This article is an excerpt from Samplecraze's new e-book release, EQ - Uncovered, by Eddie Bazil.

This book shows you, step-by-step, the intricacies of EQ, and is crammed to the hilt with walk through examples using images, audio and just about every other trick known to Samplecraze. 

To begin to understand the EQ, we need to first define the two categories it falls into; Passive and Active.

Passive EQs

These types of EQs have the distinction of being extremely simple in design and, more importantly, they cannot boost frequencies, only cut. The way they work is actually very much to do with perception. For example, by cutting low frequencies (bass) they make the mid and high frequencies sound ‘louder’.

Passive EQs do have their uses. Although they are inflexible, they can perform reduction tasks reasonably well. By cutting high frequencies, they are able to cut or lower hiss (high frequency noise). However by their very nature, passive EQs or filters then need to have the signal boosted to compensate for the cut. This in itself introduces noise into the signal path; the noise coming from the amp used to boost the signal.

In terms of circuitry, passive equalisers place the equalisation circuits either before or after a fixed-gain amplifier, in which case the amp makes up for the inherent loss in the EQ circuit, effectively boosting the frequency range(s) that haven't been cut.  
Active EQs

Because of the limitations of passive EQs, most EQs are built around active filter circuits which use frequency selective components, together with a low noise amplifier. And it is this type of EQ that we are going to concentrate on.

Fixed Frequency EQ

Pretty self explanatory, this EQ allows cut/boost of one or more frequencies. There are no additional controls over the usual components, like bandwidth, Q, etc.

Peaking EQ

A peaking EQ is an EQ which boosts a specific band of frequencies. Whereas a shelving filter has a shelf like curve, this filter has a bell shaped curve. The Q setting determines the width of the bell, while boost or cut determines the height or depth of the bell.

Figure 9.1: Peaking EQ (Filter). The shape of the frequency response resembles a ‘peak’, thus the name.

Two Band or Three Band

These types of EQ simply have two or three separate frequency ranges. Usually denoted as low, mid and high, these bands can only be cut or boosted.

Shelving Filter/EQ

We have touched on the use of tone controls as forms of EQ. These controls control a type of filter that is called a shelving filter. In the case of the bass and treble knobs, low-pass and hi-pass shelving filters are used respectively.

A low-pass shelving filter passes all frequencies below its cut-off frequency, but attenuates all frequencies above its cut-off frequency. Similarly, a high-pass filter passes all frequencies above its cut-off frequency, but affects all frequencies below its cut-off frequency.

This is the simplest type of active EQ. This EQ can shape response in a number of ways: boost/cut low frequencies, boost/cut high frequencies. This is why I have included the diagram to demonstrate what happens with the filters, low and hi-pass, in this type of EQ.

Most mixers will allow for low and high frequency EQ, and in the case of shelving filters, their mid frequencies are usually fixed.

Figure 9.2: Shelving EQ (Filter). The shape of the frequency response resembles a ‘shelf’ thus the name.

It is also common for the filter slope to be 6 dB per octave. This allows for a gentler effect. The shape is ‘shelf-like’, so the boost or cut is progressive over a range. Filters do not have zero effect at a particular frequency and then instantly jump and suddenly reappear at the next frequency; they have to get there somehow. The way they do so, and by how much, is called the gradient or slope. In the case of the shelving filter, the most common slope is 6 dB gain change per octave (doubling of the frequency). It takes time for the filter to attenuate frequencies in proportion to the distance from the cut-off point; this is the slope.

Shelving filters are generally designed to apply equal gain changes beyond the shelving frequency and have controls for selecting the shelf, cut and boost. The diagram below illustrates what happens if you cut or boost frequencies in a low-pass and a hi-pass filter.

Figure 9.3: Low Pass                                                          

Figure 9.4: High Pass

As the diagrams clearly show; in the low-pass filter diagram the frequencies below the cut-off are allowed to pass through whereas the frequencies above the cut-off are attenuated.

In the hi-pass filter diagram the frequencies below the cut-off are attenuated and the frequencies above the cut-off are allowed to pass through.

Graphic EQ

A graphic equalizer is simply a set of filters, each with a fixed centre frequency that cannot be changed. The only control you have is the amount of boost or cut in each frequency band. This boost or cut is most often controlled with sliders. The sliders are a graphic representation of the frequency response, hence the name 'graphic' equalizer.

The more frequency bands you have, the more control and accuracy you have over the frequency response.

Mixing consoles rarely have graphic EQs, but PA mixers often have a stereo graphic EQ for EQing the final stereo output.

A graphic equalizer uses a set of band-pass filters that are designed to completely isolate certain frequency bands. The diagram below shows the frequency response of a band-pass filter.

Figure 9.5: Band-pass Filter. A filter that passes frequencies between two limits is known as a band-pass filter.

This is a great filter. It attenuates frequencies below and above the cut-off and leaves the frequencies at the cut-off. It is in effect a low-pass and a hi-pass together. The cool thing about this filter is that you can eliminate the lower and higher frequencies and be left with a band of frequencies that you can then use as either an effect (as in having that real mid-range type of old radio sound), or use it for isolating a narrow band of frequencies in recordings that have too much low and high-end.

Try this filter on synthesizer sounds and you will come up with some wacky sounds. It really is a useful filter and if you can run more than one at a time, and select different cut-offs for each one; then you will get even more interesting results. Band-pass filtering is used on formant filters that you find on so many softsynths, plug-ins, synthesizers and samplers - Emu/Ensoniq are known for some of their format filters and that technology is based around band-pass filters.

It is also good for thinning out sounds and can be used on percussive sounds as well as creating effects type of sounds.

I often get emails from programmers wanting to know how they can get that old radio effect or telephone line chat effect or even NASA space dialogue from space to Houston. Well, this is one of the tools. Use it and experiment.

Notch Filter – also know as Band Reject Filter

The inverse of a band-pass is the notch filter. This is a very potent EQ/filter. It can home in on a single frequency band, and cut/boost it.  Used specifically for ‘problem’ frequencies, the notch can be one of the most useful filters.

This is the exact opposite of the band-pass filter. It allows frequencies below and above the cut-off and attenuates the frequencies around the cut-off point.

In terms of the diagram shown for band-pass filtering, the area in between the two arrows is rejected (cut out), and the remaining frequencies below and above the cut-off are allowed to pass through. This is the exact opposite of band-pass filtering.

Why is this good? Well, it eliminates a narrow band of frequencies (the frequencies around the cut-off) - that in itself is a great tool. You can use this on all sounds and can have a distinct effect on a sound, not only in terms of eliminating the frequencies that you want eliminated, but also in terms of creating a new flavour to a sound.

But its real potency is in eliminating frequencies you don’t want. Because when you select the cut-off point you are in essence selecting the frequencies around that cut-off point and eliminating them.

This is an invaluable tool when you want to hone in on a band of frequencies located, for example, right in the middle of a sound or recording. I sometimes use a notch filter on drum sounds that have a muddy or heavy mid section, or on sounds that have a little noise or frequency clash in the mid section of a sound.

Parametric

Invented by George Massenberg, this filter is one of the most commonly used today.

This filter controls three parameters; frequency, bandwidth and gain. You select the range of frequencies you want to boost or cut, you select the width of that range and then use the gain to boost or cut the frequencies (within the selected bandwidth)d by a selected amount.

The frequencies not in the bandwidth are not altered. If you widen the bandwidth to the limit of the upper and lower frequencies ranges then this is called shelving. Most parametric filters have shelving parameters.

Figure 9.6: Parametric Filters

Parametric filters are great for more complex filtering jobs and can be used to create real dynamic effects because they can attenuate or boost any range of frequencies.

Basically, the parametric EQ places several active filters across the frequency spectrum. Each filter is designated to a frequency range; low, mid, high etc. You have the usual cut/boost, resonant frequency and bandwidth. It is these qualities and the control over them that places this particular EQ in the producer’s arsenal of dynamic tools. However, you need to understand what you are doing when using a parametric EQ, otherwise things can go very wrong.

Understand frequencies and sound, and you will be in total control.

Quasi-parametric EQ

This is just another form of parametric EQ but without the bandwidth control.

Sweep EQ

This is very similar to a band-pass filter, but with variable centre frequency, and no control over the width of the filter response (Q).

You will find that most mixers will have band-pass EQ, and some will have sweep EQ (where the centre frequency can be varied, also known as ‘tuneable’), but very few, mainly digital, will have parametric EQ.

Paragraphic EQ

Another variation on the graphic EQ. This EQ provides control over the centre frequency of each band. Also regarded as a cross between the graphic and parametric EQs, this EQ offers multiple parametric peaking filters, where the gain of each is provided on a slider much like the graphic EQ.

Excerpt taken from EQ - Uncovered

Eddie Bazil (Zukan)

www.samplecraze.com