Why In a software-defined radio the signal is digitized early (at RF or at an IF/baseband stage) and everything after that is math running on a processor, FPGA or PC. That math does the bandpass/notch filtering that crystal or mechanical filters used to do, performs detection (demodulating SSB, CW, FM, AM, or a digital mode), and on transmit builds the modulated waveform before it goes to the DAC. Because those steps are code rather than fixed hardware, a firmware update can add a new mode or change filter bandwidth continuously.