At 09:43 AM 12/20/2006, A.R.S. - W5AMI wrote:
>On 12/20/06, Chuck Mayfield <[EMAIL PROTECTED]> wrote:
> > I remember an article I saw somewhere back in the 70s that described
> > a technique for building a "local" scrambler. The trick was to
> mix the audio
> > signal with another radio signal that both stations could
> receive. The only
> > way to make the resultant signal intelligible was to demodulate
> it using the
> > audio used to modulate it.
> >
>Now that is one decoder that would be hard to replicate, huh? Maybe
>that's why these signals all seem to squeeze in between AM QSO's.
>Sounds like a pretty rock solid technique, unless some listener with
>the same decoder was able to figure out what signal they used to mix
>with.
It should be noted that there ARE software programs out there that do
just this sort of signal processing (figure out what the modulation
strategy is, and descramble it). It's fairly easy with voice,
because voice has a lot of internal structure which the algorithm can
look for. The problem is akin to solving a jigsaw puzzle or an
archaeologist assembling a broken pot.. you look for structural
elements that have to "line up".
Such processing has obvious intelligence uses, so the details of
implementation tend not to be available in the open literature. It's
a classic example of something that is easy to describe, in general,
how to do it, but the hard work is in the details.
{For instance, it is conceptually easy to describe how to build a CW
contesting robot that could dominate the bands, (after all, the
exchange is standardized, etc.) tough to actually put the pieces
together and do it}
James Lux, P.E.
Spacecraft Radio Frequency Subsystems Group
Flight Communications Systems Section
Jet Propulsion Laboratory, Mail Stop 161-213
4800 Oak Grove Drive
Pasadena CA 91109
tel: (818)354-2075
fax: (818)393-6875
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