RTTY accomplished in the old fashioned ways with TTY machine or computer
in terminal mode and a dedicated TU (terminal unit) are simplistic
compared to the wheedles and whistles of the many alternative modes.
Much of the hardware (and software) for the alternative modes neglects
some of the things we learned making RTTY work such as the low pass
filter after the discriminator and the slicer after that which made a
good TU (as the ST-6 and derivatives) perform superbly, copying signals
that weren't audible. Some of that has rubbed off on PSK-31, I'm not
sure about the other modes.
30 or 40 years ago, the few hams on RTTY were highly technical for the
most part. Very few BOUGHT TU or complete RTTY machine, most assembled
from parts (no kits) and many used our own custom circuits. Ham
operators of RTTY machines were machine assemblers and adjusters first
and talkers second which made for a quite interesting group.
I'll grant these days (and I sold my best model 15 while there was a
market and I didn't have to pay to have it hauled off) that the clanking
of the printer and its pickiness about adjustments isn't missed when I
compare running an Epson dot matrix printer on commercial weather wire
service for 6 or 8 years with nothing more than ribbons and paper, while
the best of the TTY printers required annual service and lubrication,
plus ribbons and paper.
But RTTY has a modicum of simplicity compared to the multitone whatsits
of the modern digital modes. And works a whole heap better than 300 baud
packet simply because the bit length is long enough that the receiving
detector can average propagation changes over most of the bit. And with
its relative simplicity all (or nearly all) users can understand its
techniques. The diversity of the sound card based digital modes can make
it difficult to KNOW what one is using and how it works, other than it
works or it doesn't work.
I'll claim that a good TU (such as the ST-6 using toroids for input
bandpass filter and discriminator) can copy RTTY solid through QRM that
the sound card wouldn't not even notice there was RTTY signal on the
frequency simply because that toroid based TU has better dynamic range
than the sound card and better INPUT selectivity.
73, Jerry, K0CQ
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Entire content copyright Dr. Gerald N. Johnson, electrical engineer.
Reproduction by permission only.
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