Regarding the 23 Hz RTTY debate, here is an interesting article by
Brian, K6STI about RTTY shifts. Brian is the author of the program
RiTTY by K6STI, recognized by many as the best of all the soundcard
programs for decoding RTTY under poor conditions.
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(Posted to the reflector with permission of the author.)
The Case for Wide-Shift RTTY
By Brian Beezley, K6STI
Amateur radioteletype began in the late 1940s with
surplus commercial and military equipment. Hams transmitted
teletype signals over radio using frequency-shift keying. In
FSK two tones, called mark and space, alternately represent the
digital 1s and 0s. The tones were transmitted as amplitude-
modulated audio at VHF or as two carrier frequencies at HF.
Typically the tones were 850 Hz apart, a standard tone spacing
for telephone equipment since the 1930s. Early RTTY equipment
used wide channel filters or FM discriminators to demodulate
FSK, so the exact frequency shift wasn't critical.
It soon became apparent that the simple demodulators of
the day were sensitive to HF-signal fading. Unless you used
high-gain limiter and decision circuits, the demodulated output
would stick in mark or space during a deep fade and cause text
to be missed or garbled. Even high-gain circuitry couldn't help
when the mark or space channel took a dive while the other
channel remained steady. This selective fading effect fooled
the slicer circuit that determined whether a pulse was a mark or
space. If the space signal faded below the residual mark-
channel noise, the slicer would mistakenly declare the pulse to
be a mark.
Narrow Shift
Plagued with selective fading, especially on the lower
frequencies, hams began to experiment with narrow frequency
shift in the 1950s. Eventually everyone standardized on 170-Hz
shift. Narrow shift reduces selective fading because the closer
in frequency you transmit two signals, the more correlated they
remain at the receiver. Selective fading occurs when a signal
takes multiple paths through the ionosphere. The multipath
signals sum at the receiver. If two signal components are close
in amplitude but differ by about 180 degrees in phase, the
composite-signal strength will drop dramatically. The phase of
multipath components depends on path length and signal
frequency. Mark and space signals tend to fade together more
often with narrow shift because the tone frequencies are more
alike. When selective fading occurs, a signal slicer will make
the correct decision more often with narrow shift than with
wide. This can noticeably improve print.
Automatic Threshold Correction
In addition to narrow frequency shift, experimenters
developed another technique to combat selective fading.
Advanced RTTY demodulators began to employ automatic threshold
correction. ATC monitors the signal envelopes in the mark and
space channels. When they become unequal, the slicer threshold
that distinguishes mark from space is altered. The demodulator
combines the mark and space signals into a composite signal by
subtracting the space-channel waveform from that in the mark
channel. Without ATC, the slicer declares mark when the
waveform is positive and space when negative. ATC modifies the
zero threshold. For example, if the space-channel envelope
momentarily fades by half, the ATC circuit adds half this amount
to the threshold, moving it into the mark region. This new
decision point provides the most reliable discrimination between
mark and space during the momentary fade.
A good ATC circuit is tricky to design. The threshold
must not be allowed to change so quickly that individual data
pulses affect it, nor so slowly that rapid fading can't be
accurately tracked. AM-to-PM conversion in a poor ATC circuit
easily can turn benign channel-amplitude differences into
serious data-sampling timing errors. Finally, simple ATC
circuits may set the threshold to half of the mark amplitude
during a pure-mark idle. This makes the demodulator 6 dB more
sensitive to false start pulses caused by noise. False starts
can cause spurious characters and loss of synchronization. (All
of these problems are easily overcome with software ATC where
noncausal and nonlinear functions can be readily implemented.)
Despite its limitations, the introduction of analog ATC in RTTY
demodulators advanced the fight against selective fading,
particularly when combined with narrow frequency shift.
Diversity Reception
With selective fading held at bay, hams found themselves
limited mainly by poor signal-to-noise ratio during frequency-
insensitive fades. When the mark and space channels fade
together, the composite signal may momentarily drop below the
noise. This can cause a few garbled characters. Worse, a fade
may cause the demodulator or teleprinter to lose sync.
Resynchronization sometimes may take a dozen characters or more
after the signal emerges from the noise. A quick, deep fade
thus can have side effects that last much longer.
Diversity reception came to the rescue. This technique
uses two antennas, two receivers, and a combiner circuit. The
antennas are arranged so that the signal fades independently in
each receiver. You can separate the antennas by some distance
to achieve space diversity. (Often this can be accomplished
with an antenna separation of less than one wavelength.) Or,
you can orient the antennas at right angles for polarization
diversity. When the incoming sky wave arrives crosspolarized to
one antenna and yields little signal, it will match polarization
with the other antenna and maximize its output. For both space
and polarization diversity, the combiner circuit selects the
receiver output with highest S/N. (Advanced diversity systems
can coherently combine the two channels to provide better S/N
than either channel alone. These systems essentially are
adaptive beamformers.) It's much less likely that a signal will
simultaneously fade in both channels of a well-designed
diversity receiving system than in either channel alone.
Although effective against ionospheric fading, the extra
hardware required for diversity reception limited its use
primarily to commercial circuits. (There's a chance that
amateur RTTY diversity reception may finally become popular due
to the recent availability of transceivers with two independent
receiver channels and the advent of software modems. It's easy
to process and combine a second RTTY channel in software,
particularly when both analog signals can be digitized by a
single stereo sound card. Even a simple random-wire receiving
antenna can fill in many signal fades that occur on a high-gain
directional antenna.)
Mark/Space Diversity
While tuning for RTTY signals outside the ham bands, I
often wondered why the vast majority I found used wide shift. I
might come across one or two signals at 170 Hz or 425 Hz, but
most used 850-Hz shift--some even wider. Watching FFT spectra
and demodulated waveforms of wide-shift signals, I often was
struck by how frequently selective fading was clearly evident.
My FSK demodulator[1] implements robust ATC so selective fading
seldom results in bad print, but I had no idea whether the
commercial systems used something similar. I kept wondering why
the commercials didn't run narrow shift like amateur RTTY
stations. Surely the system designers were aware of the
selective fading advantages discovered so long ago.
Then one day it dawned on me that the commercial RTTY
stations must be implementing a form of frequency diversity by
using a combination of wide shift with ATC. With wide shift the
mark and space channels are far enough apart that fading usually
occurs independently in each channel. When the mark signal
disappears, ATC automatically reverts to space-only copy by
shifting the threshold over to that side of the demodulated
waveform. (Without ATC, you've stepped back 50 years to fight
the selective fading battle all over again.) The key idea is
that ATC lets you use wide-shift selective fading to combat
narrow-shift frequency-insensitive fades. It was suddenly clear
that the combination of wide shift with ATC offered superior
immunity to fading of all types. It provides automatic, built-
in frequency diversity!
Absent fading, the bit-error rates for well-designed
narrow- and wide-shift FSK systems are essentially identical
because it's possible to optimally filter the mark and space
tones independently. Therefore, wide shift with ATC should
provide better results than narrow shift because the combination
decorrelates channel fading to achieve frequency-diversity
reception without incurring a signal-to-noise ratio penalty.
Mark/space diversity won't work as well as a two-
receiver, two-antenna diversity system. When you demodulate FSK
using just one tone, you may lose up to 6 dB in signal-to-noise
ratio (the same noise power competes with half as much signal
amplitude, or one-quarter as much signal power). Since an
adaptive-combiner, dual-receiver diversity system can provide as
much as 2-dB processing gain during a single-tone fade, it
should have up to 8 dB S/N advantage over simple mark/space
diversity. Moreover, it's much less likely that all four
channels in a full-diversity system will simultaneously fade
than it is for both channels to die in mark/space diversity.
But all you need for mark/space diversity is wide shift and good
ATC.
When and Where
I think a wholesale change by amateur RTTY stations back
to 850-Hz shift would be a disaster. The bands are just too
crowded these days. In addition, simple FSK demodulators may
suffer significant S/N degradation when set for wide shift (they
may respond to noise in the region between tones). But if
you're operating RTTY on an uncrowded band and encounter severe
fading, try switching to wide shift.[2] If your demodulator has
good filters and effective ATC, you're likely to get much better
copy.
[1] I use RITTY 1.0 DSP software that runs in my PC and uses a
Sound Blaster card for analog I/O. RITTY has a limiterless
front-end, optimal matched filters, ATC, numerical flywheel, FFT
tuning indicator, and other features especially designed for
weak-signal recovery.
[2] FCC rules permit a frequency shift of up to 1 kHz.
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