Fldigi (short for Fast light digital)^[4] is a free and open-source program which allows an ordinary computer's sound card to be used as a simple two-way data modem. The software is mostly used by amateur radio operators who connect the microphone and headphone connections of an amateur radio SSB or FM transceiver to the computer's headphone and microphone connections, respectively.^[5][6]

Quick Facts Developer(s), Initial release ...

Fldigi

Fldigi main window snapshot
Developer(s)	Dave Freese (W1HKJ), et al.[1]
Initial release	2007
Stable release	4.1.19 / 24 June 2021; 2 years ago (2021-06-24)[2]
Repository	sourceforge.net/p/fldigi/fldigi/ci/master/tree/
Written in	FLTK, C, C++
Operating system	Windows, macOS, Linux, Android, FreeBSD[3]
Platform	IA-32, x64, IA-64, armel, armhf, mips, mipsel, PowerPC, s390, s390x, SPARC, Raspberry Pi
Size	About 6.5 MB
Available in	7 languages
List of languages English, Italian, Spanish, French, German, Polish, Dutch
Type	Amateur radio and DSP
License	GPL version 3.0
Website	www.w1hkj.com

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This interconnection creates a "sound card defined radio" whose available bandwidth is limited by the sound card's sample rate and the external radio's bandwidth.

Such communications are normally done on the shortwave amateur radio bands in modes such as PSK31, MFSK, RTTY, Olivia, and CW (Morse code). Increasingly, the software is also being used for data on VHF and UHF frequencies using faster modes such as 8-PSK.

Using this software, it is possible for amateur radio operators to communicate worldwide while using only a few watts of RF power.

Fldigi software is also used for amateur radio emergency communications when other communication systems fail due to natural disaster or power outage. Transfer of files, emails, and FEMA ICS forms are possible using inexpensive radio hardware.^{[7][8][9][10]}

• NBEMS: The narrowband emergency messaging system^[19][20][9]
• Support for transmitting and receiving in all languages by using UTF-8 character encoding (some modes)
• Connection to external programs via TCP/IP port 7322^[21]
• Ability to be used as a KISS modem via TCP/IP port 7342
• Dual-tone multi-frequency (DTMF) encoding and decoding
• Automatic switching of mode and frequency by use of Reed Solomon Identifier signal identification
• Inbuilt macro language and processor for programmable automated control
• Sound card oscillator frequency/skew correction
• Measure sound card oscillator's skew to atomic clock: WWV or WWVH
• Measure RF receiver frequency skew to atomic clock: WWV or WWVH
• Transmit a WWV-like time signal as a calibration reference
• Control of external transmit / receive radio hardware by using GPIO pins. (For embedded hardware)
• Simultaneous decoding of multiple morse code (CW) signals.
• Decoding of morse code (CW) by self-organizing map artificial neural network (trained artificial intelligence)

The "Fldigi Suite" consists of the Fldigi modem and all extension programs released by the same development group. Most of these extensions add more capabilities to Fldigi such as verified file transfer and message passing. Interconnection between these programs and the Fldigi modem is made over TCP/IP port 7322.^[22]

Some of the Suite are however standalone programs used for utility or testing purposes only, with no connection to the Fldigi main modem.

The Fldigi development group also releases a number of open-source programs which assist in the testing, development, and comparison of different modes within Fldigi, such as LinSim, CompText, and CompTTY.

To identify the mode being transmitted a signal called an RSID, or Reed-Solomon Identifier, can be transmitted before the data. Using this identifier the receiving software can automatically switch to the proper mode for decoding. The assigning of these identifiers to new modes is coordinated to ensure inter-operation between programs.^[36] Currently 7 sound card-digital-modem programs support this standard:

• PocketDigi
• FDMDV
• DM780
• Multipsk
• Fldigi
• AndFlmsg
• TIVAR

RSID operates by sending a short burst of a specific modulation before the data signal, which can be used to automatically identify over 272 digital modes. This burst consists of a 10.766 baud 16-tone MFSK modulation where 15 tones/symbols are sent. The burst occupies 172 Hz of bandwidth and lasts for 1.4 seconds.^[36]

For simple keyboard-to-keyboard communication Fldigi can be operated using just the main window. For more complex uses or file transfer external programs can be attached to the internal TCP/UDP ports 7322 (ARQ), 7342 (KISS), and 7362 (XML-RPC).

The image below helps to illustrate the interconnections and signal-flow within the Fldigi architecture.

Fldigi allows external programs to attach and send / receive data by connecting to port 7322/ARQ or 7342/KISS. When used this way, Fldigi and the computer's sound card are acting as a "softmodem" allowing text or data sent on one computer to be transferred using the wireless radio link in-between. Programs which have a history of use with Fldigi as the underlying modem include:

• D-Rats - easy to use chatrooms, email, and file transfer over-radio.
• PSKmail - send and receive on-internet e-mail over a remote radio connection.^[37]
• Fldigiattach - attach Fldigi as modem for Linux AX.25 and TCP/IP connections.
• UIChat - Java-based amateur radio chat program.
• LinkUP - Program for unattended operation and person to person chat.
• Linux - Fldigi can be used in Linux as a KISS (TNC) modem for AX.25 and TCP/IP connections.

• At the 2014 Dayton Hamvention the project lead, Dave Freese (W1HKJ), was recognized with the Technical Excellence Award "for his development and distribution of the Fast Light Digital Modem Application (fldigi) family of programs for use in amateur and emergency communications."^[38]
• Fldigi was selected as SourceForge's June 2017 Staff 'Project of the Month'^[39]
• Fldigi was one of SourceForge's 'Projects of the Week' for Oct 17, 2016^[40]
• Fldigi was selected as SourceForge's December 2017 Community Choice 'Project of the Month'^[41]

The broadcasts listed below are transmitted on a regular schedule and can be decoded using Fldigi.

• SITOR text forecasts and storm warnings^[63]
• WEFAX visual weather fax^[64]
• SYNOP surface synoptic observations^[65]
• NAVTEX warnings, forecasts, and safety information broadcasts^[63]
• VOA Radiogram Broadcasts
• W1AW Broadcasts

• Amateur radio
• Shortwave radio
• WSPR (amateur radio software)
• WSJT (amateur radio software)
• CW Skimmer
• Internet Radio Linking Project
• PSK31
• RTTY
• American Radio Relay League