tell you :-)

			Mark Eichin
			

[1] Speculation: a walkman(tm) jammer that simply induced a signal in
the headphones directly would probably not be illegal, because of the
(presumeably) low power needed? You can generate alot of EMF in a 5
foot antenna wire...

[2] I saw this 4-5 years ago at an IBM demo in CT somewhere, in a
division that may not exist anymore; I may have the number wrong.
Reply by mail if you want more specifics on what I think I saw.

Disclaimer: I think these are my opinions, but don't quote me on it ;-)