Types of equalizers

[intoccabile]

Stumbled on a great article today and I thought I'd share it with you all.

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Types of Equalizers

There are many different kinds of equalizers.

Here they are, in order of sophistication :

Filters

Simple filters are extremely useful on mixing and recording systems. They are more correctly known as "High (or Low) Frequency Roll Off Filters", or - with confusingly opposite names - as "Low (or High) Pass Filters". They either take the form of a single button to roll off the bottom end, or perhaps instead two rotary controls marked LF and HF. As you sweep these controls, the top and bottom end is "rolled away" from the signal. The frequency at which this happens depends on the position of the control. You can't control how steeply the sound rolls away - that is always preset. The manual either for your software or mixing hardware will say what it is preset to. It will be described as something like "6dB per octave" or "12dB per octave".

Not all mixing desks have simple, dedicated filters. On budget desks, it is assumed that the task can be done with by careful use of the main EQ, although this is a little restrictive - because having even just a single button for LF rolloff is extremely useful - especially when recording.

Shelf Equalizers

The filters described above don't raise or lower the treble and bass in general terms - the sound is actually trimmed away to nothing the higher or lower the sound gets. This makes filters good for getting rid of undesirable sounds outside of the musical range of the instrument.

But, unfortunately, filters are not very good for shaping the actual musical sound itself. That's where Shelf Equalizers come in. Just like filters, they come in two types - low frequency and high frequency. However, instead of having "Frequency" controls, they have "Gain" controls instead. The gain controls set how much boost or cut is applied to the signal. A high frequency shelf EQ will normally start working at around 8 to 12 kHz, and a low frequency shelf EQ works at around 80 to 150 Hz. The point about shelf equalizers, is that - unlike rolloff filters, they raise all frequencies above (in the case of high-frequency shelf EQ) or below (in the case of a low-frequency shelf EQ) by the same amount, which will give you much more "musical" results than rolloff filters. They are called "shelf" equalizers because of the shape of their frequency response when drawn on a graph.

The "Bass" and "Treble" controls on ordinary Hi-fi equipment are usually shelf equalizers with preset, unadjustable frequency points.

Some shelf equalizers allow you to set the frequencies at which they operate as well.

Sweep Equalizers

The problem with both filters and shelf equalizers, is that they are really only useful at either end of the audio spectrum.

So what about the middle?

Well, if you think about it, you can boost the middle by using a high and low shelf EQ to cut the bits either side, or - similarly - you can cut the middle by boosting the bits either side. But this is not very satisfactory, and difficult to control quickly and easily.

A sweep equalizer is designed to solve this problem. With a sweep equalizer, only the parts of the signal surrounding the area selected by the frequency control are affected - by the amount set using the gain control. This allows you to boost or cut selected areas of sound quite easily. It isn't possible to control the width of the sonic "area" affected by the EQ, and this width varies from manufacturer to manufacturer. Some manufacturers like to keep the area fairly broad, as this is more musical, but others prefer to keep it narrow as this is more useful for correcting harmonic problems like "ringing" on drums or other instruments.

Sweep equalizers normally come in pairs. A pair of sweep equalizers is often found on some of the better-quality "portastudio" devices, as it allows a great deal of sonic control for relatively little cost. Some manufacturers give completely different frequency ranges for each one of the pair, arguing that this allows for more precise control over the entire range of sound. This is acceptable providing that the preset "area" or "bandwidth" of the equalizer is extremely broad. If the bandwidth is smaller ("tighter"), then it is far more useful for a pair of sweep equalizers to have a generous overlap in their frequency ranges, so that you can use both of them at the same time in both the low and upper frequency ranges.

Semi Parametric

To make sweep equalizers more useful, they are sometimes fitted with a single button that changes the "bandwidth" or "area" over which they are effective.

When one or more sweep equalizers are teamed up with a couple of additional shelf equalizers for control of the very top and bottom end, the entire assembly of equalizers is referred to as a semi-parametric EQ.

Fully Parametric

On top-of-the-range mixing desks, you normally have fully parametric equalization.

With a fully parametric equalizer, you can control the frequency, the gain, and - significantly - the bandwidth of the equalizer. Typically, four of these units are packed together, and a switch on both the first and last unit, allows them to be optionally used as shelf high and low frequency EQ's respectively. Naturally, this is expensive.

For most instruments, you don't need this level of control, and semi-parametric equalizers are fine. Even in a professional set up, you can often get by quite adequately using semi-parametric equalizers, provided that there are a couple of plug-in fully parametric equalizers available for troublesome instruments. The main problem however with a mixing desk with semi-parametric equalizers throughout, is that the manufacturers preset choice of bandwidth, can result in the "sound of the desk" coloring the mix overall, to a much greater degree than it does with fully parametric EQ.

But having said that - in almost all cases - the "sound" of the EQ is probably still the major distinguishing feature that separates one mixing console from another.

Graphic Equalizers

These are by and large an overkill. They are best reserved for situations where a number of extremely subtle audio artifacts are already taking place - such as the equalization of a recording studio control room's main monitors, or the delicate final equalization of a finished mix during the mastering process when preparing a CD or other release for listening by the public at large. Graphic equalizers are much less suited to situations where the actual "correction" required is more "general" such as EQing individual instruments as part of a mix - although it must be said, with some particularly difficult sounds, you may occasionally have to resort to using a graphic equalizer to solve the problem - but not generally.

Passive and Valve Equalizers

It's worth mentioning in passing here, the subject of passive equalizers.

Most equalizers use circuitry that actively boosts or cuts the sound in the various bands using electronic feedback techniques which can (and often does) introduce audible "ringing" in the circuitry. Passive equalizers on the other hand work by already cutting the sound in all frequency bands to begin with, using simple, unpowered, passive electrical components like resistors, capacitors and inductors. A single, simply-designed amplifier stage after the equalizer usually makes up for the loss in signal level, so that the level is flat when all controls are in their "centre" position. Therefore, on such an equalizer, when you "boost" a frequency, you're not really boosting it at all - you're just allowing it to seep through unhindered by the passive circuitry. Valve equalizers often work in exactly this way.

This gives a much smoother sound. In fact a lot of the "smoothness" attributed to valve equalizers, often has little to do with the fact that they have valves in - it is instead due to the fact that the equalizer circuitry is passive rather than active.

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I will post some stuff on the creative uses of eq in a bit !

[prismatic7]

top article! great find intoccabile.

i actually really enjoyed my "o shit i really have no idea how to EQ!" moment - fixing a huge number of tracks with a few simple tweaks is an AWESOME feeling!