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{{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)}}

(contracted; show full)lter 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:
Peaking-eq.svg|thumb|right|200px|Two examples of the frequency response of a peaking EQ]][[Image:Shelving-eq.svg|thumb|right|200px|Two examples of the frequency response of a shelving EQ]]There are many kinds of EQ. Each has a different pattern of attenuation or boost. A peaking equalizer raises or lowers a range of frequencies around a central point in a bell shape. A peaking equalizer that has controls to adjust the level ([[Gain]]), [[Bandwidth (signal processing)|bandwidth]] ([[Q factor|Q]]) and center frequency ([[Hertz|Hz]]) is called a [[parametric equalizer]]. If there is no control for the bandwidth (it is fixed by the designer) then it is called a ''quasi-parametric'' or ''semi-parametric'' equalizerShelving-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.

Shelving-type equalizers increase or attenuate the level of a wide range of frequencies by a fixed amount. A ''low shelf'' will affect low frequencies up to a certain point and then above that point will have little effect. A ''high shelf'' affects the level of high frequencies, while below a certain point, the low frequencies are unaffected.⏎
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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 ====
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