Heterodyning

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"Heterodyne" redirects here. For the use of the term in poetry, see Heterodyne (poetry). For the webcomic character, see Girl Genius.

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. Mixing two frequencies creates two new frequencies, one at the sum of the two frequencies mixed, and the other at their difference. A heterodyne receiver is a telecommunication receiver which uses this effect to produce frequency shifts.

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[edit] Mathematical principle

Heterodyning is based on the simple trigonometric identity:

\sin \theta \sin \varphi = {\cos(\theta - \varphi) - \cos(\theta + \varphi) \over 2}.

[edit] Origin and use

The word heterodyne is derived from the Greek roots hetero- "different", and -dyne "power". This technique was pioneered by Canadian inventor-engineer Reginald Fessenden.

A superheterodyne receiver converts an incoming frequency to an intermediate frequency, using heterodyne action. The incoming signal is mixed with a generated signal, producing sum and difference frequencies. The signal of interest is selected by using a bandpass filter. The filter is usually fixed, common choices being 455 kilohertz and 10.7 Megahertz. The receiver's operating frequency is tuned simply by changing the frequency of the generated signal.

The term heterodyne is sometimes applied also to one of the new frequencies produced by heterodyne signal mixing.

[edit] 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] See also

[edit] References

[edit] Notations

  • Glinsky, Albert. Theremin: Ether Music and Espionage. Urbana: University of Illinois Press, 2000. ISBN 0-252-02582-2.

[edit] Footnotes

  1. ^ Videotape formats using 1/2 inch wide tape ; Retrieved 2007-01-01
  2. ^ Poynton, Charles. Digital Video and HDTV: Algorithms and Interfaces San Francisco: Morgan Kaufmann Publishers, 2003 PP 582, 583 ISBN 1-55860-792-7