[GreenKeys] Tone frequency history?

[Ralph Mowery]

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
using rtty on frequencies about 30 MHz they could use audio in the AM or FM
modes instead of just shifting the carrier.  So the demodulators were built
using the space as the high tone.  As ssb came about it was discovered if
you feed a single tone into the microphone of a SSb transmitter your signal
would be a single frequency if the tone was a pure sine wave.  This lead to
the use of lsb on all bands for rtty as the frequency shift would be
inverted .  That is if audio shift is up 170 or 850 hz and you use lsb the
RF output will be that much lower.  If usb is used then the RF output would
be higher.  The ham on the receiving end could set his receiver to the
sideband that matched his filters which was most often lsb.  Rtty really
confuses many in that you can set your receiver to either usb or lsb and
then switch the demodulator to 'normal' or 'reverse' and with the correct
combination you will receive the rtty correctly no matter what the
transmitter is doing.

Not sure if true or not,but the reasons for 2125 and 2295 is music tuning
forks of 425 hz were easy to find years ago.  The 2125 is the 5 th harmonic
and 2295 is the 7 th harmonic.  By using an oscilloscope it was easy to look
at the pattern and see if your frequency was correct.  Remember this is way
before hams had frequency counters and the receivers were as stable as they
are now.That is the reason for the 850 hz shift and the FCC had a rule the
shift could not be over 900 hz.  As things improved over the years the 850
hz shift was divided by 5 to get 170 hz shift.

Ralph ku4pt


-----Original Message-----
From: greenkeys-bounces at mailman.qth.net
[mailto:greenkeys-bounces at mailman.qth.net] On Behalf Of Harold Hallikainen
Sent: Thursday, October 07, 2021 10:31 PM
To: greenkeys at mailman.qth.net
Subject: [GreenKeys] Tone frequency history?

Back when I built my first vacuum tube TU with 88 mH loading coils, I knew
that mark was 2125 Hz and space was 2975 Hz (the 850 Hz shift days). But,
what is the history of these tones? Why were they chosen? why do we have
space high on audio and space low on RF? I've read that tones in the 2.1
kHz area would turn off the echo suppressors on long distance telephone
networks. Were these tones brought over from telephone? Bell 103 does not
use these tones. Where did they come from?

Harold
https://w6iwi.org