[GreenKeys] Tone frequency history?

[Harold Hallikainen]

On Thu, October 7, 2021 8:49 pm, Jerry Murphy wrote:
> Hi Harold:
> The WWII era AN/FGC-1 teletypewriter terminal had mark and space filters
> in it
> that were set at 2125 and 2975 Hz.  The terminal was designed and built
> by
> Western Electric.  It seems likely that Bell Labs provided some design
> help. 
> I don't see anything in the literature about why those frequencies were
> selected
> but there must have been a good reason.  I still have a pair of those
> filters.
> Â Â Â  Jerry Murphy

OK, that makes sense! A lot of 1950s amateur radio stuff was using WW2
equipment. During WW2, I doubt they were using SSB transmission (I think
it was being used for frequency division multiplex on telephone lines, but
not over radio). So the tones were probably used for RTTY demodulation
and, on the transmit side, the transmitter frequency was shifted directly
(like I did with my Viking Ranger). They probably set the receiver for CW
with the BFO on the high side. The choice of high side or low side BFO is
pretty arbitrary, but it does seem that a lot of receivers use high side
where the tone frequency increases as you increase the receiver frequency.
This would result in the inversion between the RF FSK shift and the audio
AFSK shift.

I have not yet found an online manual for the AN/FGC-1. It would be an
interesting read!

Harold



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