[GreenKeys] FRF FSK conv. (was - Two eBay Novelties)

[Rich MC Clung]

There was commercial TWINPLEX with four tones it was designed to be a form of frequency diversity assuming when one tone pair were in fade the other pair would be at peak.

The CIA's FBIS modified TWINPLEX TU's and teletype print baskets to reproduce the Cyrillic alphabet and monitored Soviet teletype circuits.

A more modern use of TWINPLEX has been used by the Pakistani Diplomatic Corps, TWINPLEX Four frequency ARQ.

RICH  WA6KNW



Date: Tue, 11 Feb 2014 09:46:30 -0500
From: "Lester Veenstra" 
Subject: Re: [GreenKeys] FRF FSK conv. (was - Two eBay Novelties)

4 frequency M-aryFSK      

The NSA units I had decoded four audio frequencies each separated (from the
next lower) by a standard shift (determined by a plug-in filter group
encased in a metal can about the size of a TV B+ capacitor with an octal
plug on one end of the assembly), and when both channels were running the
scope presented a square pattern. (As I recall, I only saw that once.)  It
had a plug-board (associated with the filter plug-in) to allow for
dynamically assigning M-M, M-S, S-M and S-S to different frequencies.  
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