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Pro Tools plug-in EQIII is a good example of a parametric EQ

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EQ is essentially a cross between a filter and a level control. It affects only specific frequencies and is used to either boost or reduce the level of said frequencies.

In times gone by, most budget EQ was very simple and limited to three bands; at least two of which were fixed-range. These days we have the facility to apply highly precise EQ to every channel, which makes it even more critical that we understand what we're doing.

The anatomy of an EQ plug-in

1. EQ Graph - A visual representation of the frequencies affected by EQ

2. Shelving - These EQs affect only frequencies above or below the cutoff frequency

3. Parametric/band-pass - This EQ type only affects frequencies within the specified range

4. Frequency - This sets either the cutoff or centre frequency to be affected

5. Gain Controls - The boost or gain applied to either the shelving or parametric EQ

6. Q - This specifies the width (parametric) or angle (shelving) of frequencies affected either side of the centre/cutoff frequency

Parametric EQ

Parametric EQ is the most powerful type of EQ for all-round use.

A fully parametric EQ strip will have three controls. The first is Frequency. All EQ is based around a curve, centred on a certain frequency, and the Frequency control determines this frequency. The next control is Gain. The Gain control specifies how much the specified frequency is made louder or quieter. If you set the Frequency control to 3kHz and apply 3dB of gain, then any frequencies at 3kHz will be boosted by exactly 3dB.

The next control is Q, and the Q control determines the amount of gain or attenuation applied to either side (industry folklore has it that Q stands for 'quality'). To understand the Q control, imagine a strip of rubber, pulled straight and attached at either end, with a row of pen marks at regular intervals. Each of the marks represents a specific frequency, and if you push any part of the rubber band up it represents gain to those frequencies, whilst pushing it down represents attenuation. The first thing you notice is that all of the notches either side of the central notch are pushed up or down too, and this is what the Q control is used for - shaping the curve either side of the centre frequency.

The Q is assigned a value, and this is derived from dividing the centre frequency value by the bandwidth of the affected signal. Of course, the frequency width is hard to define, so it's taken to mean the point on either side of the curve where the signal level drops by 3dB. High values equate to narrow frequency ranges, for precise cutting or boosting, while low values equal wide curves, for broad boost or cut.

Graphic EQ

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