[GreenKeys] RTTY receiver noise immunity?

Tue Apr 20 12:13:24 EDT 2021

Last century (1971).  I monitored the 20 meter autostart frequency.  I 
can't recall which ST series of TU I had hooked up, but the 
autostart/markhold just kept the machine quiet and shut the motor off 
after so many minutes of dead air.  (My junk box has a DT500, an ST6 
built on perf board and I haven't identified the other perfboard TU, it 
might be an ST5. I can't recall which was built at the time, but I kind 
of think it was ST5 based)

Bottom line, I think the ST6 etc.  worked pretty well at keeping noise 
from rattling the selector magnets.

---
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The above comments or recommendations are SWAG. Use at your own risk.
John, W9DDD

On 4/20/2021 11:01 AM, Nick England 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 
> <mailto: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
>>     <mailto: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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> 
> -- 
> Nick England K4NYW
> www.navy-radio.com <http://www.navy-radio.com>
> 
> 
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