Making Music
  Computer Music Software and Hardware

Rolling around

There's one more factor to consider before looking at EQ types and that's the roll-off curve or slope. This essentially describes how 'strong' or severe a filter is. The steeper the slope, the more severe it is and the greater the filtering effect as you move away from the cutoff point.

The roll-off is usually measured in dB and the distance from the cutoff point is measured in octaves. A gentle filter, therefore, might have a roll-off of 6dB per octave which means that for every octave away from the cutoff point, the signal is attenuated by 6dB. A filter with a 12dB/octave roll-off would be twice as strong and attenuate the signal by 12dB every octave.

Poles apart

In analogue synthesisers there are components within the filter circuits called poles which apply an attenuation of around 6dB per octave.

Some synthesisers use several poles and you may hear reference to a 1-pole, 2-pole, 3-pole or 4-pole filter. These would have a roll-off of 6dB/octave, 12dB/octave, 18dB/octave and 24dB/octave respectively. The more poles a filter has, the steeper the roll-off and the fewer frequencies will pass beyond the cutoff point. The famous Moog synthesisers were among the first to use 4-pole filters which contributed to their 'fat' sound.

Not all synths tell you how many poles their filters have (or, in the case of digital synths and filters, how many poles they emulate) or let you change them which ought to be easy in software. Lacking any precise information, you might assume a typical filter roll-off of 12dB/octave.

EQ type

There are a handful of common types of EQ and filters:

	Fixed EQ. This is typically a single control such as Treble or Bass. A mixer might have Low (bass) and High (treble) EQ controls. The cutoff frequency here is fixed and you control the amount of cut or boost.
	Graphic EQ. This divides the frequency range into a series of bands that you can cut and boost individually. They are easily recognised by a row of sliders, each controlling a specific frequency band. They are common on hi fi systems, as stand alone studio devices and can be found in software. Although you can technically divide the spectrum into any number of bands, typical divisions on hardware units are 15 and 31 covering two-thirds and one-third of an octave ranges respectively. Software developers are a law unto themselves and any number goes.
	Parametric EQ. Whereas with a graphic EQ the frequency bands are fixed, parametric EQ lets you change the frequency of the bands. Parametric EQs typically have three or four bands, each with three controls - cutoff frequency, bandwidth and level, allowing you to home in on specific frequencies within a very tight range.
	Paragraphic EQ. This is a relatively recent development born of the software revolution and it blurs the line between graphic and parametric. It typically offers several bands like a graphic but with user-definable frequency bands like a parametric; it's a sort of super-parametric. However, as this can become quite complex, they also tend to have highly-graphic interfaces so the user can see exactly what frequencies are being affected and by how much
	Sweeping/Semi-parametric EQ. This is a half-way house between a fixed EQ and a full parametric EQ - while the bandwidth is fixed, you can control the centre frequency (the cutoff point). It's common to many mixers. Typically you may find fixed Low and High EQ with a sweepable mid range control.

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