Fldigi
Fldigi (short for Fast light digital) 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. 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. Portability. Operating systems. 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: Additionally, Fldigi is designed to compile and run on any POSIX compliant operating system that uses an X11 compatible window system / graphical user interface. Architectures. The Fldigi software is written in highly portable C/C++ and can be used on many CPU architectures, including: Sound systems. 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. 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. Some of the Suite are however standalone programs used for utility or testing purposes only, with no connection to the Fldigi main modem. 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. Flaa . Flaa is a control program for use with the RigExpert AA-xxxx series of antenna analyzers, and does not connect to Fldigi. 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. 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. 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. Currently 7 sound card-digital-modem programs support this standard. 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. 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. 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: Notable users. Disaster relief services. The software is also utilized by some organizations for both routine and disaster/emergency relief services. Shortwave broadcasters. 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. These tests led to regular weekly digital broadcasts by the broadcasters listed below. MARS. The Fldigi suite of programs has become popular within  the U.S. Army and U.S. Air Force Military Auxiliary Radio System. Department of Homeland Security. 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" PSK Mail. Fldigi is used as the underlying modem for the PSKmail project. 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. Decodeable broadcasts. The broadcasts listed below are transmitted on a regular schedule and can be decoded using Fldigi.