Heterodyning
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In telecommunications, physics, and radio astronomy, heterodyning is the generation of new frequencies by mixing two or more signals in a nonlinear device such as a vacuum tube, transistor, diode mixer, Josephson junction, or bolometer. 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 when multiple engines of an aircraft are running at close but not identical speeds, or two musical instruments are playing slightly out of tune. For example, a frequency of 3,000 hertz and another of 3,100 hertz would beat together, producing an audible beat frequency of 100 hertz. A heterodyne radio or infrared receiver is one which uses such a frequency shifting process.
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[edit] Origin and use
The word heterodyne is derived from the Greek roots hetero-, 'different', and -dyne, 'power'.
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.
[edit] Heterodyning in analog videotape recording
Many analog videotape systems relied on a downconverted color subcarrier in order to record color information in their limited bandwidth. These systems are referred to as "heterodyne systems" or "color-under systems". For instance, for NTSC systems, the VHS (and S-VHS) system converts the color subcarrier from the NTSC standard 3.58 MHz to ~629 kHz.[1] PAL VHS color subcarrier is similarly downconverted (but from 4.43 MHz). The now-obsolete 3/4" U-matic systems used a heterodyned ~688 kHz subcarrier for NTSC recordings (as did Sony's Betamax), while PAL U-matic decks came in two mutually incompatible varieties, with different subcarrier frequencies, known as Hi-Band and Low-Band. Other videotape formats with heterodyne color systems include: Video-8, and Hi8.[2]
The Heterodyne system in these cases is used to convert quadrature phase-encoded and amplitude modulated sine waves from the broadcast frequencies to frequencies recordable on formats with sub-1 MHz bandwidth. On playback, the recorded color information was heterodyned back to the standard subcarrier frequencies for playback on televisions and for interchange with other standard video equipment.
Some U-matic (3/4") decks featured 7-pin mini-DIN connectors to allow dubbing of tapes without a heterodyne up-conversion and down-conversion, as did some industrial VHS, S-VHS, and Hi8 recorders.
[edit] Heterodyning in the human brain
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 a binaural beat will be perceived.
[edit] See also
[edit] References
- Glinsky, Albert. Theremin: Ether Music and Espionage. Urbana: University of Illinois Press, 2000. ISBN 0-252-02582-2.
- ^ Videotape formats using 1/2 inch wide tape ; Retrieved 2007-01-01
- ^ Poynton, Charles. Digital Video and HDTV: Algorithms and Interfaces San Francisco: Morgan Kaufmann Publishers, 2003 PP 582, 583 ISBN 1-55860-792-7
