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LW1DSE > RTTY 24.11.07 20:56l 333 Lines 19840 Bytes #999 (0) @ WW
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RTTY BASICS
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RTTY was first introduced to me in 1958-9. The UK hadn't used this
mode before although there were plenty of commercial RTTY stations all over
the short wave bands. Not much attention was paid to them, as it wasn't looked
upon as an amateur form of communication.
One day Bill, G3CQE, received a phone call from "Doc" Gee, G2UK. He
asked if there would be any interest in looking at some old Creed 3X
teleprinters. There was some local interest, and four of us made our way to
Lowestoft to investigate. The outcome was that three machines made their way
to Norwich. Experiments on local-loops followed and eventually Bill, G3CQE
and Doc made the first contact. Talking to Jim Hepburn, VE7KX, on CW, Bill
found that there were several stations active on HF RTTY. There was an
attraction in the sound of the mode for me, I suppose being a musician helps
here, as the sound is quite musical. I well remember the confrontations we
received from AM users of the bands telling us to "Stick our jungle-bells".
However, SSB was treated with much the same disdain from the old stalwarts
who didn't want their bands changed! Notwithstanding this, the interest
mushroomed and the rest, as they say, is history! We managed to obtain a
couple of Creed 7B machines, and were off on HF. Interfacing to the then used
home-brew rigs was fairly straightforward, with a small variable across the
VFO adjusting the frequency shift. This, in those days, was 850Hz. In fact it
was not even Hz, it was still 850kcs ( in fact I still think of it in those
terms! ) Bill, G3CQE was the first UK amateur on HF RTTY and I wasn't far
behind. Interest grew rapidly and visits to junkyards followed on a regular
basis, looking for the much-valued Creed 7B's. Many machines came onto the
scene after this, and most amateurs would have given an extremity or two in
order to own a 28ASR. These were considered the ultimate machine. There were
few about, and I never did get to own one. I ended my "noisy" days with a
Model 19 Set, the full table! Contests were laid-back affairs, most RTTY-ers
were known by name, and we took time to exchange niceties. Now it is much more
clinical, even to the extent of debating whether to send a Carriage Return at
the end of an exchange or to leave it out and save time. Personally I think
we will lose something if we take this too far, but then I am a G3....
So much for the history. Let us now look at the scene as it is now:
RTTY is a fun and easy mode to operate, but there are a few things, which may
be puzzling to the newcomer. However, in modern technology, most of the
interfacing is done with software, and very little can go wrong. It might be
a good idea, however, to understand a little of the fundamentals.
MARK AND SPACE
A RTTY transmitter sends out a continuous carrier that shifts
frequency back and forth between two distinct frequencies. There isn't
amplitude modulation, only a pure carrier similar to CW with the addition of
a frequency shift. The lower RF frequency is known as the SPACE frequency and
the upper RF frequency is known as the MARK frequency. The difference between
the two is known as the SHIFT. For amateur radio, the SHIFT has been
standardized at 170 Hz. It is customary to refer to the MARK frequency as the
frequency you are operating on. For example, if you say you are transmitting
on 14080.00 kHz, that means your MARK frequency is 14080.00 kHz and your SPACE
frequency is 170 Hz lower, or 14079.83 kHz. While 170 Hz is the standard
shift, sometimes you will find stations using a shift of 200 Hz, but don't
worry about it. MMTTY will copy either shift automatically, and the other
station will copy your 170 Hz shift as well. It isn't that critical. There
is also a contest using lower shift only, quite easy to do with the software
tweak.
FSK and AFSK
You will often hear the terms FSK and AFSK when talking about RTTY.
FSK means Frequency Shift Keying and AFSK means Audio Frequency Shift Keying.
Here is an important point: Regardless of which method is used, the RF signal
sent out over the air is identical. MARK is always the higher RF frequency
and SPACE is always the lower RF frequency. The station receiving the RTTY
signal can't tell any difference at all. The difference is the way your
transmitter generates the RF signal.
With FSK, your transmitter receives a simple on-off signal, which
causes the carrier frequency to shift back and forth. If you use MMTTY, one
of the most commonly used software programs for RTTY, the on-off signal will
come from a COM port on your computer. Other stations that don't have a
soundcard program like MMTTY would use a separate box called a TNC (Terminal
Node Controller). The TNC does the same job that MMTTY does with your
soundcard. FSK is simpler, easier and more foolproof than AFSK and is highly
recommended if your transmitter supports FSK input. Check your owner's manual
if you aren't sure.
Since not all transmitters support FSK input, there is another method
available with MMTTY, and that is AFSK. AFSK can be used with any SSB
transmitter. AFSK is a bit trickier to set up and use, but when it is done
correctly, it works just as well as FSK and will transmit a perfect RTTY
signal. Also, AFSK can do some things that FSK can't, such as Automatic
Frequency Control (AFC) of the transmitter. To operate with AFSK, you put
your transmitter in the SSB mode instead of FSK mode, and you inject an audio
signal into the microphone input (some transceivers have a rear connector for
data input). When you transmit, MMTTY causes your sound card to put out a
pair of audio tones that cause your transmitter to send the required RF
output. The tones are two sine waves but the frequency and amplitude of the
tones is critical.
Let's say you want to transmit on 14080 kHz, as in the previous
example. Remember, your MARK signal has to be on 14080 kHz. With your
transmitter in the LSB mode (Lower Side Band), whatever frequency goes into
the microphone input will be subtracted from what your dial says and be
transmitted on that frequency. For example if your dial says 14080 kHz and
you put in a 1000 Hz audio tone, your transmitter will put out an RF signal
at 14079 kHz, exactly 1000 Hz lower than your dial. So in this case, if the
1000 Hz represented your MARK signal, you would have to set your transmitter
to 14081 on the dial, and your MARK signal would be transmitted on 14080,
just as you wanted. The SPACE frequency will be transmitted 170Hz lower, on
14079.83 kHz. The audio tone that will give you 14079.83 is 14081 minus
14079.83, or 1170 Hz. So the MARK audio frequency is 1000 Hz and SPACE is
1170 Hz.
There you have the basics of AFSK. MMTTY generates the two audio
frequencies and your transmitter converts them into two RF frequencies. For
technical reasons related to harmonic generation, audio frequencies of 1000
Hz and 1170 Hz are NOT recommended. They are used in this example just to
keep the math simple. The recommended audio frequencies are 2125 Hz for the
MARK audio frequency and 2295 Hz for the SPACE audio frequency. Making the
frequencies higher like this will reduce any second harmonics that might be
generated in your transmitter. At one stage lower tones were used and this
very problem caused a MAJOR problem on the air. You may have noticed the
SPACE audio frequency is higher than the MARK audio frequency - just the
opposite of the RF frequency you actually transmit. This happens because
you're using lower sideband. If you happen to forget and set your transmitter
to USB instead of LSB, two things will happen. Because your MARK and SPACE
are now reversed in your receiver, any RTTY signals you hear will not print
correctly. All you will see is random characters that make no sense at all.
The other thing is that YOUR transmissions will also be nonsense to the other
guy, so just remember - always use LSB. In the real world of course, sometimes
USB gets selected accidentally. This is why MMTTY has a button marked REV.
When you have a station tuned correctly but all you see is nonsense printing,
click on REV and your transceiver will be reversed. Now you can print the
other fellow and tell him he is "upside down", as it's commonly called. After
he reverses himself, just click REV again and you will both be back to normal.
Note: By default when using AFSK, REV reverses both your receiver and
transmitter. If you want REV to reverse only your receiver, go to
Option/Setup MMTTY, click the TX tab and click "Disable REV". When using FSK,
REV reverses only your receiver. If you want to reverse your transmitter and
receiver with FSK, your transceiver should have a way of doing that.
Also, you should know that in some parts of the world, especially
Europe, the standard is to use USB instead of LSB. This works fine as long as
you also reverse the MARK audio frequency and the SPACE audio frequency.
MMTTY defaults to LSB, and it is recommended to leave it there, even in
Europe, since your signal will be identical. If you prefer to use USB, leave
REV on all the time. This is no big deal on most modern transceivers, as the
memories can be set up in such a way that modes such as RTTY will come up
correct each time when selected via the memories.
The really critical part about AFSK is the amplitude of the signal
fed into the microphone connector (or rear panel connector), together with
the microphone gain setting. You must NOT overdrive your transmitter or
spurious signals will be transmitted. In general, keep the audio drive low
enough that your transmitter doesn't generate any ALC voltage. Never try to
drive your transmitter to maximum output. Around 80 to 90 percent of maximum
is about right. Consult your owner's manual for more information on how to do
this. If you ever hear a station at two or more frequencies at the same time,
the cause is almost always overdriving. None of this applies to FSK, of
course. With FSK, you can run full power and not worry about overdrive.
FIGURES SHIFT and LETTERS SHIFT
RTTY uses the Baudot code, invented before radio even existed, and
still widely used throughout the world. The Baudot code uses data bits to
represent letters, numbers and punctuation, much like your computer does.
Unlike your computer, which uses eight bits for each character, the Baudot
code uses only five, plus a start bit and stop bit. Using fewer bits is good
because it speeds up transmission and reduces the chance of errors, but there
is a complication. Five data bits can only represent 32 different characters.
Since there are 26 letters in the English alphabet plus ten numbers, plus
some punctuation, 32 different characters isn't enough, even if you only use
capital letters, which Baudot does.
Mr. Baudot could have chosen to use six data bits or even more, but
he found a better solution. He reasoned that most of what would be sent would
be letters rather than numbers or punctuation, so he assigned all the letters
to the basic 32. He then had six characters left over and he did a very
clever thing with two of them. He made one of them a FIGURES SHIFT and
another a LETTERS SHIFT. The way it works is this: When sending one of the
basic 32 characters, nothing special happens. But when a number or punctuation
is to be sent, a FIGURES SHIFT character is sent first (it's a non-printing
character - you won't see it on your screen).
Whatever follows will still be one of the basic 32 characters, but the
receiver will interpret it differently. For example the letter Q uses the
same five data bits as the number 1, but when the receiver gets a FIGURES
SHIFT first, it prints the next character as a 1, not a Q. This continues
until a LETTERS SHIFT character is received, at which time the receiver goes
back to "normal" printing. All of this shifting is done by the system - there
is no key marked LETTERS SHIFT or FIGURES SHIFT. It's all automatic and you
will scarcely notice it happening. In fact, the only reason to mention it at
all is because we are using radio instead of wires, and radio is susceptible
to interference from various sources such as lightning static, man-made noise,
etc.
If a burst of static should happen to wipe out a LETTERS SHIFT or
FIGURES SHIFT character, the characters following will not print correctly
until another LETTERS SHIFT or FIGURES SHIFT is received. For example,
suppose you are sending a signal report of 599, but a burst of static wipes
out the FIGURES SHIFT character. Instead of printing 599, the other fellow's
computer will print TOO. TOO is exactly the same as 599, without the FIGURES
SHIFT. We all got used to interpolating "shift" transmitted reports and
serial numbers in the early days!
Using MMTTY however, there is an easier way to read wrong-shifted
characters. With the right mouse button, just click on the word and it is
instantly changed to the opposite shift. Right-click again, and it's shifted
back. Easy as can be.
BANDWIDTH and FILTERS
When the bands are nearly empty, you can use practically any receiver
bandwidth with good success. Your SSB filters are probably between 2.1 and
3.0 kHz wide and as long as no other stations are nearby, copy will be fine.
For optimum performance however, less bandwidth is better, in fact MUCH
better. 170 Hz shift RTTY only needs about 250 Hz for proper copy. If you
don't have a 250 Hz filter, 500 Hz will do pretty well, but anything wider
than that will not be satisfactory in the long run. Further discussion on
this subject can be found elsewhere.
For amateurs, the ARRL handbook is a good source. Depending on your
transceiver, you may or may not be able to use a narrow filter for RTTY. Some
of the less expensive transceivers don't have an FSK mode, and also are
unable to select a narrow filter while in the LSB mode. Using an outboard
audio filter between the speaker output and the soundcard input can make some
improvement, but unfortunately, that will not prevent a strong adjacent signal
from causing the receiver's AGC circuit to reduce gain, often to the point
where the desired signal disappears. The best solution is to upgrade to a
transceiver that has an FSK mode built in, AND which allows you to select a
narrow filter while in that mode. Modern DSP filtering helps enormously here
and using both on-board and external DSP filtering can be very useful.
BAND PLANS
It's easy to remember the band plans for RTTY. Most activity will be
found between 80 and 100 kHz up from the bottom edge of the band, except for
80 meters, which goes an additional 40 or 50 kHz higher, and 160 meters. 160
meter RTTY activity is rare, but when found, it is usually between 1800 and
1820. Avoid the CW DX window between 1830 - 1840. At present, there isn't much
activity on the WARC bands, although 30 meters can be active at times.'
Here is where you will find most of the RTTY activity:
80 meters: 3580 - 3650 (3520 - 3525 in Japan)
40 meters: 7080 - 7100
in the US (see note below) 30 meters: 10110 to top of band 20
meters: 14080 - 14099 (avoid the beacons at 14100) 15 meters: 21080
- 21100 10 meters: 28080 - 28100
RTTY allocations for 40 meters vary greatly all over the world. In the US,
RTTY is permitted between 7000 and 7150, although most US activity is between
7080 and 7100. DX activity is often found between 7020 and 7040. As you see,
RTTY in the USA on 40 meters covers half as much again as we have in total
bandwidth for ALL modes! About time we had another 400kHz of allocation on
this band.
Check your BR68 for the UK allocations.
RTTY DX
Chasing DX on RTTY is highly popular with the RTTY crowd. As you
might guess, 20 meters is the premier DX band for RTTY, and most rare DX
stations and especially DXpeditions operate on 14080. Just like with CW or
phone, if the DX is calling CQ and getting no answers, you can feel safe in
calling him right on his frequency. If things are busy however, he will often
work split, which means you should call him on a different frequency, usually
2-10 kHz higher. He will say "up 2-10" or something similar at the end of his
transmission, and that's your clue. Your transceiver owner's manual will
explain how to do "split". Split operation in the early days wasn't possible
on any mode! Now it is common practice and even on RTTY quite easy to do.
RTTY CONTESTS
RTTY contesting is a passion with a lot of operators. There are about
a dozen major RTTY contests each year and when they are on, the bands will be
full! Even if you don't care to compete, it's a great way to pick up new
states or countries. Many of the rare DX stations are serious contest
operators. A list of RTTY contest times and rules can be found on the web at:
http://home.online.no/~janalme/RTTY.html or http://www.sk3bg.se/contest/
Contesters are in a hurry, of course, so please don't send your name,
QTH or anything except what is required by the contest. Plenty of time for
chatting after it's over.
If you are really serious about contesting then take a look at
WRITELOG. This program, coupled with MMTTY is one of the most commonly used
combo's on the bands today. There are numerous RTTY software programs, all
running from the sound card on the PC. If you look at the RTTY site of AA5RU,
you will find lots of help here, together with suggested downloads etc.
YOUR FIRST RTTY QSO
For your first time on RTTY, try the 20 meter band. 20 has the lion's
share of RTTY activity and you can usually find someone, day or night. Try
calling CQ between 14080 and 14087 kHz. A typical RTTY CQ would go like this:
CQ CQ CQ CQ DE G3LDI G3LDI G3LDI AR PSE K
Practically all RTTYers add the "PSE" at the end. Some will add their
name and QTH, some will add the time and date. You'll find a lot of variety
and it's all ok - just get on the air and try it out! Again, with all the
modern software, you can construct macro's (we used to call them "brag tapes"
due to the fact that we had to make a punched tape of something we wanted to
use over and over again!) to hold constantly used exchanges. If you're
familiar with CW procedures, you'll be fine with RTTY. RTTY'ers use most of
the Q-codes, as well as DE, K, KN, and all the rest. And if you accidentally
find yourself "upside down", don't get embarrassed - we've all done it!
RTTYers are some of the nicest people you'll ever meet, and things like
jamming and profanity are almost unheard of.
RTTY FOREVER
In spite of the newer digital modes like PSK, Pactor, G-Tor and
others, RTTY remains the favorite of contesters and DXers alike. RTTY doesn't
use error correction, handshaking, or synchronizing, all of which slow things
down. When quick back-and-forth exchanges are important, RTTY is the mode of
choice. Roundtable discussions and nets, which would be difficult or slow
with other modes, are a natural for RTTY, and RTTY is likely to be around
for a long time to come. I hope you enjoy it and look forward to talking to
you on the green keys! Oh yes, another leftover from 50 years ago. ALL
teleprinters had green keys. I suppose I should say - See you on the "Data
Communications Main Index"
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º Downloaded from Internet. Translatted to ASCII by LW1DSE Osvaldo º
º F. Zappacosta. Barrio Garay, Almirante Brown, Buenos Aires, Argentina. º
º Made with MSDOS 7.10's Text Editor (edit.com) in my AMD's 80486. º
º June 09, 2007 º
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