[GreenKeys] Dovetron TU question - keying a transmitter

[Jim Haynes]

Yeah, in amateur RTTY it was long understood that you had a send-receive
switch to control the transmitter keying and whatever other functions
you needed to do to keep the demodulator from interfering with the
transmitted signal.

What you're talkin about doing might be pretty irritating to the operator
you're talking with, if your transmitter goes off every time you pause
in keyboarding.  It was normal to operate pretty much like an AM voice
transmission - you turn the transmitter on and it stays on until you say
"over" or "K" in the case of RTTY.  In olden times if you paused while
transmitting you just sent a steady mark.  Or you might peck at the
LTRS or blank keys just to let the guy on the other end know you are
still alive and thinking what to say next.  It was definitely not geared
to the equivalent of "break in" operation in CW.

One thing that was used for a while was called bell break.  The idea was
to put a switch contact on the bell, and a two-state relay operated by the
contact, so that when you sent a bell your transmitter would go off and
the other guy's would go on.  Choice of the bell was that it was attention
getting, and also there was a mechanism there where you could easily 
attach a switch.  So in that operation you would say a few words and then
BK (bell) and the other guy would talk back to you.  And of course every
so many minutes you had to identify with call signs and at first with
call signs in Morse code as well.

The idea that the carrier goes on only when you are transmitting is
something done with the TTD that the deaf people use.  That was because
transmitting space tone only is good enough, and you don't need to
have a send-receive switch.  The TTD also uses a mark tone but it is
not part of the demodulation process.  The demodulation is single tone
on the space tone.  The mark tone is only to suppress echoes that were
a problem on phone lines of a certain length.

There are a lot of things you have to work out in a "traditional" RTTY
setup.  The Irv Hoff demodulators all depend on the keyboard and printer
being connected in series in a single loop, so you get local copy right
from the loop.  So when you are transmitting you need to kill the normal
output from the demodulator lest it interfere with the signal transmitted
from the keyboard.  Other designs got local copy in other ways, such as
monitoring your own transmitted signal and getting local copy from the
demodulator output.  (which only works if you are transmitting and
receiving on the same frequency)  Or putting the keyboard and printer
into separate loops but getting local copy some other way.  Then sometimes
you might want to retransmit the signal you are receiving, which requires
that the demodulator output does go to the transmitter input but you are
receiving on a different frequency from the one you are transmitting on.