Modulation refers to the conversion of information signals (such as voice or data) into a form that can be transmitted on radio frequencies. Demodulation refers to converting the modulated signal back to the original information signal.

Commonly used modulation methods are: frequency modulation (FM), amplitude modulation (AM) and digital modulation (such as QAM). In FM, the frequency of signal changes with the change of information signal, while in AM, the amplitude of signal changes with the change of information signal.

The process of demodulation is to detect the characteristics of the modulated signal and convert it back to the original information signal. For FM signal, frequency demodulator is usually used for demodulation, and for AM signal, amplitude demodulator is usually used for demodulation.