Still, at the
rates they were sending data in those days, FSK
could have been as simple as a solenoid on a capacitor, tied to
the frequency reference.
That was the normal way of doing things at that time. Some of us
can still do it that way...
Modem technology of the late 1930s and World War 2 era was far more
advanced that the simple trick described above. FSK and tone
modulation were both characterized by fairly complex equipments for
the time with active, tube circuitry. Many sets could also cram a few
telegraph or teleprinter channels onto one voice channel or pair of
wires.
--
Will