[GreenKeys] RTTY receiver noise immunity?

[Nick England]

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