Digital Modes Overview


Radio Teletype or RTTY

What is RTTY?

RTTY (short for RadioTeleTYpe) is the first digital mode to be used by radio amateurs.

Teleprinter machines were first used for landline communication in the mid nineteenth century. Before that time, all telegraph communications had used Morse Code. This required a qualified Morse operator at each end of the circuit, and the data rate was slow because the speed was limited by the ability of the operators to copy Morse signals by ear.

The original teleprinters included a keyboard that could generate a series of electrical impulses corresponding to whichever key was typed. For printing, teletype machines used a motor-driven mechanism that controlled a printing head or a set of typehammers like those used in ordinary typewriters. These machines were capable of sending data at a much faster rate than was possible using Morse, and messages could be printed out automatically at the receiving end. (click picture above for a larger view)

Baudot, Murray and ASCII Codes

From the late nineteenth century, the standard coding system for teleprinters was the 5-unit Baudot code (named after its inventor, Emile Baudot). When a key was pressed, the teletype machine would generate a series of pulses by switching a DC voltage on or off. The "on" state was referred to as the "mark" state, and the "off" condition was called the "space" state. The first pulse in the Baudot code was a "start" bit, then five data bits, followed by a "stop" bit which marked the end of each character. When these pulses reached the receiving machine, they would determine which character was printed.

The 5-unit Baudot code allowed the transmission of 32 different characters. This was not enough to allow each character - letter, numeral, or punctuation sign - to have its own unique code. The way around this problem was a "shift" key that allowed the machine to print either letters or figures. The 5-bit code was later modified by Donald Murray, and the resulting "Murray Code" became the standard code used in commercial teletype links. This code was later superseded by the ASCII code, which remains standard today.

Teletype on the Air

To send teletype signals on the air, the transmitter generates a continuous carrier that is shifted slightly between two different frequencies that correspond to the mark or space states. This technique is known as "frequency shift keying" or FSK. Nowadays FSK transmission is normally achieved by feeding an audio tone generator (or, more usually, a computer sound card) into an SSB or FM transmitter. This technique is known as AFSK (audio frequency shift keying).

On the receiving end, RTTY stations originally used decoders known as "terminal units". The terminal unit would decode the incoming audio from the receiver and convert the two-tone signal to a series of pulses that were then sent to the teleprinter. Nowadays the decoding can be done by a computer sound card, so there is no need for a separate terminal unit.

Amateur RTTY operation originally depended on the availability of surplus ex-commercial or military teletype machines. The two most popular teleprinters used by Australian amateurs were the Model 15 Teletype (made in USA), and the Creed teleprinter, which was made in England. But RTTY was not a mode for the faint-hearted, because these machines were very bulky and noisy, and they needed a constant supply of paper.

Back in the 1970s there was a major revolution in RTTY operation, as amateurs replaced the old teletype machines with computer keyboards and monitors, connected to small terminal units that generated and decoded the FSK signals. These techniques brought RTTY within the reach of all amateurs. Then in the 1980s, RTTY received a further boost from the development of home computers and software that could generate and decode FSK signals using the computer's sound card.

Newer Digital Modes

As more amateurs explored the uses of computers, this led inevitably to the development of new digital modes, all of them derived from RTTY but offering faster speeds and more up-to-date features such as error correction. One of the earliest of these new modes was AMTOR (AMateur Teleprinting Over Radio).

The most popular new mode was Packet Radio, which was adapted by amateurs from a commercial computer networking protocol. Packet allowed faster data rates than RTTY, while using a narrower bandwidth (500 Hz for a 300 baud packet signal, as opposed to 1.2 kHz for a 50 baud RTTY signal). Packet became extremely popular and quickly became the most widely used amateur digital mode.

The 1990s saw the development of an ever-increasing number of new modes coming into use. These included PSK (phase shift keying), and amateur implementations of commercially used modes such as MFSK (multi-frequency shift keying). Software is now available for a bewildering variety of modes, including many developed specifically for amateur use. These include QRSS, VFSK CW, MT63, and WSJT, to name just a few.

The computer revolution also saw increased activity using image modes such as fax and SSTV, and an all-electronic implementation of Hellschreiber, which was another mode originally generated and decoded by machines.

Increasing use of the Internet has led to a decline in packet radio activity. But the ready availability of computer sound card software has caused an upsurge of interest in digital modes, and a revival of interest in older modes such as CW and RTTY. Probably the most popular of the newer modes is PSK31, which allows stations to chat at normal typing speed.

The best way to explore this variety of digital modes is to download some of the many free programs that are available on the Internet. There are a number of programs that can generate and decode a variety of modes including CW, RTTY, PSK and MFSK, so there is no need to run a different program for each mode. Popular multimode software includes:
  MultiPSK by F6CTE
  MixW by UT2UZ
  Stream by IZ8BLY

RTTY Activity Today

RTTY signals can be found on all HF bands - check the band plans and tune through the "Digital Modes" segments. A number of different modes are used in these band segments, but these modes can be distinguished by ear:
 RTTY signals are characterised by a regular shifting back and forth between two tone frequencies.
 Packet signals have a buzzing sound.
 AMTOR sounds like a rapid series of chirps.
 MFSK is a whistle of constantly changing pitch.
 PSK sounds like a continuous warbling with no change of pitch.

You are most likely to find RTTY activity on or around the following frequencies:

  40 metres: 7.030 - 7.040 MHz (7.080 for American stations)
  30 metres: 10.140 MHz
  20 metres: 14.080 - 14.090 MHz
  15 metres: 21.080 - 21.100 MHz
  10 metres: 28.080 - 28.100 MHz

Since the arrival of packet, there has been less RTTY activity on the VHF bands. RTTY signals are most likely to be found on 2 metres, using AFSK with FM transceivers. There are two national RTTY frequencies in the band plan - the traditional one on 146.600 MHz, and the newer national channel on 145.600 MHz.

There is a weekly RTTY broadcast from ANARTS (The Australian National Amateur Radio Teleprinter Society) on 7042 kHz at 0030 UTC on Sunday mornings.

The most common data rate for amateur RTTY is still the traditional 45.45 bauds. For many years the standard frequency shift was 850 Hz, using tone frequencies of 2125 and 2975 Hz or the alternative "low tones" 1275 / 2125 Hz. Nowadays the most commonly used frequency shift is 170 Hz, using tone frequencies of 1275 / 1445 or 2125 / 2295 Hz.

Further Information

 The ANARTS web site - click this Link
 Software downloads: the Amateur Radio Soundblaster Software Collection, by DL9QJ - click this Link. This site also includes a large amount of software for RTTY, CW, PSK, MFSK, packet etc.
 Digital Amateur Radio Software: another useful software download site - Link
 Stream and other multimode software by IZ8BLY - click this Link
 The World of Fuzzy Modes by ZL1BPU (mainly MFSK, MT63 and Hellschreiber) - click this Link.


Page Last Updated: Thursday 22 May 2008 at 15:8 hours