A software-defined radio (SDR) system is a radio communication system which uses software for the modulation and demodulation of radio signals.
The hardware of a software-defined radio typically consists of a superheterodyne RF front end which converts RF signals from and to analog IF signals, and analog to digital converter and digital to analog converters which are used to convert a digitised IF signal to and from analog form.
One of the first software radios was a military project named SpeakEasy. The primary goal of the SpeakEasy project is to utilize programmable processing to emulate more than 10 existing military radios, operating in frequency bands between 2 and 200MHz. Further, another design goal was to be able to easily incorporate new coding and modulation standards can in the future, so that military communications can keep pace with advances in coding and modulation techniques.
Software-defined radio can currently be used to implement simple radio modem technologies. In the long run, software-defined radio is expected by its proponents to become the dominant technology in radio communications.
GNU Radio is a project to implement software-defined radio as free software.
See also: PACTOR, AMTOR