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In audio processing, equalization (EQ) is the process of restoring the frequency envelope of a sound. In passing through any channel, data is temporal/frequency spreading occurs. Etymologically, it means to correct, or make equal, the frequency response of a signal. more...

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The term "equalizer" is eroneous applied as a general term for audio filters. DJ mixing equipment and Hifi audio components often include so called graphic equalizers/equalisers or simply equalizer/equaliser. These are in fact general all purpose filters, which can be arranged to produce the effect of low pass, high pass, band pass and band stop filters. Only when these filters are arranged so as to reverse the effects of the internal circuitry on sound output, are they operating as equalisers.

Overview

There are many kinds of EQ. Each has a different pattern of attenuation or boost. A peaking equaliser raises or lowers a range of frequencies around a central point in a bell shape. A peaking equalizer with controls to adjust the level (Gain), bandwidth (Q) and center frequency 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 equalizer.

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 how quickly the filter attenuates frequencies past the cut-off point. A band-pass filter is simply a combination 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.

One common type of equalizer is the graphic equalizer, which consists of a bank of sliders for boosting and cutting different bands (or frequencies ranges) of sound. Normally, these bands are tight enough to give at least 3 dB or 6 dB maximum effect for neighboring bands, and cover the range from around 20 Hz to 20 kHz (which is approximately the range of human hearing). A simple equalizer might have bands at 20 Hz, 200 Hz, 2 kHz and 20 kHz, and might be referred to as a 4-band equalizer. A typical equalizer for live sound reinforcement might have as many as 24 or 31 bands. A typical 31-band equalizer is also called a 1/3-octave equalizer because the center frequencies of sliders are spaced one third of an octave apart.

Read more at Wikipedia.org