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Fldigi (Fast Light Digital modem application) is a free, open-source digital mode program for amateur radio, developed by Dave Freese (W1HKJ) and a community of contributors. It runs on Windows, macOS, and Linux, and supports a remarkably wide range of digital modes — from PSK31 and RTTY to Olivia, MT63, MFSK, Contestia, Thor, and dozens more. If you want a single piece of software that can operate nearly any keyboard-to-keyboard digital mode on HF, Fldigi is the go-to choice.
Fldigi turns your computer into a multi-mode digital modem. It takes audio from your radio (via a sound card interface or built-in USB audio), decodes the digital signals, and displays the decoded text on screen. When you type a message, Fldigi generates the appropriate audio tones and feeds them to the radio for transmission.
The interface centres on a waterfall display that shows a visual representation of all signals within the radio's passband. You click on a signal in the waterfall to tune to it, and the decoded text appears in the receive window. The transmit window lets you type text or trigger macros for CQ calls, exchanges, and other common messages.
Fldigi supports an extensive list of digital modes, including:
Phase Shift Keying: PSK31, PSK63, PSK125, PSK250, PSK500, QPSK31, QPSK63, and multi-carrier variants (PSK-R modes)
Frequency Shift Keying: RTTY (Baudot and ASCII), NAVTEX
MFSK (Multi-Frequency Shift Keying): MFSK4, MFSK8, MFSK16, MFSK32, MFSK64, MFSK128
Olivia: A robust mode designed for difficult HF conditions, with multiple bandwidth and tone configurations (Olivia 8/250, 8/500, 16/1000, 32/1000, and others)
MT63: A wideband keyboard mode with strong error correction, popular in some emergency communication networks
Thor: A robust MFSK variant with forward error correction
Contestia: Similar to Olivia, optimised for contesting
DominoEX: An incremental frequency keying mode
Feld Hell / Hell modes: Hellschreiber, a fascinating mode that transmits text as a visual bitmap pattern
WEFAX / Weather Fax: Decoding weather charts transmitted on HF
CW: Fldigi can decode (and send) Morse code, though dedicated CW operators typically prefer other methods
This breadth of mode support is one of Fldigi's greatest strengths. You can experiment with obscure modes, decode unusual signals you encounter on the bands, and try something new without installing additional software.
Fldigi is available from the Fldigi/W1HKJ website as pre-built packages for Windows, macOS, and Linux. On Linux, it is also available through most distribution package managers.
When you first launch Fldigi, a configuration wizard walks you through the essential settings:
Enter your callsign, name, QTH (location), and grid locator. These are used in macros and logging.
Select your sound card interface or USB audio device as the audio input (capture) and output (playback) device. If your radio has built-in USB audio, select the radio's audio device.
Fldigi uses Hamlib (also called RigCAT or FLrig) for radio control. You can either:
Select your PTT method: CAT (via Hamlib/FLrig), serial port RTS/DTR, or hardware VOX. CAT PTT through FLrig or Hamlib is recommended.
The waterfall is the heart of Fldigi's interface. It scrolls from top to bottom, showing signals as coloured traces. Different modes have distinctive visual signatures:
Click on a signal in the waterfall to place your receive cursor on it. Fldigi will begin decoding whatever mode you have selected. If you're not sure what mode a signal is, Fldigi's RSID (Reed-Solomon Identification) feature can automatically identify many modes by detecting a special identification burst that some stations transmit at the beginning of their transmission.
Fldigi includes a row of macro buttons along the top or bottom of the window. Macros are programmable message templates that insert your callsign, the other station's callsign, signal report, and other variables. Typical macros include:
You can customise macros extensively using Fldigi's macro language, which supports variables like <MYCALL>, <CALL>, <RST>, <NAME>, and many others.
Fldigi includes a built-in logging system that records contacts. It can also interface with external logging software (Log4OM, N3FJP, HRD, etc.) via network connections. When you complete a QSO and click the log button, Fldigi records the callsign, frequency, mode, signal reports, and other details.
Fldigi is part of a suite of related programs by W1HKJ:
FLrig — Standalone radio control software. Many operators run FLrig alongside Fldigi for more flexible radio management.
FLmsg — A forms and message handling program designed for emergency communications. It creates structured forms (ICS-213, radiograms, etc.) that can be transmitted as compressed data bursts via Fldigi.
FLamp — File transfer software that sends files (images, documents, etc.) over Fldigi using ARQ (Automatic Repeat Request) for reliable delivery.
FLnet — Net control logging software for managing amateur radio nets.
Together, these programs form a powerful, free, open-source toolkit for digital communication, message handling, and net management.
Fldigi and its companion programs (particularly FLmsg) are widely used in emergency communications. The combination provides:
Many ARES and RACES groups train on Fldigi as part of their digital communications capability.