Revision 427421404 of "Equalization" on enwiki

{{Merge from |Blind equalization |discuss=Talk:Equalization#Merge parametric and blind equalization into equalization article|date=September 2010}}

{{about|electronic compensation for systems' uneven frequency responses|equalization particularly as used in sound recording and reproduction|Equalization (audio)}}
{{other uses|Equalization (disambiguation)}}

'''Equalization''', (British: '''equalisation''') is the process of adjusting the balance between [[frequency]] components within an electronic [[Signal (electronics)|signal]]. The most well known use of equalization is in [[sound recording and reproduction]] but there are many other applications in electronics and telecommunications. The circuit or equipment used to achieve equalization is called an equalizer.  These devices strengthen (''boost'') or weaken (''cut'') the energy of specific frequency [[Frequency range|bands]]. 

In telecommunications equalizers are used to render the [[frequency response]]—for instance of a telephone line—''flat'' from end-to-end. When a [[communication channel|channel]] has been "equalized" the [[frequency domain]] attributes of the signal at the input are faithfully reproduced at the output. Telephones, [[DSL]] lines and television cables use equalizers to prepare data signals for transmission.

In the field of audio electronics, the term "equalization" has come to include the adjustment of frequency responses for practical or aesthetic reasons, often resulting in a net response that is not truly equalized.  The term EQ specifically refers to this variant of the term.<ref>Ballou, pp.875-876.</ref> Stereos typically have adjustable equalizers which boost or cut [[Bass (sound)|bass]] or [[treble]] frequencies. Broadcast and recording studios use sophisticated equalizers capable of much more detailed adjustments, such as eliminating unwanted sounds or making certain instruments or voices more prominent.

In addition to adjusting the relative amplitude of frequency bands, an equalizer (and the system it is correcting) will alter the relative [[phase (waves)|phase]]s of those frequencies. While the human ear is not sensitive to the phase of audio frequencies (involving delays of less than 1/30 second) in most non-audio applications the actual [[waveform]] of the transmitted signal must be preserved, not just its frequency content. Thus these equalizing filters must also cancel out any phase shifts (unequal delay) between different frequency components.

== Uses ==

=== Audio and music ===
{{main|Equalization (audio)}}
[[Image:Shelving-eq.svg|thumb|right|200px|Two first-order shelving filters: a -3dB bass cut (red), and a +9dB treble boost (blue)]]
[[Image:Peaking-eq.svg|thumb|right|200px|Second-order [[linear filter]] functions. Blue: a 9 dB boost at 1 kHz. Red: a 6 dB cut at 100 Hz having a higher [[Q factor|Q]] (sharper bandwidth)]]
Although the range of equalization functions is governed by the theory of [[linear filter]]s, the adjustment of those functions and the flexibility with which they can be adjusted varies according to the topology of the circuitry and controls presented to the user. Shelving controls are usually simple first-order filter functions which alter the relative gains between frequencies much higher and much lower than the [[corner frequency|corner frequencies]]. A ''low shelf'' such as the bass control on most [[hi-fi]] equipment, is adjusted to affect the gain of lower frequencies while having no effect well above its corner frequency. A ''high shelf'' such as a treble control adjusts the gain of higher frequencies only. These are coarse adjustments more designed to increase the listener's satisfaction than providing actual equalization in the strict sense of the term.

A [[parametric equalizer]], on the other hand, has one or more sections each of which implements a [[linear filter|second-order filter]] function. This involves three adjustments: selection of the center frequency (in [[Hertz|Hz]]), adjustment of the [[Q factor|Q]] which determines the sharpness of the [[Bandwidth (signal processing)|bandwidth]], and the level or gain control which determines how much those frequencies are boosted or cut relative to frequencies much above or below the center frequency selected. In a ''quasi-parametric'' or ''semi-parametric'' equalizer there is no control for the bandwidth (it is preset by the designer) or is only selected between two presets using a switch.

A pass filter attenuates either high or low frequencies while allowing other frequencies to pass unfiltered.  A [[high-pass filter]] modifies a signal only by taking out low frequencies; a [[low-pass filter]] only modifies the audio signal by taking out high frequencies.  A pass filter is described by its ''cut-off point'' and ''slope''.  The cut-off point is the frequency where high or low-frequencies will be removed.  The slope, given in decibels per octave, describes a ratio of how the filter attenuates frequencies past the cut-off point (e.g. 12 dB per octave).  A [[band-pass filter]] is a combination (in series) of one high-pass filter and one low-pass filter, which together allow only a band of frequencies to pass, attenuating both high and low frequencies past certain cut-off points.

Variable equalization was first used by [[John Volkman]] working at [[RCA]] in the 1920s.  They were used to equalize a motion picture theater playback systems.<ref>H. Tremaine, Audio Cyclopedia, 2nd. Ed., (H.W. Sams, Indianapolis, 1973)</ref>

=== Analog telecommunications ===

==== Audio lines ====
Early telephone systems used equalization to correct for the reduced level of high frequencies in long cables, typically using [[Zobel network]]s. These kinds of equalizers can also be used to produce a circuit with a wider bandwidth than the standard telephone band of 300&nbsp;Hz to 3.4&nbsp;kHz.  This was particularly useful for broadcasters who needed "music" quality, not "telephone" quality on landlines carrying program material.  It is necessary to remove or cancel any [[loading coil]]s in the line before equalization can be successful.  Equalization was also applied to correct the response of the transducers, for example, a particular [[microphone]] might be more sensitive to low [[frequency]] sounds than to high frequency sounds, so an equalizer would be used to increase the volume of the higher frequencies (''boost''), and reduce the volume of the low frequency sounds (''cut'').

==== Television lines ====
A similar approach to audio was taken with television landlines with two important additional complications.  The first of these is that the television signal is a wide bandwidth covering many more octaves than an audio signal.  A television equalizer consequently typically requires more filter sections than an audio equalizer.  To keep this manageable, television equalizer sections were often combined into a single network using [[ladder topology]] to form a [[Cauer equaliser|Cauer equalizer]].

The second issue is that phase equalization is essential for an analog television signal.  Without it [[material dispersion|dispersion]] causes the loss of integrity of the original waveshape and is seen as smearing of what were originally sharp edges in the picture.

=== Digital telecommunications ===
Modern digital telephone systems have less trouble in the voice frequency range as only the local line to the subscriber now remains in analog format, but [[Digital Subscriber Line|DSL]] circuits operating in the [[MegaHertz|MHz]] range on those same wires may suffer severe [[attenuation distortion]], which is dealt with by automatic equalization or by abandoning the worst frequencies.  [[Videophone|Picturephone]] circuits also had equalizers.

==See also==
*[[Blind equalization]]
*[[Electronic filter]]
*[[Adaptive equalizer]]
*[[Loudness compensation]]
*[[Weighting filter]]

==References==
{{Reflist}}
<div class="references-small">
:*Glen Ballou, "Filters and equalizers", ''Handbook for Sound Engineers'', Fourth edition, Focal Press, 2008 ISBN    0240809696.
</div>

==External links==
{{Wiktionary|equalisation|equalization}}
*[http://www.audiocheck.net/engineertraining_bands_difficult.php Discriminating EQ frequencies by ear]
*[http://www.sengpielaudio.com/calculator-bandwidth.htm Calculator: bandwidth per octave ''<math>N</math>'' to quality factor ''<math>Q</math>'' and back]
*[http://www.idc.ul.ie/idcwiki/index.php/Equalisation EQ Condensed Overview]
*[http://www.wikirecording.org/EQ WikiRecording's Guide to Equalization]
*[http://www.musicdsp.org/files/Audio-EQ-Cookbook.txt Audio EQ Cookbook]

{{Music technology}}

[[Category:Audio effects]]
[[Category:Linear filters]]
[[Category:Tone, EQ and filter]]

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