Fldigi

Fldigi (short for Fast light digital) is a free and open-source program that allows an ordinary computer's sound card to be used as a simple two-way data modem. Fldigi is intended for Amateur Radio Digital Modes of operation using a PC (Personal Computer). Fldigi operates (as does most similar software) in conjunction with a conventional HF SSB radio transceiver, and uses the PC sound card as the main means of input from the radio, and output to the radio. These are audio frequency signals. The software also controls the radio by means of another connection, typically a serial port.

Fldigi is multi-mode, which means that it is able to operate many popular digital modes without switching programs, so you only have one program to learn. Fldigi includes all the popular modes, such as DominoEX, MFSK16, PSK31, and RTTY. 

FLDIGI works great with the Narrow Band Emergency Messaging Software (NBEMS). There are other modes we use such as VaraC, Winlink, Varim, and Garim but FLDIGI and JS8Call remain the backbone of our communications.