[GreenKeys] on-off radio teletype (not FSK)

[Ralph Mowery]

Just like many things, there is usually a trade off.

The wider shift will give a better copy on selective fading, but it takes up
more band width.  You can use a narrow shift and a more selective filter to
help keep other signals from causing interference.

I got into rtty too late for the wide shift.  I did build up a st-6 and it
will copy on either mark or space.  I have watched even the 170 hz shift
fade from one tone to the other on some occasions.  At one time several of
us in the local area were copying pix off 20 meters.  I did not have a very
good receiver or antenna, but could usually copy better than those that had
the st-4 and other demodulators that needed both tones to copy rtty when the
band was not in good condition.




-----Original Message-----
From: greenkeys-bounces at mailman.qth.net
[mailto:greenkeys-bounces at mailman.qth.net] On Behalf Of Robert Downs via
GreenKeys
Sent: Saturday, February 17, 2018 12:19 AM
To: greenkeys at mailman.qth.net
Subject: Re: [GreenKeys] on-off radio teletype (not FSK)

I would say "less likely" rather than "unlikely" but selective fading was
why I disagreed with the statement in an earlier post that wide shift was
found to be unsatisfactory on HF but narrow shift worked just fine.  With a
converter capable of copying mark-only and space-only you have a frequency
diversity system that uses only one receiver and one antenna.  It would
probably perform better that a narrow shift system under frequency selective
fading conditions.

Robert Downs - Houston
WA5CAB


-----Original Message-----
From: greenkeys-bounces at mailman.qth.net
[mailto:greenkeys-bounces at mailman.qth.net] On Behalf Of Jim Haynes

Another reason for 850 Hz shift, realized later, is that the two tones can
be detected separately and the detector outputs combined, giving the effect
of frequency diversity.  It was unlikely that both frequencies would fade at
the same time.