Digital modes

Digimodes — from RTTY and PSK31 to FT8 and FreeDV: worldwide radio contacts using low power and simple antennas, thanks to sound cards and free software.

Radio modes such as PSK31, Olivia, JT65 and FT8 enable global contacts even with small antennas and low transmit power. The long list of digital transmission modes — the ‘digimodes’ — reflects the history of technology: from mechanical teletype machines through the first computer-based methods to modern algorithms capable of decoding signals far below the noise floor. The original methods were used solely for transmitting text; modern methods allow the transmission of any digital sequence — hence the collective term ‘digimodes’.

Note: A comprehensive overview of the digimodes found on shortwave (including military and commercial ones), complete with waterfall screenshots and audio samples, is available on the website sigidwiki.com — HF Signals.

The IARU Region 1 band plan specifies segments for digimodes with bandwidths of up to 500 Hz, 2,700 Hz and 6,000 Hz respectively — the frequency should be selected to match the transmission bandwidth. Some of these segments are marked ‘automatic digital stations’ and are reserved for gateways and mailboxes; individual QSOs should not be initiated there. Separate activity centres are provided for image transmission — see Image Transmission. Digital operation is not permitted in the bands marked as CW.

How do you go QRV?

For digital modes, a simple SSB transceiver is usually sufficient — only a few modes require particularly high frequency stability. Modulation and demodulation are handled by sound card software such as FLdigi, DM780 or MixW. This is why these digital modes are also referred to as sound card modes.

It is worth using a soundcard interface between the soundcard and the transceiver: it adjusts the levels, ideally provides galvanic isolation between the two devices (preventing ground loops) and enables the transmitter to be keyed up via a PC signal or a built-in AF-Vox.

Caution: Check the transceiver’s manual to see what transmit power is permitted for digital modes (‘continuous wave’). If in doubt, reduce the power to 50 per cent of peak power or less — otherwise the power amplifier may be damaged.

Recommended reading:

FT8, FT4, JT65 and JT9

This family is based on WSJT, a computer programme by Nobel Prize-winning physicist Joe Taylor (K1JT). WSJT enables QSOs even when the signal is many decibels below the noise floor in a 2,500 Hz-wide SSB channel — by standardising the data and reducing the transmission rate. Originally intended for troposcatter and Earth-Moon-Earth links, the method was soon adopted for QRP QSOs on shortwave.

  • FT8 — introduced by Joe Taylor (K1JT) and Steve Franke (K9AN) in June 2017. Transmission duration just 15 seconds, bandwidth 47 Hz. Slightly less sensitive than JT9 / JT65, but four times faster. Now the dominant digital mode on shortwave — see the dedicated page on FT8.
  • FT4 — introduced in 2019 by Joe Taylor (K1JT) and Steve Franke (K9AN) as a faster variant specifically for contest operation (part of WSJT-X). Pass length just 7.5 seconds, bandwidth around 90 Hz. Slightly less sensitive than FT8, but twice as fast — optimised for high QSO rates.
  • JT65 — 65 tones, 175 Hz bandwidth. Very sensitive (over 10 dB below CW), developed for VHF/UHF with an almost constant signal strength; successfully used on shortwave by QRP stations since 2006.
  • JT9 — 9-FSK, bandwidth of less than 16 Hz at 1-minute intervals. Originally intended for long-wave and medium-wave; also available in 2-, 5-, 10- and 30-minute versions for even weaker signals.

VARA and VarAC

VARA is a powerful software modem developed by José Alberto Nieto Ros (EA5HVK), which transforms the sound card into a fast OFDM modem. There are two variants: VARA HF for shortwave and VARA FM for high-speed data transmission via FM repeaters on VHF/UHF. VARA continuously adjusts the data rate and robustness to propagation conditions and achieves several thousand bits per second on shortwave, depending on the bandwidth.

Its most important application is the global Winlink network for email transmission without the internet — VARA HF and VARA FM have largely replaced the older PACTOR and WINMOR modems there. VARA HF also forms the basis for VarAC, a free application for keyboard-to-keyboard connections (live chat, beacons, automatic CQ calls), which has gained considerable popularity in recent years.

The VARA software is shareware: the basic version runs free of charge at reduced speed; a licence is required for the full data rate. VarAC itself is free.

PSK family

PSK31 has become so popular that it has overtaken RTTY as the leading mode. Particularly on the 20-metre band, stations from all over the world can be received almost round the clock above 14,070 kHz. Compared to CW, PSK31 requires less bandwidth; the software’s narrow DSP filter can still decode signals when practically nothing else can be heard. On the waterfall display, several dozen stations often appear side by side within the SSB filter — a single mouse click selects the target station.

Newer variants, PSK63 (63 baud) and PSK125 (125 baud), offer double or quadruple the transmission rate, but require double or quadruple the bandwidth and consequently have a signal-to-noise ratio that is halved or quartered.

Advantages: very narrowband; popular worldwide (QSO partners always available); no TCXO required; worldwide QRP QSOs possible with 5 W. Disadvantages: predominantly macro QSOs (‘F1 to F12’), hardly any ragchew; no error correction; susceptible to selective fading and multipath reception due to phase modulation — only of limited suitability on 160 m / 80 m and on VHF/UHF.

SIM31 (also SIM-BPSK, by ON4NB et al.) is a newer hybrid of PSK31 and JT9 with a bandwidth of just 45 Hz (USB). More robust against interference and fading than PSK31 — well suited to QRP/DX. MFSK16 (Murray, ZL1BPU) uses 16 tones spaced 15.625 Hz apart (total bandwidth 316 Hz) and operates with an effective FEC at a net data rate of 42 WPM. Similar in sensitivity to PSK31, but significantly less susceptible to Doppler effects.

Olivia

Olivia was developed in late 2003 by Pawel SP9VRC — a combination of MFSK and an FEC code based on Walsh functions. Olivia can still decode signals up to 15 dB below the white noise floor. Available bandwidths: 125, 250, 500, 1,000 and 2,000 Hz; the most common combinations are 4/125, 8/250, 8/500, 16/500 and 32/1,000 (tones/bandwidth).

Tones / BandwidthBaudWPMS/N threshold
OLIVIA 32/1,00031.2524.4−12 dB
OLIVIA 16/50031.2519.5−13 dB
OLIVIA 8/50062.529.3−11 dB
OLIVIA 8/25031.2514.6−14 dB
OLIVIA 4/12531.259.8−15 dB
The most common Olivia formats.

Advantages: highest tolerance to Doppler shift, fading, aurora and multipath propagation; SSB filters are sufficient; well-suited to Ragchew QSOs (8/500, 16/500, 32/1000); no TCXO required (frequency deviation > 100 Hz is still corrected). Disadvantages: moderately widespread; higher bandwidths than PSK31.

Classic and older methods

In addition to the modes discussed above, other digital modes can occasionally be observed on the bands:

  • RTTY — the classic teletype mode. Characters are encoded using 5 bits, transmitted via FSK with a 170 Hz or 850 Hz shift at 45.45 baud. When transmitted as AFSK via an SSB transmitter, this is known as the AFSK method.
  • AMTOR (Peter, G3PLX, 1981) — radio teletype with high transmission reliability thanks to ARQ (Automatic Repeat reQuest) or FEC. Characteristic ‘chirp-chirp’ sound caused by rapid switching between transmit and receive modes. Now largely replaced by PACTOR.
  • PACTOR (DL6MAA, DF4KV, from the 1990s) — developed specifically for data transmission on shortwave. PACTOR-4 achieves speeds of up to 10,500 bits/s with compression. Widely used as a transmission protocol in WinLink for sending e-mail via shortwave. Requires a special controller (TNC).
  • WINMOR (ARRL/TAPR DCC 2008) — an ARQ transmission protocol for WinLink that does not require an expensive TNC — a sound card and an SSB transceiver are sufficient. Up to 1,300 bits/s at a bandwidth of 500/1,600 Hz. Software: free RMS Express.
  • MT63 (Pawel SP9VRC) — 64 tones spaced 15.625 Hz apart, DBPSK-modulated, highly robust against QRM and fading. Bandwidth 500 Hz to 2,000 Hz. Transmission is displayed with a delay of up to 6 seconds — the mode still operates without errors even if 25 per cent of the information is missing.
  • Packet Radio — AX.25 protocol, derived from the commercial X.25. Commonly used on VHF/UHF at 1,200 or 9,600 baud, on LW at only 300 baud — now largely superseded by other modes on shortwave.
  • DominoEX (Murray ZL1BPU) — MFSK-based, primarily for shortwave on the low-frequency bands (NVIS — Near Vertical Incidence Skywave). Can cope with propagation delays of up to 80 ms and a drift of 200 Hz/min. The standard is DominoEX11 (262 Hz, approx. 70 WPM).
  • FSQ (ZL1BPU, ZL2AFP, 2015) — “Fast Simple QSO”, 33 tones spaced 9 Hz apart, 300 Hz bandwidth. Includes the FSQCALL protocol for chatting with individual or multiple stations, file reception and status enquiries. Sensitivity approx. −13 to −16 dB SNR.
  • Contestia (Nick UT2UZ, 2005) — derived directly from Olivia, roughly twice as fast but 3 dB less sensitive. Sounds almost identical to Olivia.
  • CLOVER (HAL Communication, 1993) — 8-bit data transmission with adaptive modulation; measures transmission conditions and automatically adjusts the modulation format and data rate.
  • ROS (José Alberto Nieto Ros) — a CDMA scheme, theoretically decodable down to −35 dB SNR, but requiring high bandwidth. In the USA, ROS is classified as spread spectrum by the ARRL/FCC and is not permitted on shortwave or 2 m bands.
  • THOR — related to DominoEX, 18 tone frequencies in a constant phase relationship, very robust for weak HF links. Speeds from 14 to 78 WPM with a bandwidth of 173 to 524 Hz.
  • THROB (Lionel G3PPT) — MFSK with 9 tones, 72 or 144 Hz bandwidth, 10–40 WPM. ‘Throbharmonika’ sound. Robust against noise and impulse interference, but slow.
  • V4Chat — Keyboard client based on the WINMOR modem; up to 55 WPM at a bandwidth of just 200 Hz. Supports ASCII and UTF-8, as well as FEC and ARQ modes. Successor H4 (Hamming Encoded 4FSK) in beta.

WSPR — Weak Signal Propagation Reporter

WSPR (pronounced like ‘whisper’) is another programme by Joe Taylor (K1JT). The transmitter transmits very narrowband signals which, thanks to a highly specialised coding scheme, can be detected and decoded well ‘below the threshold’ — down to −28 dB below noise. Primarily intended for beacon operation.

It transmits the call sign, grid locator and transmit power in dBm. Receiving stations calculate an SNR value and transmit the data to wsprnet.org, where it is made available worldwide for propagation studies and antenna comparisons. Transmit power is typically 5 W or less. This means that even stations with limited antennas can receive worldwide reception reports from exotic countries.

Hellschreiber

Hellschreiber was the first successful direct-print transmission system, comparable to a fax. Letters are transmitted as small images within a coarse 7×7 grid — whilst interference may degrade the image, it rarely renders the character completely illegible. This redundancy is the key advantage over the classic teletype (RTTY).

  • Feldhell (1935) — 122.5 baud, 360 Hz bandwidth, quasi-synchronous. Used for mobile field operations during the Second World War.
  • Siemens GL (1955) — start-stop system, 300 baud, 1,000 Hz tone frequency, 600 Hz bandwidth. Primarily for interference-free VHF FM links.
  • HELL 80 (1965) — frequency-shift keying at 1,625 / 1,925 Hz, 315 baud, 900 Hz bandwidth. Reliable transmission even over heavily jammed radio links.
  • CP-16 (CRAC, 2011) — Chinese characters in a 16×16 dot matrix with a bandwidth of just 400 Hz. 2–5 characters per second.

Today, a range of free soundcard programmes is available for morse code transmission. Preferred frequencies (IARU Region 1) are 3,584, 7,085, 10,144, 14,063, 18,104, 21,063, 24,924 and 28,063 kHz (centre frequencies in each case). The Aurora beacon DK0WCY also transmits on 10,144 kHz.

FreeDV — digital voice

FreeDV is an open-source GUI application for Windows, Linux and macOS that transmits digital voice (‘DV’ — Digital Voice) at a low bit rate on an SSB channel using any SSB radio transceiver. The microphone signal is compressed to 1,400 bit/s (Codec 2 by David Rowe, VK5DGR) and modulated onto a 1,100 Hz-wide QPSK signal. Unlike D-STAR, FreeDV is completely open source.

Meeting frequencies

Within the IARU Region 1 digimode band segments, the following meeting frequencies have become established as the preferred operating frequencies. For digimodes, the ‘dial frequency’ (USB carrier) is always specified — in many modes, the tones actually transmitted lie 1,400–1,600 Hz above this.

CW (reserved — digimodes not desired): 1,810–1,838, 3,500–3,570, 7,000–7,040, 10,100–10,130, 14,000–14,070, 18,068–18,095, 21,000–21,070, 24,890–24,915 and 28,000–28,070 kHz.

PSK31 (USB) above 1,838, 3,580, 7,040, 10,140, 14,070, 18,100, 21,070, 24,920 and 28,120 kHz. The lowest ~1 kHz are preferred for QRP; PSK63 / PSK125 from 2 kHz above.

SIM31 (USB): 1,839, 3,596, 7,045, 10,142, 14,067 (not in accordance with the band plan), 18,098, 21,100, 24,916, 28,127 kHz.

AMTOR, MFSK16, MT63, OLIVIA, PACTOR, RTTY, THROB etc. (USB): 1,838–1,842, 3,583–3,600, 7,043–7,050, 10,143–10,150, 14,080–14,099, 18,103–18,109, 21,080–21,120, 24,923–24,929 and 28,080–28,150 kHz. Olivia 32/1000 is mainly found at 14,107.5 kHz (centre frequency).

Hellschreiber method (USB, centre frequency in each case): 3,584, 7,085, 10,144, 14,063, 18,104, 21,063, 24,924 and 28,063 kHz. On the 20 m band, 14,063 kHz (up to 14,069) is the preferred calling frequency of the Feld Hell Club.

FAX/SSTV: 3,735 (LSB; DRM-SSTV 3,733), 7,165 (LSB; DRM-SSTV 7,058), 14,230 (USB; DRM-SSTV 14,233), 21,340 (USB; DRM-SSTV 21,233), 28,680 kHz (USB).

FreeDV (digital voice): 7,190 and 14,236 kHz (USB).

JT65A (Dial Frequencies, USB): 0.13613, 0.4742, 1,838.0, 3,576.0, 5,357.0, 7,076, 10,138.0, 14,076.0, 18,102.0, 21,076, 24,917 and 28,076 kHz.

JT9 (Dial Frequencies, USB): 0.4742, 1,840.0, 3,578.0, 5,359.0, 7,078.0, 10,140.0, 14,078.0, 18,104.0, 21,078, 24,919 and 28,078 kHz.

FT8 (Dial Frequencies, USB): 1,840.0, 3,573.0, 7,074, 10,136.0, 14,074.0, 18,100.0, 21,074, 24,915 and 28,074 kHz — see the FT8 page.

WSPR (Dial Frequencies, programmed; transmit frequencies 1,400–1,600 Hz above): 0.136, 0.4742, 1.8366, 3.5926, 5.3647, 7.0386, 10.1387, 14.0956, 18.1046, 21.0946, 24.9246, 28.1246 MHz, as well as 50.293, 70.091 and 144.489 MHz.

FSQ: 3,588, 5,355, 7,044 and 10,144 kHz (USB). For experiments on higher bands, 14,074, 18,104, 21,074, 24,924 and 28,124 kHz are recommended.

Note on frequency specifications

For digital modes, the ‘dial frequencies’ are always specified — that is, the suppressed carrier frequency in USB mode. The tones generated by the software or sound card are 1,400–1,600 Hz higher, depending on the mode; so with a dial frequency of, for example, 3,572.6 kHz, transmission actually takes place at 3,574.0–3,574.2 kHz.