fldigi is a free, open-source digital modem application for amateur radio operators on macOS, enabling real-time transmission and decoding of dozens of digital radio communication modes through a connected transceiver.
What is fldigi?
fldigi — short for Fast Light Digital Modem Application — is the gold standard software for ham radio digital modes on the desktop. If you hold a licence and have spent any time on HF, you have almost certainly encountered its distinctive waterfall display while chasing DX or checking into a digital net. It bridges the gap between your Mac's sound card and your transceiver, letting you send and receive text-based signals ranging from the venerable RTTY and PSK31 to modern, noise-resistant modes like Olivia and THOR.
The project has been under continuous development for well over a decade by W1HKJ and a global community of contributors. Unlike commercial modem software, fldigi costs nothing to run and carries no artificial limits on modes, connections, or features.
What does fldigi do best?
fldigi excels at the real-time waterfall — the frequency-domain view of everything alive on the band — and its robust multi-mode decoder, which can lock onto and decode a signal with a single click on the waterfall display.
The mode library is vast: PSK31, PSK63, BPSK, QPSK, RTTY, CW, Olivia, Contestia, Thor, Throb, DominoEX, MFSK, MT63, Navtex, and more — each with tunable parameters to match propagation conditions. The integrated logging panel captures callsigns, frequencies, modes, and signal reports, and can export directly to common log formats or talk to external loggers via its built-in XML-RPC and Hamlib interfaces.
I rely on fldigi every weekend during RTTY contests. The macro system alone saves dozens of keystrokes per QSO: one macro fires my callsign exchange, another logs the contact and fires a 73. When conditions are rough, switching from PSK31 to Olivia 8/500 takes three clicks — the waterfall redraws instantly, and signals I couldn't read before snap into clear text.
Is fldigi free?
Yes — fldigi is completely free and open-source under the GNU General Public License. There are no subscription tiers, no feature gates, and no nag screens. The project is maintained by volunteers and funded entirely by donations.
Who should use fldigi?
fldigi is purpose-built for licensed amateur radio operators. If you are a Technician who has upgraded to General or Extra and wants to explore HF digital modes beyond FT8, fldigi is the natural next application to install. It complements WSJT-X (which owns FT8/FT4) by covering everything else: rag-chewing on PSK31, emergency-traffic nets on Olivia, RTTY contesting, and slow-scan image decoding when you add companion tools like flrig and flmsg from the same W1HKJ suite.
Experienced operators building an Winlink gateway or a digital-mode contest station will also find fldigi's Hamlib integration and macro engine indispensable. Newcomers to digital modes should be prepared for a steeper learning curve than FT8 — fldigi requires you to understand modes, bandwidth, and audio levels — but that learning pays off fast.
What are the best fldigi alternatives?
The closest alternative on macOS is Multipsk (Windows-only, requires Parallels or CrossOver), and MixW (also Windows-centric). For the narrow slice of FT8 and FT4 specifically, WSJT-X is the undisputed standard and runs natively on macOS. For CW-only work, CWget and RufzXP handle decoding with less overhead. None of these match fldigi's breadth of native macOS modes in a single free package.
How does fldigi compare to WSJT-X?
WSJT-X and fldigi serve fundamentally different use cases. WSJT-X is the definitive tool for weak-signal modes like FT8, FT4, JT65, and MSK144 — it is optimised for minimal bandwidth, automated logging, and working deep into the noise floor. fldigi is for everything else: conversational digital contacts, contesting on RTTY, and modes where readable text flows in real time rather than in 15-second timed bursts. Most active HF operators run both side by side.
- fldigi — real-time text decoding, 30+ modes, contest macros, Hamlib CAT control
- WSJT-X — FT8/FT4/JT65, automated sequences, extreme weak-signal capability
The two applications are complementary, not competitors.
