[GreenKeys] Hal ST-6 using it in modern times

[Jim Haynes]

On Sun, 22 Mar 2020, Richard Knoppow wrote:

>    I don't remember the details but it seems to me that in the early days of 
> spark keying was done by detuning the arc thus shifting its frequency. So, 
> what was on the air was a sort of FSK. Nothing is new.
>
Spark was done with on/off keying, but the Poulsen arc transmitters could
not easily be started and stopped so they shifted the frequency to send
telegraphy.

Moving on to the era of vacuum tube technology, E. H. Armstrong proposed
FSK for telegraphy in 1928.  This was well before he worked out the
principles of wide band FM for broadcasting speech and music.  About
the same time Schmitt of Teletype got a patent on FSK for radio
telegraphy.  Both of these inventors proposed limiterless two-tone
detection rather than FM detection, hoping that noise would affect mark
and space channels equally and would cancel out, while the desired signal
would be detected as mark or space.