Equalization is a form of signal processing controlled via an equalizer. The equalizer influences the relative amplitudes of various frequencies within the audible spectrum. Attenuating or boosting the amplitudes of different frequencies changes the timbre of an instrument or a mix. Equalization may need to be applied to a single recorded channel, to a group of channels, or an entire track.
There are four primary types of Equalizer, including filters, shelving, graphic and parametric:-
Filters
The simplest of all equalizers are filters. Filters usually take the form of a single button or, as a rotary knob or slider which allow various filtering amounts. The low pass filter attenuates the level of those frequencies above a set threshold and rolls the top end away from the signal. The high pass filter attenuates the level of those frequencies below a set threshold and rolls the bottom end away from the signal. Band pass filters pass a set range of frequencies between two pre-selected points.
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| High Pass Filter |
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Low Pass Filter
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It should be noted that the affected frequency range on filters can not be adjusted, and that filters can only attenuate a signal.
Shelving
The shelving equalizer is similar to the high pass and low pass filters in that they work on fixed frequency ranges. They come in two types; the low frequency shelf and high frequency shelf. Unlike the high pass and low pass filters, the shelving EQ allows for cutting and boosting of the signal using a gain control.
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| Shelving: Bass and Treble Boost |
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| Shelving: Bass and Treble Attenuation |
In general, a high frequency shelf will start rolling the signal off at about 12 kHz, and a low pass shelf at around 150 kHz. The shelving EQ is more commonly found on home HI FI as the bass and treble controls.
Graphic
The graphic equalizer affects a range of frequencies evenly spaced through the audio spectrum (20-20,000 Hz). The frequencies are changed using slide controls which boost or attenuate the relative signal amplitudes. A simple graphic EQ will have 2 or 3 bands. These can be found on guitar amplifiers and small Hi Fi. A more advanced graphic EQ can provide more than 30 bands.
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| 32 Band Graphic Equalizer |
Using a graphic EQ for mixing is generally not that effective, as there is no control over the bandwidth of each individual band. Obviously, more sliders provide more control, but ultimately, for optimum control a parametric EQ should be used (see below).
Parametric
The parametric equalizer is the most flexible form of equalization and usually has three controls; one for choosing a centre frequency to work on, one for boosting or attenuating the frequency signal gain, and one for controlling the bandwidth of frequencies influenced.
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| Parametric EQ with high Q setting |
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| Parametric EQ with low Q setting |
The bandwidth control has command over the width of the frequency range and is normally labelled 'Q' (short for 'quality'). A sharp peak is usually determined from a high Q, while a gentler curve has a low Q.
Q is worked out from the equation: Q= centre frequency/bandwidth
The Parametric EQ offers full control over the audio spectrum and is the best type of equalizer used for mixing. Another name for a parametric EQ is a ‘bell EQ’ because it produces bell shaped curves.
Last Updated ( Monday, 23 June 2008 16:40 )
