[GreenKeys] RTTY receiver noise immunity?

Tue Apr 20 15:09:48 EDT 2021

     Presumably we are talking about FSK. In FSK one carrier or 
signal is always on. This is not the same as a carrier, which 
would be a third frequency, but is similar to a carrier in that 
there is always something on one of the two frequencies. One 
could design a squelch or mute that would turn off the motor when 
_neither_ signal was present. Another kind of mute would be if 
only one signal was present for longer than a specified time, 
then one could shut off the motor. It would take some activity to 
start the machine when the signals resume. This could be 
anything, say a few line feeds or carriage returns. Enough to 
give the listening machines time to come up to speed. I am sure I 
am describing something that exists.

On 4/20/2021 10:57 AM, steve bennett via GreenKeys 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 
> spaces
> to 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 strategies
> 1) 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 signal
> 3) 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 <http://www.navy-radio.com>
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