The filter is arguably the element of a synthesizer that does the most to define its character. Words like squelchy, brassy, creamy, gritty and so on are often used to describe their impact, and reading those adjectives in this context probably made you think of a few specific synths. Perusing the various offerings in both the fixed architecture and eurorack markets presents a perplexing plethora of jargon. To make matters worse, many of these terms overlap, as manufacturers will often use different names to describe the same feature. So what makes all these filters different and how does that affect the way a synthesizer will sound?
First, let’s brush up on the basic filter types and attributes. The low pass (sometimes referred to as high cut) filter is the most common type of synthesizer filter. As its name suggests it allows low frequencies to pass through, which can be useful for taming an abrasive top end. High pass (or low cut) filters do the opposite, letting the high frequencies through while cutting out the more rumbly tones. Mix a high pass with a low pass and you get either a band pass or a notch filter, depending on how they are arranged. A band pass allows only a narrow band of the frequency spectrum to pass through while the rest is filtered out. Notch is the opposite, filtering out only a narrow band of the frequency spectrum. A coffee filter, on the other hand, separates the ground beans from your coffee (I threw that one in just to see if you were paying attention).
There is also the more rare and mysterious All Pass filter. If everything is allowed to pass through then what exactly is filtered? In this case the entire signal is passed through but shifted in phase and blended with the original signal. At the filter cutoff point the phase shift is ninety degrees. The resulting phase cancellation causes an effect that sounds similar to a phaser or flanger. Similarly, comb filtering, which is the term for the actual result of a phaser, flanger, or chorus, is caused by blending the original signal with a slightly delayed version of itself which creates a series of notches through phase cancellation. The length of the delay determines where in the frequency spectrum the notches occur. Varying the amount of the delay time is what creates the swishy jet plane effect.
The cutoff frequency is the point in the spectrum where the filtering takes effect. Resonance is an emphasis added through feedback at the cutoff frequency. Sometimes the filter resonance parameter is labelled as Q, standing for Quality. In some cases adding resonance causes a dip surrounding the resonant peak. Some filters are able to create enough feedback that they produce a tone without any input signal at high resonance settings. These are referred to as self oscillating filters because they can effectively function as sine wave oscillators.
The number of poles, or slope of a filter determines how steep or gradually material is filtered beyond the cutoff frequency. Each pole represents a reduction of six decibels per octave, so a four pole filter would have an attenuation of 24db per octave.
While most synth aficionados are familiar with the above types of filters, they are also aware how much a low pass filter can vary from synth to synth, or even between different versions of the same model. Now that we’re on the same page, let’s take a look at some of the different architectures used to create some of our favourite filters.
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