[GreenKeys] RTTY receiver noise immunity?

[Richard Knoppow]

    I am far from an expert at this but it seems to me that the 
situation is analogous, actually the same, as an FM receiver. As 
long as the signal is above a threshold, called "quieting", there 
is no noise output. Audio receivers also have a "squelch" to mute 
the receiver when the input signal is below a set threshold. I 
don't know if this would be applicable to FSK for RTTY

On 4/20/2021 8:36 AM, Nick England 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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-- 
Richard Knoppow
1oldlens1 at ix.netcom.com
WB6KBL