[GreenKeys] RTTY receiver noise immunity?

Tue Apr 20 15:41:50 EDT 2021

 I'll stop beating the dead horse after saying... remember that transmitting FSK before SSB was done with an oscillator and a power amplifier. Two-tone audio only was an artifact of trying to get the same answer using an SSB transmitter.  

RegardsGerry

    On Tuesday, April 20, 2021, 12:15:19 PM PDT, steve bennett via GreenKeys <greenkeys at mailman.qth.net> wrote:  

  Gary,
  Well yes and no.I think there is no difference between viewing the signals as two audio tones on SSBvs viewing it as two distinct carriers.It amounts to the same thing. In practice I think it is dealt with as two audio tone on SSB. But yes you could view that is two carriers somaybe my claim about carrier detection was correct after all.
Isn't radio crazy?
-Steve

    On Tuesday, April 20, 2021, 02:11:17 PM EDT, Gerry Block <gblock at sbcglobal.net> wrote:  

  Actually RTTY is all carrier.Your description of the mark-hold as a carrier detector is actually a pretty good description.  In reality it does carrier detection on two carriers (mark or space) and activates when either one is not inactive (i.e. an OR gate).There are many ways you could spoof this system, but not many of them are natural occurrances.Gerry

    On Tuesday, April 20, 2021, 10:59:32 AM PDT, steve bennett via GreenKeys <greenkeys at mailman.qth.net> wrote:  

  Wait...RTTY is SSB so no carrier.Strike that sentence about detecting a carrier.There must have been some way of timing the length of marks and spacesto determine if it was legitimate RTTY.
-s

    On Tuesday, April 20, 2021, 01:50:28 PM EDT, steve bennett <raleigh_ranger at yahoo.com> wrote:  

  I was just reading about Autostart in a manual for a TU.This particular TU could tell the difference between voice, CW and RTTY by the difference in duty cycle.Of the three RTTY is the only one with a 100% duty cycle.So in other words detect a carrier wave that is constantly on or else do not print.Not sure if this helps in their situation.
-Steve AD8CI
    On Tuesday, April 20, 2021, 11:41:36 AM EDT, Nick England <navy.radio at gmail.com> wrote:  

 I’ve been in a discussion with the USS Iowa guys about what to expect when there’s no RTTY signal just receiver noise. They are concerned because they get a bunch of garble when there’s no signal and the AGC turns up the gain. 
Note. This discussion is not about noisy RTTY signals but responding to noise when there is NO RTTY signal.

I am fairly new to RTTY but use vintage equipment.  It seems normal to me that some noise will trigger the space filter and the TTY will print some random characters every now and then, more often or almost continuously if the noise level is high. The same discussion holds true for autostart I guess. 
OK, software or complicated digital hardware can do all kinds of things to mitigate random characters, but what about earlier TUs? What is your experience from the olden days (or today using vintage gear)?
I can see a couple of non-software strategies1) SELCAL with a M28 stunt box. The TTY still cycles but doesn’t start printing until the correct sequence of characters arrives. 2) manually riding the receiver RF gain to keep noise low when there is no signal3) having a receiver with squelch or COR (carrier operated relay)4) use a multivibrator to Mark hold unless a space pulse is at least 22ms long. 5) use a multivibrator to Mark hold until there is a  long Space or Mark signal. Typical autostart circuit I guess. 
Or???

-- 
Nick England K4NYW
www.navy-radio.com
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