RTTY: The History and Technology of Radio Teletype Data Transmission

The evolution of RTTY: from noisy machines to quiet, modern data transmission

RTTY or Radio Teletype is one of the oldest data modes in radio technology, not only in amateur radio. Originally, RTTY was operated with mechanical teleprinters, i.e. electromechanical systems that are very similar to typewriters. The only difference is that in addition to the keyboard, there is also the possibility to send and receive data via a serial line and to print out the data. Such electromechanical teleprinters were a true marvel of precision engineering.

The writing speed was at the limit of what was possible at the time (early to mid 20th century) with reasonable effort: initially around 6 characters/second, later 13 characters/second. These mechanical teleprinters were used in business communication as the so-called "telex" system. After the Second World War, there was a surplus of many used teleprinters on the market. Unsurprisingly, radio amateurs quickly considered using these devices for radio data transmission as well.

The challenge of dual character levels in RTTY data transmission

As with wired telex systems, amateur radio teletype uses a very simple serial data code. It uses 5 bits per character. This results in a maximum of 32 possible codes (25 = 32). This is just enough to represent the alphabet and a few special characters, there is no more room for the numbers! This was improved by defining two "levels" of characters, i.e. using certain codes twice. Two codes were used for switching the levels, certain other codes had the same meaning on both levels (line feed, carriage return, space, etc.).

Thus, to transmit the simple string "VY 73", the following was sent:

<Switch to letter level> <V> <Y> <space> <Switch to digit level> <7> <3>

And woe betide you if a change of levels was ever lost during the transfer. Then the characters were printed in the last selected level and the text could only be converted into readable content with a little experience.

By the way: This procedure does not distinguish between upper and lower case letters, but only CAPITAL LETTERS.

The challenges of error correction in the RTTY Baudot code and its historical significance

The serial data code used in RTTY (called Baudot code) does not use any backup or error correction. This decision was made to keep the code as short as possible for each character, i.e. to increase speed. This is because each backup of the data transmission requires redundant, additional data bits, which reduce the transmission speed.

Operating without error correction was still viable for wireline transmission; wireline switching was similar to the telephone and was quite reliable. But radio transmission, especially on short wave, is quite different! Here, a short crack is enough to falsify a bit. An "A" then becomes an "S" or "Z". This makes reliable data transmission more difficult, and the operator's influence makes a big difference between a successful QSO and a NIL (Not In Log).

Of course, there are better, safer and faster coding methods available today. But you have to take into account when RTTY was created. At that time, around 1930, there was no other practicable and economical method. The coding on the transmission line is therefore also as simple as possible: a high tone for a logical one, a low tone for a logical zero (or current/no current on a wire line). This very simple, binary coding can be created easily with amateur means, both when transmitting and receiving.

Radios with integrated RTTY decoder:

If your Radio have not an integrated decoder you can get an external sound card and use it with specialized Software. Here some alternatives:

The technique of Frequency Shift Keying (FSK) in RTTY and its parameters

This very simple transmission method is called FSK - Frequency Shift Keying. You can still hear this type of data transmission quite often on shortwave, it is mainly used by commercial and military services. More accurately, the modulation should be called 2FSK, which describes a shift keying of the transmitted signal between two frequencies. This is perfectly suited for digital data - after all, 0 or 1 is transmitted. For historical reasons, these signal layers are also called 'mark' and 'space' in RTTY. In a superheterodyne receiver (i.e. in the 'SSB' position), these two signals produce tones that differ in pitch.

The technical parameters of a 2FSK transmission are determined by two parameters - the distance between the two tones and the scanning speed. In amateur radio, the distance ('shift') is fixed at 170 Hz, the scanning speed at 45 baud (exactly 45.45 Bd). These two values also have a historical background. In the commercial radio service, there were and are many other 'shifts' and scanning speeds. For example, the German Weather Service uses 450 Hz shift and 50 baud for its shortwave transmissions.

Low Tones 1275 and 1445 Hz
High Tones 2125 and 2295 Hz

The technique of Frequency Shift Keying (FSK) in RTTY and its parameters

Long before computerisation began, people used so-called "converters". This was an electronic circuit with two AF filters. These filters were fixed exactly to the two tones of the signal. So you had to set the SSB receiver exactly so that the received signal produced exactly the pitches to which the filters of the converter were tuned. This was unproblematic, but also quite inflexible. In order to agree on a uniform frequency specification, the pitches were standardised. There are two layers, the so-called "low tones" and the "high tones".

The pitch was chosen so that the low tones lie comfortably in the speech band and thus in the IF filter of the SSB receiver. This means that the same equipment as for voice transmission can continue to be used with the existing filters. However, some converters had problems with the first harmonic overtones of the low tones, which are also still in the passband of the filter. Therefore, a second standard was created, the High Tones. They lie at the upper edge of the LF range of speech, any overtones are cut off by the IF filter.

Today, these pitches don't really matter any more, because we can set the computer's decoder to any tone we want. However, we still encounter these tones in the settings of even very modern transceivers, because they also offer DSP-controlled AF filters that are optimised for these fixed pitches.

For decoding, the 5-bit Baudot data must be converted into the 8-bit ASCII (or UTF-8) data used by modern computers. This was also done by a converter in the past, the data was then output via a classic RS-232 serial interface. The output was generated by a computer.

RTTY transmission method: FSK vs. AFSK and their effects on data transmission.

To generate the 2FSK described above on the transmitting side, there are two methods. Either the transmitter is keyed directly, where the radio then has a special key input for this purpose, or a separate operating mode for RTTY is set on the transceiver. This method is usually referred to as FSK.

Or an audio signal is modulated in the LSB/USB position. In this case, an interface or the sound card of the computer generates two tones, which then produces a corresponding modulation with two tones. This procedure is called AFSK (Audio Frequency Shift Keying).

For the receiving side, it does not matter how the transmit signal is generated. If everything is set correctly, the receiver cannot tell whether the signal is generated by direct keying of the transmitting oscillator (FSK) or by modulation of the SSB signal (AFSK). However, each method has certain advantages and disadvantages:

Advantages Disadvantages
  • Goes with any SSB radio
  • Goes with any programme and modem
  • Overmodulation easily possible
  • Ambiguous frequency display
  • No overmodulation possible
  • The frequency display is 'correct'
  • Precise position of the tones and the shift
  • Special 'RTTY' mode on many radios with own filter settings
  • Requires radio capable of FSK
  • The programme or modem must offer direct keying
  • Possible timing issues when using virtual COM ports via USB

RTTY operation and equipment:

A detailed guide to transmitting and receiving RTTY data

Many modern SDR transceivers have an RTTY decoder built in, and memory can be used to send short, predetermined texts. So no additional equipment is needed here at all. But this is not very flexible, though with a little effort it gets much more convenient.

For RTTY operation, you need a modem, i.e. an interface. The modem takes over the modulation or FSK keying of the transceiver as well as the PTT control. The received signal (the AF) is digitised and offered to the computer via a virtual sound card emulated by the modem. Optionally, many interfaces also offer a CAT interface.

Matching cables from the interface to the transceiver. The cable connects:

  • TX audio (or FSK keying)
  • RX audio
  • PTT keying
  • CAT control (optional, not mandatory)
  • A PC with suitable software
  • A shortwave transceiver, in principle any model will do.

The interface is connected with the cables either to the microphone socket and an AF output (loudspeaker socket), or (better) to an existing connection for accessories. This connection is often called 'ACC', Accessory. ACC connections have the advantage of providing and expecting constant AF levels. They also have lower distortion because the receive signal is picked up before the radio's audio amplifier, the transmit signal is fed in independently of the microphone amplifier.

The software is set up to control the interface (which behaves like a sound card). Alternatively, an unused pin of a (virtual) serial interface is selected as the FSK keying signal. The PTT keying is done either via another unused pin of a (virtual) serial interface or (if connected) via a CAT command to the radio.

Now set it to "RTTY 45 baud" and see if anything is received. Most RTTY signals can be heard at 20 m around 14085 kHz. And if no radio amateur is active at the moment, you can use some commercial signals in Europe for testing. The German Weather Service (DWD) constantly broadcasts a CQ loop or weather reports on several frequencies on shortwave. Be careful, the decoder must then be set to 50 baud and a different shift (85 or 450 Hz).

The frequencies of the DWD are:
  • 147.3 kHz, call sign DDH47 (50 Bd, 85 Hz shift).
  • 4583.0 kHz, call sign DDK2 (50 Bd, 450 Hz shift)
  • 7646.0 kHz, Call sign DDH7 (50 Bd, 450 Hz Shift)
  • 10100.8 kHz, Call sign DDK9 (50 Bd, 450 Hz Shift)
  • 11039,0 kHz, Call sign DDH9 (50 Bd, 450 Hz Shift)
  • 14467.3 kHz, call sign DDH8 (50 Bd, 450 Hz shift)

The IARU has defined the position of the RTTY tones on the radio channel according to ITU recommendations: According to these, the mark tone should always be the higher tone in HF terms. In the HF position, it looks like this when operating with low tones in the lower sideband:

The 'Mark' tone is thus 1275 away from the (suppressed) carrier, 'Space' is 1445 Hz (1275 + 170 Hz) below the carrier.

Again, 'Mark' is the higher tone in RF terms, as given in the IARU recommendation.

The result is that the position of the two tones is always the same. This shows one of the problems of this old mode: The signal is easily received 'upside down', the screen then only shows 'bum'. This is due to the very simple coding of the signal: high tone = 1, low tone = 0. If instead of USB you accidentally set it to LSB, the high and low tones are reversed, 0 becomes 1 and vice versa - and then no programme can decode anything. Practically, most programmes offer functions to carry out this reversal in the software (normal/reverse switching).

If the reception works, you can carefully test the transmitting side. This is best done on a dummy load, with the lowest transmitting power. If AFSK is used, i.e. the modulation of the SSB signal with audio tones, the level of the audio signal must be adjusted very finely. Too high a level quickly leads to an overloaded signal, the HF signal becomes unnecessarily wide and no one can decode anything.

Next to phonics and telegraphy, RTTY is the most used mode at big contests on shortwave. Well-known contests with thousands of participants are the WPX RTTY, CQWW RTTY, the WAEDC RTTY and many others.

Well-known contest programs (loggers) such as N1MM also support RTTY, usually with the help of a plug-in such as MMTTY.

RTTY contests

RTTY QSOs: How to establish contacts outside contests and the band ranges for RTTY operation

Outside of contests, you can usually only find a QSO partner on weekends or in good conditions. Nevertheless, you should give it a try, only if we also call CQ sometimes, the activity on our bands increases.

RTTY is used on all shortwave bands, the respective ranges in Region 1are (each +- a few kHz, based on the IARU band plan):

  • 80 m: 3580 - 3590 kHz
  • 40 m: 7040 - 7047 kHz
  • 30 m: 10130 - 10150 kHz
  • 20 m: 14080 - 14089 kHz
  • 17 m: 18095 - 18105 kHz
  • 15 m: 21080 - 21090 kHz
  • 12 m: 24915 - 24925 kHz
  • 10 m: 28080 - 28120 kHz and 28150 - 28190 kHz

The procedure is the same as for any radio communication: You introduce yourself, give a signal report, describe your station. Most of the time that's it. Recurring texts can be conveniently assigned to function keys, so-called "macros". Ideally, you should also make an effort to "smash the keys"; this makes the QSO much more personal. And the low speed of the transmission nicely hides the fact that you are not a professional on the keyboard.

RTTY vs. Modern Digimodes: Why the challenge of RTTY is appreciated

One often hears the question why such an obsolete data transmission without any error correction is still used at all. There are plenty of other "digimodes" that offer perfect, error-free transmission, and which also require much lower transmitting power.

Right, that's true. PSK31 (for example) is error-free, works with the same equipment and software. Other digimodes are also extremely robust and often require only a few watts of transmitting power. Technical progress and the inventiveness of radio amateurs have produced enormous things here in recent years.

The difference is that RTTY challenges the operator precisely because of its weaknesses. By skilful operation of the equipment, an experienced RTTY operator can save a QSO. One recognises typical misspellings, knows how contest participants behave and can thus score more points. RTTY poses a challenge of its own, which can only be mastered with experience and a little dedication. This is in stark contrast to very modern modes, which work very reliably but have also become a bit more boring. Perhaps you can compare it to riding a motorbike or driving a car. Why freeze in the rain in the wind on two wheels when you can sit comfortably in a warm car? Because you can and because people love challenges. That's why RTTY.


Why should I use an interface between PC and radio?

To avoid ground loops. Ground loops occur because the computer and the PC each use their own power supply units and thus small potential differences can occur at ground level. A few millivolts are already enough to conjure up a distinct hum on the modulation. Complete galvanic isolation between the PC and the radio prevents such ground loops, and that is exactly what an interface like this does. An interface uses optocouplers and transformers to achieve galvanic isolation. This breaks up the unwanted ground connections and prevents ground loops.

What software is available for RTTY?

There are a lot of programmes. One of the best known is "FLDIGI" by W1HKJ. It can be used for normal QSOs as well as for contesting. A typical contest program is "N1MM Logger". Other well known programs are MixW, MMTTY, MultiPSK and many more.

Which is better for RTTY - AFSK or FSK keying?

FSK is preferable if the transceiver supports it. FSK is less problematic, does not allow overmodulation and the frequency display is correct. More details can be found above.