RTTY INTRO
by Roger Cooke - G3LDI

RTTY BASICS

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RTTY was first introduced to me in 1958-9. The UK had not 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 was not 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 did not 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 was not 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.

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 cannot 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 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, 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 cannot, 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 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 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 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 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 do not 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 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. 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 was not possible on any mode! Now it is common practice and even on RTTY quite easy to do.

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.

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.

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