For HF the tones do not matter. Just that they are either 850 Hz apart for the old standard or 170 Hz apart for the new standard. Hams shifted low on HF, so when SSB became common LSB was used to invert the tones. On VHF either AM or FM was used. The tones had to be set to a ‘standard’ of 2125/2995 or 2125/2295. The receiver frequency could not be shifted to compensate for the tones.
When SSB was used by feeding the tones into the microphone the transmitters would not pass the higher tones equally. So lower tone pars were used. The tones should be above 1000 Hz so if there was any 2nd harmonics of the tones the ssb filter would help eliminate that.
Commercial RTTY often used 450 Hz shift and the tones were reversed from what the hams used.
One other note is that for hams on HF to start with there were no two tones but off and on keying similar to CW as per the FCC rules.
Ralph ku4pt
From: [email protected] [mailto:[email protected]] On Behalf Of Nick England
Sent: Friday, August 25, 2023 3:26 PM
To: Greenkeys
Subject: [GreenKeys] RTTY - High tones - Low tones?
I'm attempting to get a Robot 800 TU going. The manual (circa 1981) says Low Tones (1275/1445) are used for HF and High Tones (2125/2295) are used for VHF. "Return to factory" if you want to change....
Now I'm a newbie when it comes to RTTY, but I really would like to know if there was ever a time when people agreed to one standard and stuck with it for more than 6 months?