RTTY BASICS

RTTY is a fun and easy mode to operate, but there are a few things
which may be puzzling to the newcomer.  Please take a few minutes to
read the following and you will probably save yourself some time and
frustration, as well as becoming more knowledgable about the basics of
RTTY.

MARK AND SPACE

A RTTY transmitter sends out a continuous carrier which shifts
frequency back and forth between two distinct frequencies.  There is
no 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 is not that critical.

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 not tell any
difference at all.  So what is the difference?  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.  Since you'll be
using MMTTY, the on-off signal will come from a COM port on your
computer.  Other stations which do not 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're not sure.

Since not all transmitters support FSK input, there is another method
available with MMTTY.  You guessed it - 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 FSK can
not, 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 put 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 which cause your transmitter to send the required RF output.
Sounds simple, right?  Here's the tricky part:  The tones are two sine
waves - nothing fancy - 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.
How do you get there with AFSK?  Here's how.  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.  Ok so far?
Now, what about SPACE?  Remember, you want your SPACE signal to be
transmitted 170 Hz lower, on 14079.83 kHz.  What audio tone will give
you 14079.83?  Simple - 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 which might be
generated in your transmitter.  

If you've been paying very close attention, 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.  See your owner's manual.

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.

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 does not 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 overdrive.  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 is not 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 the
FIGURES SHIFT character gets wiped out by a burst of static.  Instead
of printing 599, the other fellow's computer will print TOO.  TOO is
exactly the same as 599, without the FIGURES SHIFT.  So how can he
read what you intended to send?  It's easy if he knows the secret.
Here it is:  Look at the top row of letter keys on your keyboard -
QWERTYUIOP.  Now look just above each key and to the left.  Each of
those number keys is the same as the letter key below and to the
right, plus the FIGURES SHIFT.  In our example, TOO = 599.  Likewise,
the word PIPE, if the LETTERS SHIFT were missed, would print as 0803.
If 0803 lost its FIGURES SHIFT, it would print as PIPE.  

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.
  
You may wonder why, if the shift is 170 Hz, do you need a 250 Hz
filter?  Why not 170?  The reason is that shifting the frequency
generates sidebands adjacent to the actual signal and if the sidebands
are attenuated, the signal will be degraded.  RTTY is actually a form
of FM, and if you'd like to understand more FM theory, there are a
large number of books available.  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 do
not have an FSK mode, and also are unable to select a narrow filter
while in the LSB mode.  Some improvement can be made by using an
outboard audio filter between the speaker output and the soundcard
input, 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 which has an FSK mode built in, AND
which allows you to select a narrow filter while in that mode.  

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 is not 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.  

For US operators, remember that RTTY is not allowed in the phone
portions of the HF bands except on 160 meters, where it is legal
anywhere in the band.

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".

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 get serious about RTTY contesting, you will probably want to
get a program designed specifically for it.  In the meantime, MMTTY
will let you try it out and see how you like it.

YOUR FIRST RTTY QSO

Ok, you've learned a bunch of stuff and you're ready to get on the
air!  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 CQ CQ DE W7TI W7TI W7TI PSE K  
Practically all RTTYers add the "PSE" at the end.  Kind of a friendly
touch.  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!  

If you're familiar with CW procedures, you'll be right at home with
RTTY.  RTTYers use most of the Q-signals, 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
does not 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.  

So there you have most of what you need to know to become a proficient
RTTY operator.

Welcome to RTTY!

Bill, W7TI
