[GreenKeys] Tone frequency history?

[Jim Haynes]

On Fri, 8 Oct 2021, Ralph Mowery wrote:

> For the space high/low the reason is that at first rtty was mostly sent by
> putting a capacitor across the tuned circuit of the transmitter oscillator.
> This caused the frequency to shift down for the space. As people started

That may be, but I think amateur RTTY started with AFSK at VHF before it
got into HF.  And the commercial and military services were using LSMFT
before the hams.  The standard commercial/military FSK generator was a
200 KHz L-C oscillator frequency modulated with a reactance tube.  The
output of that was heterodyned with a crystal or stable VFO to produce
the desired operating frequency.  Or a sub-multiple of it if the 
transmitter was going to frequency multiply.  I assume maybe the simple
diode modulator was not very linear frequency versus voltage, and maybe
they wanted linearity so they could transmit fax as well as tty.

Then there were the demodulators that operated from the receiver IF
rather than from audio.  Maybe this was to keep BFO drift out of the
equation.  But with that kind of demodulator you don't get a choice
of which frequency is higher at the receiver, and in fact it could be
either way depending on the RF to IF conversion scheme of the receiver.