Ever get stuck wondering which filter to use when making patches? We break synthesizer filters down, from cutoff to key tracking, 2-pole to multimode, bandpass to SEM.
Filters are arguably the most important part of a synthesizer for shaping the sound. Watch a few YouTube synth comparison videos and the oscillators all kind of sound the same. A sawtooth is largely a sawtooth with only small differences between manufacturers. But the filter? That makes a big difference.
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n this beginner’s guide, we’ll break down what filters are, the different types and - most crucially - when to choose one over the other. Confused about the difference between an MS-20 and acid filter? Don’t know what a highpass is for? We have you covered.
Note that this is intended to be a quick and dirty guide. For something a little more in-depth, revisit this article from the Attack archives.
What Is A Filter?
In traditional subtractive analog synthesis, the filter removes certain frequencies from the audio signal. The clue is in the name: it filters out harmonics. In this way it’s a lot like the shelf on an equalizer but with a few major differences. Whereas EQs are meant to be largely transparent, filters can be very colorful, with overdrive often part of the character. They’re also modulatable, with envelopes and LFOs contributing to the change in tonal content over time.
Although filters are most associated with subtractive synthesis, these days it’s common to find them in all kinds of synthesizers, both hard and soft, including even FM.
Basic Filter Controls
With only a few basic controls, filters are relatively simple to use.
The main knob to focus on is the cutoff. Sometimes called frequency, this sets the point at which the filter begins attenuating frequencies. Cutoff is massively important for tone shaping and so you may see a hardware synth with an oversized cutoff knob, making it easy to grab during performances.
The next parameter to know is resonance (also known as emphasis on some synths, like those from Moog, or Q). Resonance sets the amount of boost at the cutoff point. This results in feedback in the signal and a telltale squelchy sound. With high enough resonance settings some filters are capable of self-oscillation. Note that not all filters have resonance control.
Another common filter control is keyboard tracking. Increasing this parameter causes the filter to respond in a way that mirrors the behavior of acoustic sounds, with lower notes darker and higher notes brighter.
Lastly, envelope controls how much of an associated envelope affects the shape of the filter over time. For more on envelopes and other forms of modulation, see this article.
Types of Filters
There are a number of different filters that all affect the incoming signal differently. A synthesizer could have just one filter, it could have two arranged serially (in a row) or in parallel, or it could have a multimode filter, which allows you to choose from a variety of different types.
By far, the most common type of filter is the lowpass filter. As the name suggests, this allows low frequencies to pass through. In other words, it cuts high frequencies, darkening the sound as you lower the cutoff frequ