NetNews Usenet Archive 1993 #1/NN_1993_1.iso/spool/sci/crypt/6375

Internet Message Format | 1993-01-04 | 1.8 KB

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From: [e-mail] (Phil Karn)
Newsgroups: sci.crypt
Subject: Re: Motorola 'Secure-Clear' Cordless Telephones
Message-ID: <[e-mail]>
Date: 4 Jan 93 22:03:03 GMT
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In article <[e-mail]> [e-mail] (Rob Warnock) writes:
>2. If the FM "deviation" is large, there will be significant AM-like sidebands
>   that might be listened to with an SBB receiver with sufficiently narrow
>   filters. [O.k., my math's weak on this one. Maybe Phil Karn or some other
>   radio-knowledgable type could comment further on this case.]

Actually, it's if the FM deviation is *small*. At arbitrarily small
values of deviation, the FM signal closely resembles an AM signal
except that the sidebands are 90 degrees out of phase with the carrier
(instead of being in phase). However, this doesn't matter to a SSB
receiver since they ignore any transmitted carrier anyway.

However, the canonical way to "decode" frequency inverted speech is
with a "ring modulator". The "speech descrambling" kits you see in the
back of electronics magazines are nothing more than this: two audio
transformers, each with two windings, one center-tapped, four diodes
and an audio oscillator.  You wire the diodes as a ring (all four
diodes pointing the same way, NOT the same as a bridge rectifier).

Another way to decode frequency inversion is with DSP (digital signal
processing; NEXTstation users take note). You simply invert the sign
of every other input sample, and vary the sampling rate to vary the
inversion frequency.

Phil