PPC Guide to EQ Types - Part 1
When doing a mix or 'adjusting' tracks, one of the first tools most people head for is the EQ section. Ian Waugh shows you how
EQ in modern software has become very sophisticated and goes far beyond the Treble and Bass controls on hi fi systems and even the row of EQ sliders. Here we look at the main types of EQ and their controls.
EQ is used to cut or boost specific frequency bands in the audio and different types of EQ do this is different ways.
There are five common EQ terms you need to understand.
Cutoff frequency or cutoff point
This is the point at which the EQ kicks in. Depending upon the EQ type, frequencies above, below or either side of the cutoff-frequency may be affected.
This is sometimes known as Q, bandwidth, emphasis or peak. It's the range of frequencies either side of the cutoff point. By increasing the resonance, the range of frequencies around the cutoff point narrows so if this range was boosted there would be a peak in the frequency band as shown below:
The top example shows a frequency boost with a very wide bandwidth. The lower example shows the result of narrowing it.
The cognoscenti term for reduction, the opposite of amplification.
Roll-off or slope
This is the rate at which the filter's effect increases as it moves further away from the cutoff point. It's usually measured in dB (decibels) and the distance from the cutoff point is measured in octaves. A typical slope will be something like 6dB/octave.
You can see from the figure that the 'stronger' curves (with higher dB values) have a greater filtering effect. If the roll-off of a filter is not quoted it will probably be 12dB/octave.
A higher roll-off curve does not necessarily make the filter better. If that were the case an 'ideal' filter would have a vertical slope but this would cut the frequencies dead at that point which would sound very unnatural indeed.
This determines the amount by which a signal is cut (attenuated) or boosted.
There are two common types of EQ - graphic and parametric.
This divides the frequency range into several bands which can individually be cut or boosted. Theoretically, the spectrum can be divided into any number of bands but common divisions are 5, 8 and 10.
It's common for each band to be twice that of the preceding one. A doubling of the frequency represents a doubling of the octave so musically that's very useful. A graphic EQ is useful for shaping the overall tone of the sound.
This lets you specify the cutoff frequency you wish to process. It can, therefore, home in quite accurately on any part of the spectrum.
They typically have three controls - cutoff frequency, bandwidth and level - although some have additional features to make them more flexible. It's not uncommon to see a parametric EQ with more than one band. Some developers call these a paragraphic EQ.
The dividing line between an EQ and a filter is a thin one if, indeed, one still exists at all.
Traditionally, EQ was used on mixers to help 'equalise' a recording, to compensate for the inaccuracies of recording equipment. Filters were used in synthesisers to shape the tone of the sound. As developers have produced more powerful and more specialised 'tone modification' software, they have borrowed features from one to include in the other, hence the blurring of boundaries.
There are six common filter types whose operation is fairly easy to understand.
This passes frequencies below the cutoff point while attenuating the higher one. It's the most common filter type and the most natural as it removes high frequencies which are usually the first to go in most environments.
The opposite of the low pass filter. It passes the high frequencies and attenuates the lower ones.
Cuts or boosts the frequencies above the cutoff point.
Cuts or boosts the frequencies below the cutoff point. Shelf filters are used to change a broad spectrum of the sound.
This removes frequencies either side of the cutoff point.
Band reject or notch
The opposite of the band pass filter which attenuates frequencies around the cutoff point and passes the rest. Sometimes the two are combined into a filter which can cut or boost at the selected frequency.
Watch out for our collection of EQ Tips, coming soon...