Heterodyne

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In telecommunications and radio astronomy, 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 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 3,001 hertz would beat together, producing an audible beat frequency of 1 hertz. A heterodyne radio or infrared receiver is one which uses such a frequency shifting process.

Contents

1 Origin and use
2 Heterodyning in the human brain
3 Heterodyne in Analog Videotape Recording
4 See also
5 Popular culture
6 References

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.

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 is produced inside the brain.

Some people claim that this phenomenon can be used to alter their brain wave frequencies. As the human ear cannot detect the low frequencies required for this phenomenon, a pair of higher frequency tones must be used. The brain wave frequencies range from 1 to 30 Hz or so, while the ear is sensitive to no signal lower than 20 Hz. (For example, a 102 Hz signal mixed with a 100 Hz signal will produce a beat frequency of 2 Hz, the difference of the two; but its tone will sound like 101 Hz, the average of the two. One would hear a 101 Hz tone with an amplitude (or volume) which oscillates twice per second. While a 2 Hz non-heterodyne signal could be produced, it would be impossible to hear.) Optical stimulation is usually more successful, as much more of the human brain is devoted to processing visual information, and it can utilize amplitude modulation, rather than beat frequency. 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.

Heterodyne 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 http://www.lionlmb.org/quad/format.html#12incomposite ; Retrieved 2006-07-13 . PAL VHS color subcarrier is similarly downconverted (but from 4.43 MHz). The now-obsolete 3/4" "Umatic" systems used a heterodyned ~688 KHz subcarrier for NTSC recordings (as did Sony's Betamax), while PAL Umatic 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 Poynton, Charles. Digital Video and HDTV: Algorithms and Interfaces San Francisco: Morgan Kaufmann Publishers, 2003 PP 582, 583 ISBN 1558607927 .

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-1MHz 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 Umatic decks (Sony 3/4" duplication recorders) featured 8-Pin DIN connectors to allow dubbing of tapes without a heterodyne up-conversion and down-conversion, as did some industrial VHS and S-VHS recorders.

Popular culture

"Heterodyne" is also the name of a famed family of mad scientists in the webcomic Girl Genius. The name is probably in reference to the telecommunications term.
"Heterodyne" is also the name of the monsters in the anime series Dai-Guard, so named because they are "created" by the convergance of energy waves.
David Webers "The Stars at War" series features heterodyne lasers, although the concept is never explained in sufficient detail.

References

Glinsky, Albert. Theremin: Ether Music and Espionage. Urbana: University of Illinois Press, 2000. ISBN 0252025822.