Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!zaphod.mps.ohio-state.edu!sol.ctr.columbia.edu!emory!athena.cs.uga.edu!mcovingt
From: mcovingt@athena.cs.uga.edu (Michael A. Covington)
Newsgroups: sci.electronics
Subject: Re: Phone scrambler
Keywords: Phone scrambler
Message-ID: <1991Mar24.012754.9816@athena.cs.uga.edu>
Date: 24 Mar 91 01:27:54 GMT
References: <1991Mar23.215050.24738@sugar.hackercorp.com>
Organization: University of Georgia, Athens
Lines: 23

Although I haven't tried it, you could make an audio scrambler out
of an NE602 mixer-oscillator chip. Run the oscillator at about 5 kHz
and mix it with the audio using the double balanced mixer. Out come the
sum and difference frequencies. Low-pass-filter it so that you get only
the difference frequencies; this gives you audio which is "scrambled"
because its spectrum is backward. (It will sound like an SSB radio
signal does when you're tuned for the wrong sideband.) Run the sound
through the same setup again to descramble it.
  
  In sum:
    
    300-to-3000-Hz bandpass filter on input
    Oscillator 5000 Hz or so
    (no, make that 3500 Hz)
    Doubly balanced mixer
    300-to-3000-Hz bandpass filter again on output


-- 
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Michael A. Covington | Artificial Intelligence Programs
The University of Georgia  |  Athens, GA 30602   U.S.A.
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