Heterodyne
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In telecommunications, to heterodyne is to generate new frequencies by mixing two or more signals in a nonlinear device such as a vacuum tube, transistor, or diode mixer. The mixing of each two frequencies results in the creation of two new frequencies, one at the sum of the two frequencies mixed, and the other at their difference. A low frequency produced in this manner is sometimes referred to as a beat frequency. A beat frequency, or "beating," can be heard for example when multiple engines of an aircraft are running at close but not identical speeds, or two musical instruments are playing slightly out of tune.
A superheterodyne receiver converts any selected incoming frequency by heterodyne action to a preselected common intermediate frequency, for example, 455 kilohertz or 10.7 megahertz, and provides amplification and selectivity, or filtering.
The term heterodyne is sometimes also applied to one of the new frequencies produced by heterodyne signal mixing.
Heterodyning is not confined to electrical signals, but can occur in any medium where signals of different frequencies are mixed, such as sound vibrations in the aircraft engine example given above. The human brain heterodynes stereo signals; by playing two tones with different frequencies (or the same audio with different phases), a beat frequency called binaural beat is produced inside the brain. As the human ear cannot detect the low frequencies required for this phenomenon, a pair of higher frequency tones must be used.
People can use this phenomenon to alter their brain wave frequency, as the brain is an oscillator and can be externally driven. Optical stimulation is usually more successful, as much more of the human brain is devoted to processing visual information. Stimulation in this manner can produce changes in consciousness, as different brain wave frequencies are associated with unique states of mind. Epileptics and people otherwise sensitive to flashing lights should use caution with any brain wave stimulation technique.