Okay, so if it's several hundred times per minute, that would definitely
change everything. However, if that's the case, would it not be most
likely they're employing some sort of sinewave (or sawtooth, or
stepwave) 'seed' signal into a VCO which supplies the mixer freq?

It's not impossible, but it may be unlikely the mixer freq changes that
often. If it changes too much, then it itself starts to become a
problem, because the switching itself will begin to encroach into the
audio spectrum. The mathematical limit would be 3000 seed changes per
minute, as this would impose a 50Hz noise on the information. I suppose
part of the problem is my not ever hearing this type of signal, so I'm
only able to imagine what it sounds like. Is the system is truly an
analog mangling of the source material? Also, there was another system
out back in the 70's where they took the passband, inverted it, sliced
it up, and shifted the pieces about. It think Radio-Electronics did a
feature on it, including a project for one that broke 300Hz-3000Hz into
4 pieces, mangled them a bit and out popped your encoded audio. It was a
symmetrical system.

The more I think about it, it could be a nasty chore to straighten it
out. Harris has a system for HF that's straight analog but certainly not
easy to decipher, but their's is not straight inversion either. If
however, the mixer truly shifts about that much, it's simply not worth
the effort, unless you don't mind placing your sanity at risk...






Ralph A. Schmid, DK5RAS wrote:
bob wrote:


roward to decode." I was not suggesting it would not be labour
intense. It could turn out to be a royal pain in the a$$, however, if
it's simply inversion with a variable mixer freq, there's no real
security there.


The problem may be that the tick usually has nothing to do with the
number of frequency changes. The inversion frequency may change
several hundred times per minute, and the tick is just used to keep in
sync.



regards - Ralph