Which of these functions is performed by software in a software-defined radio (SDR)?

A Software-Defined Radio (SDR) replaces traditional analog hardware components with digital signal processing (DSP) software. Functions that historically required physical capacitors, inductors, crystals, and diodes are instead performed by mathematical algorithms executing on a computer or dedicated processor. Therefore, filtering, detection (demodulation), and modulation can all be handled entirely by the software once the RF signal has been digitized or before it is converted to analog. This immense flexibility is exactly what makes SDRs so powerful and easily upgradeable.