Electroacoustics / Psychoacoustics
In a radio RECEIVER, the process of combining in a non-LINEAR device a received SIGNAL with a locally generated signal of slightly different CARRIER frequency. The two combining frequencies produce SUMMATION and DIFFERENCE frequencies (see COMBINATION TONES, BEATS). The difference frequency that is produced is called the intermediate frequency (455 kHz for AM radio) which then is demodulated to retrieve the original program signal. See: DEMODULATION.
In electronic SOUND SYNTHESIS, it is a term describing a passive non-linear mixing or combination of signals to produce certain sound modifications characterized by the addition of PARTIALs.
Compare: DISTORTION, MIXING, MODULATION, SIDEBAND.
Some writers prefer the use of the term heterodyne components instead of AURAL HARMONICS and COMBINATION TONES to refer to the added partials heard when two tones are combined as a result of non-linear processing both mechanically in the inner ear and neurologically in the auditory nerve.
The most easily recognized components generated by frequencies x and y are (2x), (2y), (x + y), and (x - y). Next come the components (3x), (3y), (2x +y), (2x - y), (2y +x), (2y - x). These frequencies can in some cases cause BEATS, and in other cases be heard as a missing FUNDAMENTAL. The removal of roughness caused by heterodyne components is an aid in TUNING.
Compare: LAW OF SUPERPOSITION.