[GreenKeys] RTTY receiver noise immunity?

[Gerry Block]

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 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
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