BI4OAE

Beyond FT8: A Practical Guide to PSK31, RTTY, Olivia and JS8Call

2026-10-18 · Digital Modes

The gap FT8 leaves behind

Over the past few years FT8 has become shorthand for “digital mode” on the shortwave bands. It is a remarkable piece of engineering: a 77-bit message inside a 15-second cycle, two stations trading callsigns, grids and reports in ninety seconds, working at signal levels the human ear cannot touch.

It also cannot talk. There is no room for a sentence in a 77-bit message, and no mechanism for a conversation. If what you want is to type a few lines to someone in another country, chase a ragchew, or push a block of text through when everything else on the band is failing, FT8 is simply the wrong tool.

Fortunately, the family of HF digital modes is much larger. The useful ones fall into clear roles.

PSK31: built for keyboard chat

PSK31 was introduced in 1998 by Peter Martinez (G3PLX), who wanted a mode that beat RTTY on weak signals without needing its bandwidth. Two ideas did the work. First, Varicode — a variable-length character encoding in which common letters get short codes and rare ones long ones, rather like Morse. Second, modulation by phase: the sound card shifts the audio phase by 180 degrees for a zero and leaves it unchanged for a one, at 31.25 baud.

The result occupies about 31 Hz. That number matters more than it sounds. RTTY typically spreads across 170 Hz or more, so a single SSB channel can hold many parallel PSK31 conversations — a real advantage on a crowded band.

Operating feel is the mode’s other selling point. Transmission is real time: you type, and the characters appear on the other station’s screen almost as fast as you send them. It is the closest thing to a chat room that HF has ever had, and it was the mainstream HF data mode for years before FT8 arrived. It is also efficient on power — where RTTY may want tens of watts to hold a path, a few watts on PSK31 often suffices — and its phase-based modulation tolerates the selective fading of long paths better than frequency-shift modes do. The trade is speed: fine for conversation, hopeless for files.

Software is easy: fldigi is the standard cross-platform choice, with DigiPan and MultiPSK as alternatives.

RTTY: the oldest mode that still matters

RTTY — radioteletype — predates personal computers by decades. Characters are encoded in the five-bit Baudot code, and the simplest modulation shifts the audio tone 170 Hz between mark and space. The standard rate is 45.45 baud: each bit lasts about 22 milliseconds, each seven-and-a-half-bit character about 165, which works out to roughly 60 words per minute. Enough to keep up with most typists, which was the entire design goal in an era when the receiving equipment really was a hundred-pound teleprinter clacking out characters on a paper roll.

Software decoding replaced the machinery long ago — MMTTY, fldigi and contest loggers such as N1MM+ all do the job — but the signal on air has barely changed in seventy years. Weak-signal performance is no longer competitive with the newer modes. What keeps RTTY alive is a combination of simplicity, reliability, mature tooling, and an outsized role in contesting: the ARRL RTTY Roundup, the CQ WW DX Contest’s RTTY leg, and events such as the BARTG sprints guarantee hundreds of RTTY signals on the air at once. If you want to compete in digital modes, RTTY is unavoidable.

Olivia: for when conditions are dreadful

Olivia has one job: get data through when almost nothing else will. It is an MFSK mode — multiple tones, one sent at a time — wrapped in a forward error correction layer based on Walsh functions, so a corrupted symbol costs you one bit spread across 64 bits of a character block rather than a lost character.

The bandwidth and tone count are configurable, from 125 Hz up to 2 kHz, with common presets like 8/250, 16/500 and 32/1000 (tones/bandwidth). The default sends 32 tones within 1,000 Hz at 31.25 baud, yielding about 15 words per minute. The payoff is robustness: the designer’s own simulations show decoding with the signal around 10 dB below the noise, and field reports agree. That is why emergency communications groups list Olivia among their recommended disaster modes. When propagation has collapsed but a message must still get out, this is the mode that earns its keep.

The same family includes MFSK16 — about 316 Hz wide, similarly noise-immune — and Contestia. fldigi supports all of them.

JS8Call: FT8’s skeleton, a conversation’s skin

JS8Call is the most interesting recent entry. Jordan Sherer (KN4CRD) took the FT8 protocol and added what it lacks: a messaging layer with free text. You type normally, and the other station sees your words decoded on their screen, with FT8-grade weak-signal performance underneath — the software claims decoding down to about -24 dB SNR.

Two features make it more than a chat toy. The first is store-and-forward: messages can be relayed through intermediate stations, or left in an inbox for someone who is not currently on the air. The second is an optional heartbeat and auto-reply system, so a station can announce its presence and answer simple queries without an operator at the keyboard. Together these turn a handheld into a node in an ad-hoc messaging network that needs no infrastructure — no repeater, no server, no internet.

That combination has made JS8Call popular with two overlapping crowds: operators who want to ragchew when the band is barely alive, and the emergency communications community, where some ARES groups have adopted it for exactly the reason described above. It received a Hamvention Special Achievement Award in 2020. Speeds are adjustable — Slow (25 Hz, 30 seconds per frame), Normal (50 Hz, 15 s), Fast (80 Hz, 10 s) and Turbo (160 Hz, 6 s) — trading robustness for throughput as conditions allow.

Two neighbours worth knowing

SSTV sends still images over an ordinary SSB channel using nothing but a sound card, in formats such as Martin, Scottie and Robot36, each taking roughly half a minute to a couple of minutes per picture. The 20-metre calling frequency, 14.230 MHz, is one of the most recognisable spots on the band — once you hear that rhythmic warble you know someone is sending a picture. The ISS also transmits SSTV during scheduled events, usually on 145.800 MHz FM.

FT4 is FT8’s contest-oriented sibling: the same design compressed into 7.5-second cycles with about 90 Hz bandwidth, giving up some weak-signal margin for twice the contact rate.

Which software, and which sideband

Every HF digital mode runs in USB. If your transceiver has a built-in USB sound card — most modern ones do — a single USB cable carries audio and frequency control, which is by far the tidiest setup.

For software: WSJT-X covers FT8, FT4, JT65 and WSPR; fldigi covers PSK31, RTTY, Olivia and the MFSK family; MMTTY or N1MM+ are the RTTY contest standards; JS8Call is a standalone program.

The customary frequencies are worth memorising. On 20 metres, FT8 sits at 14.074 MHz, JS8Call at 14.078, PSK31 at 14.070 and RTTY at 14.080; 40 metres has its own set, including FT8 at 7.074. These are long-standing conventions coordinated through the IARU band plans — voluntary agreements, not regulations — so the polite move everywhere is to listen for a few minutes and see where activity actually is before transmitting.

Choosing

These modes are not competitors. Most active digital operators keep three or four programs installed and pick based on the band and the evening — which is exactly how the hobby is supposed to work.

References

Chinese-language archive of this site: /zh/