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.
Fldigi is based on the lightweight portable graphics library FLTK and the C/C++ language. Because of this, the software can run on many different operating systems such as:
Multiple sound systems are supported by Fldigi, allowing the program to abstract the sound card hardware across differing hardware and operating systems.
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.[21]
Some of the Suite are however standalone programs used for utility or testing purposes only, with no connection to the Fldigi main modem.
Flamp
Flamp implements the Amateur Multicast Protocol by Dave Freese, W1HKJ[22] and is a tool for connectionless transferring of files to multiple users simultaneously without requiring any existing infrastructure. The program breaks a given file into multiple smaller pieces, checksums each piece, then transmits each piece one or more times. When all parts are correctly received the sent file is re-assembled and can be saved by receiving stations.[23] This program is useful for multicasting files over lossy connections such as those found on High frequency (HF) or during emergency communications.
Flmsg allows users to send, receive, edit, and create pre-formatted forms. Such a system speeds the flow of information during emergency communications. The software has a number of forms built-in including FEMAICS forms, MARS reports & messages, Hospital ICS forms, Red Cross messages, IARU and NTS messages.[26]
Flwrap
Flwrap is a tool for the sending of files using a simplified drag and drop interface. Data compression is available also, which reduces data transfer times.[27]
FLNet
FLNet assists net control operators in keeping track of multiple stations during digital amateur radio nets.[28]
FLLog
FLLog is a logging software which keeps track of conversations between amateur radio operators in a database format known as ADIF.[29]
FLWkey
FLWkey is a simple interface to control an external piece of hardware called a Winkeyer.[30] This is a Morse codekeyer which is adjustable via computer commands over USB.[31]
Flcluster
This is a telnet client to remote DX cluster servers, which is a real-time reporting of stations heard transmitting, and their frequencies. It does not connect to Fldigi.[32]
Flaa
Flaa is a control program for use with the RigExpert AA-xxxx series of antenna analyzers, and does not connect to Fldigi.[33]
Flrig
FLRig is a component of the FLDigi suite of applications that enables computer aided control of various radios using a serial or USB connection.[34]
Using FLRig in combination with FLDigi, events such as frequency, power level, receiver gain and audio gain may be adjusted from the computer automatically or by user intervention.
Test tools
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.
LinSim
CompText
CompTTY
RSID
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.[35] 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.[35]
Software architecture
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 softmodem flowchart
Community-provided extensions
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.
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."[36]
Fldigi was selected as SourceForge's June 2017 Staff 'Project of the Month'[37]
Fldigi was one of SourceForge's 'Projects of the Week' for Oct 17, 2016[38]
Fldigi was selected as SourceForge's December 2017 Community Choice 'Project of the Month'[39]
Notable users
Disaster relief services
The software is also utilized by some organizations for both routine and disaster/emergency relief services.
Following the successful tests by the Voice of America's VOA Radiogram program, international and government shortwave broadcasters began testing and experimenting with digital data over shortwave broadcast channels using the Fldigi software.[48]
These tests led to regular weekly digital broadcasts by the broadcasters listed below.
VOA Radiogram, service terminated in 2017 and continuing as Shortwave Radiogram.
Fldigi is being used in-testing as part of the Department of Homeland Security Shares program, which utilizes "existing HF radio resources of government, critical infrastructure, and disaster response organizations to coordinate and transmit emergency messages"[56][57]
PSK Mail
Fldigi is used as the underlying modem for the PSKmail project.[58] PSK Mail allows users to retrieve and send normal emails over radio.
AirChat
In 2014, the group Anonymous released a communications tool named AirChat, which used Fldigi as the underlying modem. This provided a low speed yet reliable data connection using only moderate radio hardware. The AirChat software allows for anonymous transmissions of both encrypted and unencrypted messages over unencrypted channels.[59][60][61]
Decodeable broadcasts
The broadcasts listed below are transmitted on a regular schedule and can be decoded using Fldigi.
