Editing Frequency extender

{{short description|Electronic device used in telephone systems}}{{More citations needed|date=May 2024}}

In [[broadcast engineering]], a '''frequency extender''' is an [[Electronics|electronic]] [[Information appliance|device]] that expands the usable frequency range of [[Plain old telephone service|POTS]] telephone lines.<ref name=":0">{{Cite book |last=White |first=Glenn D. |title=The Audio Dictionary |last2=Louie |first2=Gary J. |date=May 2005 |publisher=University of Washington Press |isbn=9780295984988 |edition=3rd |pages=165}}</ref> It also allows [[high-fidelity]] [[analog audio]] to be sent over regular [[telephone line]]s, without the loss of lower [[audio frequencies]] ([[Bass (sound)|bass]]). It is an extended [[concept]] of a [[telephone hybrid]].

The [[concept]] uses frequency shifting to overcome the [[narrowband|narrow]] [[Bandwidth (signal processing)|bandwidth]] of regular telephone systems, extending the usable range by approximately two [[Octave|octaves]].<ref name=":0" /> The input [[signal]] is sent on one telephone line as-is, or in some cases upshifted to provide extra low-frequency response, and sent on a second line shifted down by 3 [[kHz]], which is normally the upper [[bandpass]] limit in telephony.  Thus, an audio frequency of 5&nbsp;kHz is sent at 2&nbsp;kHz.  A receiver on the other end then shifts the second line back up and [[audio mixing (recorded music)|mixes]] it with the first.  This results in greatly improved audio, adding a full [[octave]] of range, and pushing the total bandpass to 6&nbsp;kHz.  The sound is then acceptable for [[Human voice|voice]], if not for [[music]].

It is also possible to add other lines, each increasing the bandpass by another 3&nbsp;kHz.  However, the [[Diminishing returns|law of diminishing returns]] takes over, because each successive octave is double the size of the last.  A third line  pushes the bandpass up 50[[%]] to 9&nbsp;kHz, equivalent to [[AM radio]].  A fourth line would push it up 33% to 12&nbsp;kHz.  [[FM radio]] quality would require five telephone lines to be installed, pushing the bandpass up 25% to 15&nbsp;kHz.  The audio is shifted down by 6,9, and 12&nbsp;kHz respectively for each additional line.

Frequency extenders have been nearly eliminated by [[POTS codec]]s.

==See also==
* [[Remote broadcast]]

== References ==
{{reflist}}

[[Category:Broadcast engineering]]
[[Category:Telephony]]
[[Category:Frequency|E]]


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