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on its first century and it will probably be around for quite
a few more years.
Frequency-Shift Keying (FSK)
The two tones used by RTTY are a type of modulation
known as frequency-shift keying or FSK ( en.wikipedia.org/
wiki/Frequency-shift_keying). In FSK, shifts in frequency
are the signaling event and the period during which a tone
is transmitted is the symbol. There are several common
methods of modulating a carrier to create FSK:
1. Shifting the frequency of an RF oscillator back and forth.
2. Modulating an SSB transmitter with tones instead of speech.
3. Modulating an FM transmitter with tones instead of speech.
On the air, methods 1 and 2 sound exactly the same.
An unmodulated carrier and a sideband created by a single
tone are identical signals. Method 3 creates a standard
many-sidebands FM signal, but the audio output from the
receiver sounds the same as the audio from methods 1 and
Method 1 is called direct FSK because the RF carrier
frequency is controlled directly by the digital data signal
input to the transmitter. Methods 2 and 3 are called audio
frequency-shift keying or AFSK because the digital signal is
used to create an audio signal (the tones) that modulates
the transmitter. The end result of all three is a set of tones
that represent the 1/0 of a digital signal.
RTTY uses a pair of tones (mark and space, mentioned
earlier), so it is binary FSK. The difference between the two
tones is called the shift. Most ham radio RTTY on the HF
bands (shortwave frequencies below 30 MHz) has a 170
Hz shift. On the VHF and UHF bands, hams use AFSK
over an FM link with a 1,000 Hz shift between the tones.
You can hear a sample of what FSK/AFSK signals sound
like online; see the sidebar, “Tuning In to Digital Modes.”
Figure 1 shows a typical AFSK RTTY signal on the HF
bands using lower sideband by convention. Note that the
spectrum has to “spread out” due to the extra sidebands
created by the rapid switching between tones.
Two-tone binary FSK is the simplest form, but more
tones can be used now that we have more processing
power to throw at the problem. By using more tones,
the encoding can be more sophisticated to overcome
noise, distortion, and interference from other signals.
Amateurs use multi-tone FSK or MFSK with the number of
tones ranging from four to more than a dozen. The most
common on the HF bands are Olivia, ALE, MFSK8 and
16, and JT9 and 65. On the VHF and higher frequencies,
MFSK441 is used to communicate via ionized meteor trails,
and JT65 is used to bounce signals off the Moon!
The Radio Channel
It’s worth mentioning at this point why having all these
modes is useful. The radio channel — the physical over-the-air portion of the signal’s journey — is quite a bit different
than one composed of wires. Fading, noise, reflections, and
a multitude of other things affect the radio signal in ways
that confuse the demodulators and decoders. Different
frequency ranges have different characteristics as well,
PRACTICAL TECHNOLOGY FROM THE HAM WORLD
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Have you ever wondered where the term “break” came
from? It is one of the oldest communications terms still in use
(along with packet and checksum), with its roots in the early
1840’s landline telegraph system.
Those early systems were constructed “party-line style”
with the wires connecting one station to another through a
sounder. Having all stations “in series” meant that they all
heard all of the traffic on the line. The wire also ran through a
telegraph key, and closing the key allowed the current to flow
through wire. If a station wasn’t transmitting, it was expected
to close the key — that’s what the shorting bar is for on a
So, what if a station needed to jump into the ongoing
traffic with an important message, like the bank was being
robbed? The operator would remove or open the shorting
bar to break the current path, and all stations would hear the
line suddenly go dead. The sending operator would wait long
enough that every other operator was paying attention, then
send the message. This is “breaking in” and on voice circuits,
the operator would wait for a pause, then say “break!”
Now you know “who’s the breaker?”
n FIGURE 1. A typical ham radio RTTY signal spectrum
for an AFSK signal. The transmitter is set to LSB and
audio tones with fMARK and fSPACE are input in place of
microphone speech. A receiver set to LSB and tuned
to the same carrier frequency would recover the audio
tones with the same mark and space frequencies.