is a signal processing technique invented in 1901 by Canadian inventor-
engineer Reginald Fessenden that creates new frequencies by combining or mixing
two frequencies. Heterodyning is used to shift one frequency range into another, new one, and is also involved in the processes of modulation and demodulation. The
two frequencies are combined in a nonlinear signal-processing device such as a vacuum tube, transistor, or diode, usually called a mixer. In the most common application,
two signals at frequencies f1 and f2 are mixed, creating
two new signals, one at the sum f1 + f2 of the
two frequencies, and the other at the difference f1 − f2. These new frequencies are called heterodynes. Typically only one of the new frequencies is desired, and the other signal is filtered out of the output of the mixer. Heterodynes are the mathematical dual of the phenomenon of "beats" in acoustics.
A major application of the heterodyne process is in the superheterodyne
radio receiver circuit, which is used in virtually all modern
radio receivers.