[GreenKeys] on-off radio teletype (not FSK)

[David I. Emery]

On Fri, Feb 16, 2018 at 07:45:41PM -0600, Jim Haynes wrote:

> the inked slip into hard copy.  In my childhood we could turn to the
> shortwave bands on the family radio receivers and hear tremendously strong
> droning sounds that we thought were airplanes.  I was told that this was
> high speed Morse.

	Nope... at least in the mid to late 50s through the 70s....

	Those signals were VFT ("tone pack") FDM multiplexed
multichannel FSK signals... most of the strong ones you'd hear in NA
would have been US or Canadian military or government.

	Typically 16 FSK'd tones spaced 170 Hz apart and shifted by 85
Hz occupying most of the normal voice bandwidth of a VF (voice
frequency) audio channel.

	Mostly transmitted over SSB or more commonly ISB rigs from audio
modem racks... often with voice trunks, or fax, or some early data
modem, or another VFT mux on the other sideband.   Some systems used 4
audio channels, two on the upper sideband and two on the lower.

	Early systems transmitted a partially suppressed carrier down a
few db and used a motor tuned demodulator that tracked that carrier to
handle receiver drift (often with R390As as the receivers).

	The famous Navy R-1051 family was specifically designed to work
with these signals which were the primary mechanism for transmitting the
US Navy Fleet RTTY broadcast from the early 60s until the UHF milsats
took this over in the late 70s.   Receiving these signals with the
narrow 85 Hz shift reliably required considerable transmitter and
receiver stability and the 1051 was designed to provide that by
frequency synthesis from a stable OXCO standard, way better than say
R-390As... and allowed elimination of the pilot carrier tone.

	They were also used for fixed point to point circuits in those
pre-satellite, pre undersea cable days... depending on where you lived
some of the most powerful ones might have been some of those circuits
from around the DC area and various other places.   Quite a few were
used to support the Eastern and Western Test ranges (eg launches from
Cape Canaveral and Vandenberg AFB)

	They did have a sound like the slowly shifting drone of
multi-engine piston aircraft flying overhead - when tuned on an AM
receiver.. a careful observer tuning them on a SSB receiver would have
noticed that there seemed to be some FSK signals in there somewhere...

	And yes when I was a little boy I too thought they were
transmissions from airplanes - especially when the garbled SSB voice for
the ISB channel on the other sideband was audible and allowed one to
believe that was the pilot talking to some ground station over the roar
of the engines.

	I eventually figured out how to copy the RTTY from these signals,
much of the traffic was encrypted, but there were news wires and weather
circuits and some others in the clear on some of them.

-- 
  Dave Emery N1PRE/AE, die at dieconsulting.com  DIE Consulting, Weston, Mass 02493
"An empty zombie mind with a forlorn barely readable weatherbeaten
'For Rent' sign still vainly flapping outside on the weed encrusted pole - in 
celebration of what could have been, but wasn't and is not to be now either."