A typical amateur software radio uses a direct conversion receiver. Unlike direct conversion receivers of the more distant past, the mixer technologies used are based on the quadrature sampling detector and the quadrature sampling exciter.
The receiver performance of this line of SDRs is directly related to the dynamic range of the analog-to-digital converters (ADCs) utilized. Radio frequency signals are down converted to the audio frequency band, which is sampled by a high performance audio frequency ADC. First generation SDRs used a 44 kHz PC sound card to provide ADC functionality. The newer software defined radios use embedded high performance ADCs that provide higher dynamic range and are more resistant to noise and RF interference.
The SDR software performs all of the demodulation, filtering (both radio frequency and audio frequency), and signal enhancement (equalization and binaural presentation). Uses include every common amateur modulation: morse code, single sideband modulation, frequency modulation, amplitude modulation, and a variety of digital modes such as radioteletype, slow-scan television, and packet radio. Amateurs also experiment with new modulation methods: for instance, the DREAM open-source project decodes the COFDM technique used by Digital Radio Mondiale. ~Wikipedia.