Much of that intervention became overcome with the use of a pilot

carrier.

Back in the heyday of the VOA use of SSB/ISB transmitters for

program feeds and the RTTY schedules, the SSB/ISB transmitters

had a pilot frequency, essentially, the carrier frequency, at about 20dB

down from the PEP of the transmitter.  

The receivers used would lock on to that carrier pilot signal and via

the AFC circuit, would keep the receivers on frequency.

73

Sheldon

On Sat, 4 Feb 2023, Harold Hallikainen via GreenKeys wrote:


It's my belief, without any solid evidence, that 850 shift was originally
chosen to get some tolerance for frequency drift in radio equipment of the
1930s-50s.  170 Hz shift for amateur RTTY came into wide use when we got
better equipment, thanks to the needs of SSB operation.

In amateur service it was tolerable to carry on a QSO with one hand on
the receiver tuning knob; but in commercial/government/military service
there was a need for reception without manual intervention over a period
of hours at a time.