A SHORT TALK ON DIGITAL MODES
by Laurie VK3EBN


placeholder only The recent decision to remove Morse Code or CW (Carrier Wave) from amateur examination requirements has in no way reduced the presence of digital modes on the amateur bands. It should merely be viewed as a step in their evolution.

Whilst admittedly reducing the number of 'fuzzy modes' to HFFAX, SSTV & HELL, i.e. modes which are visually or audibly decoded rather than by the use of a computer, the overall number of modern digital (radio operating) modes has increased dramatically since they (computers) began appearing in the shacks of many amateurs.

Two types of digital mode are:

(a) Frequency Shift Keying (FSK) which shifts the signal between two states, both of which are known; either as a carrier frequency (F1B) signal or a modulating SSB signal with 2 shifting audio tones (AFSK).


(b) Phase Shift Keying (PSK) which changes the phase of a signal against a pre-determined reference, and usually transmits a complex audio waveform using SSB. This requires more critical tracking, and thus more frequency stability.

However, the use of PC sound cards for DSP (Digital Signal Processing) has encouraged the growth of both FSK & PSK modes by enabling programmers to use Fast Fourier Transform technology to
decode ASCII characters & produce waterfall displays, and Constant Phase Frequency Shift Keying (CPFSK) to send coded signals, as is the case in both THROB & MFSK16, for example.

Older FSK (Frequency Shift Keying) modes, such as RTTY (Radio Teletype), AMTOR (Amateur Teleprinting Over Radio), and PACKET are now giving way to newer FSK & PSK modes, like

  1. CLOVER – a PSK mode with full duplex simulation. Clover II is now the current release,requiring a different, DSP-based modem which offers better bandwidth efficiency & higher error-correction rates. Constant monitoring of the signal allows Clover to adapt to the current conditions, and based on this monitoring, the best modulation scheme.

  2. MT63 – a newer DSP-based mode for sending keyboard text over paths that experience fading & interference from other signals. Uses a complex scheme of encoding text in a matrix of 64 tones over time & frequency. This provides a 'cushion' of error correction for reception whilst still maintaining 100 WPM. The downside is the bandwidth, vis. 1 KHz, which makes it less desirable on crowded ham bands.

  3. THROB – yet another DSP-based mode that uses Fast Fourier Transform technology to decode a 5 tone signal in an attempt to succeed where other methods fail due to sensitivity or propagation difficulties, but at the same time, maintaining a reasonable speed.

  4. MFSK16 – an advancement to THROB using 16 tones, FFT & CPFSK to send the coded signal. Continuous Forward Error Correction (FEC) sends all data twice, with an interleaving technique to reduce errors from impulse noise & static crashes. Uses a new improved Varicode to increase the efficiency of sending extended ASCII characters, thus allowing the transfer of short data files between stations under fair to good conditions. The relatively wide bandwidth (316 Hz) for this mode allows faster baud rates & greater immunity to multipath phase shift. This mode is becoming popular for reliable keyboard operation, and is available in several popular programs.

  5. PACTOR & PACTOR II - are both robust & powerful PSK modes that operate well under varying conditions. Pactor II uses stronger logic, automatic frequency tracking, is DSP-based and up to 8 Times faster than Pactor. Both use the same protocol handshake, making them compatible.

  6. PSK31 – is the first new digital mode to gain popularity on the HF bands in many years. It combines the advantages of a simple variable length text code with a narrow bandwidth PSK signal using soundcard-based DSP techniques. It is designed for 'real time' keyboarding, and at a baud rate of 31, is only fast enough to keep up with the typical amateur typist. PSK31 has effectively replaced Baudot (RTTY) as the standard for 'live' keyboard communications. A second version having quad phase shifts (QPSK) is available that provides FEC, but at the cost of reduced S/N ratio.

  7. HELLSCHREIBER - an old FUZZY text mode using facsimile technology made new again by the recent use of PC sound cards as DSP units.

Amateurs working EME and weak signals such as aircraft enhancement, meteor scatter & digital DX have also been responsible for developing many of the newest digital modes, including JT43, JT65, FSK441, EME-1, 2 & 3, LHL-7, LTI & PUA43, the latter being in beta stage, but employing no less than 43 tones, each equalling an alphanumeric character, sent one at a time & falling into a spectral band of between 2 & 9 Hz in width. Other modes are more or less just high speed CW, and often they are developed for a specific purpose, such as line-of-sight, ground waves, meteor scatter, or simply for the thrill of working stations that you cannot hear!
They are constantly pushing the boundaries of both the frequencies used, the modes used to work them, and the bandwidth employed. If you are computer literate, and are looking for a new challenge in Amateur Radio, look no further. PC software like WSJT, & MixW will put you in the digital mode/weak signal picture.

Apart from the codes developed by amateurs for amateurs, there are numerous other codes in use commercially, amongst which are PACTOR III (not used on amateur bands due to its wide bandwidth), POCSAG, GOLAY, MFSK (Conquelet, Piccolo), ARQ, ACARS, TDM and a whole host of other modes employing computers to synchronise their signals.

Digital modes are no different to any other technology; they are constantly changing, and as some are finally laid to rest, others step up to take their place.
So the next time you hear a strange signal on the band you are working, chances are that it might be any one of a number of digital modes currently in use. If you want to hear more, then the Australian Amateur Band Plans section of the current (2006) WIA Callbook will tell you where to listen.

If you would like a copy of the software that was used to accompany this talk, then give me a call (or send me a blank CD-R) and I will get one to you.


Laurie Dare VK3EBN
11.7.2006