[GreenKeys] RTTY receiver noise immunity?

Tue Apr 20 13:47:23 EDT 2021

 Nick, 

The filters were very sharp.  There was of course an input limiter.  The output level of the filters was very low on random noise.  I believe there was a diode-or'd threshold detector on both tones with a time constant (measured in seconds).  It was all very effective.  The motor control would shut the motor off after 60 seconds of mark-hold.  This system was used on the 3620 (later 40 meter) autostart net.  We were using crystal controlled transmitters and receivers and it was just like texting today.  With Sel-Cal (done eletronically or by stunt-box) your motor would not turn on until a message was directed to you or via all-call.  When nobody was on the frequency, the machine just sat there doing nothing, with its motor off.   It was on 24 hours a day.

Gerry

    On Tuesday, April 20, 2021, 9:04:19 AM PDT, Nick England <navy.radio at gmail.com> wrote:  

 Ok if there is no audio, Mark hold is straightforward. But what about when you’ve got a enough noise so that both Mark and Space filters are getting pulses?
Was your experience that the TTY was quiet unless you had a real RTTY signal?
Here’s what mine looks like -https://youtu.be/fwMSE2hZNCk
Nick

On Tue, Apr 20, 2021 at 11:54 AM Gerry Block <gblock at sbcglobal.net> wrote:

Nick if I recall the ttl demodulators went into ‘Mark hold’ on the absence of both tones through a simple time constant.  I think Frederick did the same. Gerry

Sent from my iPhone

On Apr 20, 2021, at 8:40 AM, 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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-- 
Nick England K4NYW
www.navy-radio.com
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