Fldigi
Free sound-card modem software for amateur radio worldwide.
Fldigi, an acronym for "Fast light digital," is a free, open-source program that turns a computer's sound card into a basic two-way data modem. It is mainly used by amateur radio operators, who link the microphone and headphone jacks of an SSB or FM transceiver to the computer's headphone and microphone ports. This setup creates a "sound card defined radio," where the available bandwidth is limited by the sound card's sample rate and the external radio's bandwidth.
These communications typically occur on shortwave amateur radio bands using modes like PSK31, MFSK, RTTY, Olivia, and CW (Morse code). The software is also increasingly used on VHF and UHF frequencies with faster modes such as 8-PSK. With Fldigi, amateur radio operators can communicate worldwide using only a few watts of RF power. It is also employed for emergency communications when other systems fail due to natural disasters or power outages, enabling the transfer of files, emails, and FEMA ICS forms using inexpensive radio hardware.
Fldigi supports numerous digital modes and is highly portable. Built on the FLTK graphics library and written in C/C++, it runs on Microsoft Windows (2000 or newer), macOS, Linux, FreeBSD, OpenBSD, NetBSD, and Solaris. It is designed to compile and run on any POSIX-compliant operating system with an X11-compatible window system. The software works on many CPU architectures, including amd64, i386, armhf/armel, ia64, mips, mipsel, powerpc, s390, s390x, sparc, and Raspberry Pi.
Multiple sound systems are supported, such as Open Sound System (OSS), PortAudio, PulseAudio, and reading/writing to WAV files. Key features include the narrowband emergency messaging system (NBEMS), support for UTF-8 character encoding in some modes, connection to external programs via TCP/IP port 7322, and use as a KISS modem via TCP/IP port 7342. It also offers DTMF encoding and decoding, automatic mode and frequency switching using Reed Solomon Identifier signals, an inbuilt macro language, sound card oscillator frequency/skew correction, measurement of sound card and RF receiver skew against atomic clocks (WWV or WWVH), transmission of a WWV-like time signal, control of external radio hardware via GPIO pins, simultaneous decoding of multiple Morse code signals, and Morse code decoding using a self-organizing map artificial neural network.
Quick Facts
- Developer
- Dave Freese (W1HKJ), et al.
- Released
- 2007
- Latest Release Version
- 4.2.05
- Latest Release Date
- 2024-04-23
- Programming Language
- FLTK, C, C++
- Operating System
- Windows, macOS, Linux, Android, FreeBSD
- Platform
- IA-32, x64, IA-64, armel, armhf, mips, mipsel, PowerPC, s390, s390x, SPARC, Raspberry Pi
- Language
- English, Catalan, Dutch, French, German, Greek, Italian, Polish, Spanish, Russian
- Language Count
- 10
- Genre
- Amateur radio and DSP
- License
- GPL version 3.0
- Repo
- https: · sourceforge.net/p/fldigi/fldigi/ci/master/tree
Facts from the source article.
Lore & Background
Fldigi was developed by Dave Freese (W1HKJ) and is based on the lightweight portable graphics library FLTK and the C/C++ language, allowing it to run on many operating systems and CPU architectures. The software supports multiple sound systems including OSS, PortAudio, PulseAudio, and WAV file I/O. It can decode multiple Morse code (CW) signals simultaneously and uses a self-organizing map artificial neural network for CW decoding.
The Fldigi Suite includes several extension programs: Flamp implements the Amateur Multicast Protocol for connectionless file transfer to multiple users; Flarq implements the ARQ specification for unicast, connection-based transfer of emails, text, images, and binary files; Flmsg allows sending, receiving, editing, and creating pre-formatted forms including FEMA ICS forms; Flwrap provides simplified drag-and-drop file transfer with data compression; FLNet assists net control operators; FLLog logs conversations in ADIF format; FLWkey controls an external Winkeyer Morse code keyer; Flcluster is a telnet client to DX cluster servers; Flaa controls RigExpert antenna analyzers; and FLRig enables computer-aided control of radios via serial or USB.
The software uses RSID (Reed-Solomon Identifier) to automatically identify over 272 digital modes by sending a short burst of 16-tone MFSK modulation at 10.766 baud, occupying 172 Hz of bandwidth for 1.4 seconds. Fldigi can connect to external programs via TCP/IP ports 7322 (ARQ), 7342 (KISS), and 7362 (XML-RPC).
Reader's Guide
Fldigi's significance lies in its role as a versatile, free, and portable sound-card modem that has become a standard tool in amateur radio digital communications. By allowing ordinary computers to function as data modems using only a sound card and a transceiver, it lowered the barrier to entry for digital modes such as PSK31, MFSK, RTTY, Olivia, and CW on shortwave bands, as well as faster modes like 8-PSK on VHF and UHF. Its use in emergency communications—enabling file transfers, emails, and FEMA ICS forms when other infrastructure fails—demonstrates its practical value beyond hobbyist use.
The Fldigi Suite extends the modem's capabilities with verified file transfer, message passing, and form handling, creating an integrated ecosystem for both routine and disaster-relief operations. The software's cross-platform support (Windows, macOS, Linux, BSD, Solaris, and many CPU architectures including Raspberry Pi) and its ability to be used as a KISS modem for AX.25 and TCP/IP connections have contributed to its widespread adoption. Recognition at the Dayton Hamvention and multiple SourceForge project awards reflect its impact on the amateur radio community.
Did You Know?
- The RSID system in Fldigi can automatically identify over 272 digital modes using a 1.4-second burst of 16-tone MFSK modulation.
- Fldigi was selected as SourceForge's June 2017 Staff 'Project of the Month' and December 2017 Community Choice 'Project of the Month'.
- The software can measure sound card oscillator skew and RF receiver frequency skew to the atomic clock signals from WWV or WWVH.
Frequently Asked Questions
What is Fldigi and what does it actually do?
Fldigi (short for 'Fast light digital') is a free, open-source application that repurposes a computer's sound card as a simple two-way data modem. Amateur radio operators typically connect the mic and headphone jacks of their SSB or FM transceiver to the PC's audio ports, creating what's called a 'sound card defined radio.'
Who created Fldigi?
Fldigi was written by Dave Freese, who holds the amateur radio call sign W1HKJ. The codebase is built in C/C++ and distributed under a free, open-source license.
Which digital modes can I run through Fldigi?
The program handles a range of shortwave-band modes including PSK31, MFSK, RTTY, Olivia, and CW (Morse code). Practical bandwidth is constrained by both the sound card's sample rate and the external transceiver's own bandwidth.
What operating systems and hardware does Fldigi support?
It runs on Microsoft Windows (2000 or later), macOS, Linux, FreeBSD, OpenBSD, NetBSD, Solaris, and essentially any POSIX system with X11. Supported architectures span amd64, i386, ARM (armhf/armel), ia64, MIPS, PowerPC, s390/s390x, SPARC, and even Raspberry Pi.
Has Fldigi earned any recognition in the ham radio community?
Yes—Fldigi received the Technical Excellence Award at the 2014 Dayton Hamvention, one of the most prominent gatherings in the amateur radio world.
More in Radio Modulation Modes, Part 2 1-24
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