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

[Lester Veenstra]

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.