Fl-digi is a free, open-source digital modem software for amateur radio operators, turning a computer's sound card into a data modem for modes like PSK31, RTTY, CW, MFSK, Olivia, and Hellschreiber, enabling text, image, and Morse code communication over the air by connecting to an amateur radio transceiver. It features integrated logging, contest modes, transceiver control (Hamlib/RigCAT), and supports various operating systems like Windows, macOS, and Linux, with companion software like flrig for radio control. 

Key Features & Uses:

Digital Modes: Supports numerous modes (BPSK, MFSK, RTTY, CW, DominoEX, MT63, Olivia, Thor, FSQ, Hellschreiber) for data communication.

Sound Card Modem: Uses the computer's sound card for audio input/output, creating a "sound card radio".

Transceiver Control: Integrates with flrig (a separate control program) and Hamlib/RigCAT for automated frequency and mode changes.

Logging & Contesting: Built-in logbook, supports contest modes (like RTTY Roundup) with duplicate detection, and integrates with logging systems.

Station Management: Can query online databases (like QRZ), send reports, and manage operator info.