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 frequency. The classic use is on bass; reduce the cutoff to get rid of unnecessary high frequencies.
The Moog-style lowpass filter on U-he Diva opening on a bassline:
Another common filter type is the highpass. This cuts out the bass, useful for shaping pads and strings and other sounds that don’t need a lot of low-frequency information. Highpass filters are also highly effective on drum loops in breakdowns.
A stock Ableton Live highpass filter progressively removing the bass from a drum loop:
Between the lowpass and highpass is the bandpass. This allows a midband section of frequencies through, cutting above and below it. In this example, we’re modulating a bandpass over a Reese bass.
A bandpass filter on Kiloheart’s Phase Plant modulating a Reese bass:
The last common type is the notch filter, which cuts only a sliver of frequencies. Less immediately useful than other types, it works well when sweeping through sustained sounds like pads, creating a phaser-like effect.
Arturia’s Filter M12 filter effect plugin set to notch mode sweeping over a pad:
Filter Slopes
Like EQ shelves, filters have slopes. The slope is how steeply the curve rolls off the frequencies towards zero. These are expressed in multiples of six and measured as decibels per octave (db/Oct). The larger the number, the steeper the curve.
Most filters are either 12dB/Oct or 24dB/Oct or, expressed in another way, 2-pole or 4-pole, respectively, with a ‘pole’ representing six decibels. There are others of course, including 6dB/Oct and 18dB/Oct filters – or even steeper in some soft synths.
In terms of sound, 12db/Oct filters are generally thought of as more gentle than 24db/Oct ones but the steepness of the curve is only one part of the equation. The design of the filter also plays a part.
The stock Ableton Live filter set to lowpass at 12dB/Oct opening and closing on a drum loop:
The same loop and filter now with a 24dB/Oct slope:
Filter Design
Filters are not one size fits all. In hardware synthesizers, there are any number of different ways to create a filter using components. This, more than anything, is the greatest determiner of the sound of the filter.
Here are some examples of famous filter designs in action.
Moog Ladder Filter
One of the most famous filters ever made, Bob Moog’s 24dB/Oct transistor-based Ladder filter – so-called because the shape of the circuit design looks like a ladder – first appeared in his Model 12 modular system in the form of the 904A lowpass filter. It was its use in the Minimoog Model D, however, that made the sound legendary. Creamy and chewy, it’s the defining element of the Moog sound.
Best for: Typically Moogy sounds, bass, leads
The Moog Ladder filter in Moog’s Minimoog plugin in action on a bass sound:
Sequential Prophet-5 Filter
Another famous filter is the one in Sequential’s Prophet-5 synthesizer. To say the ‘one’ is a bit of a lie, as the original hardware had two different chips, first the SSM2040 and later the CEM3320. Both are 24dB/Oct OTA (Operational Transconductance Amplifier) filters known for their smooth, low-distortion sound.
Best for: Pads, big poly sounds
The filter in U-he’s Repro-5 at work on a pad:
Korg MS-20 Filter
Like the Prophet-5, the Korg MS-20 had two different filter topographies during its original lifetime. However, unlike the Prophet, it is definitely not known for being smooth. When people talk about the MS-20 filter, they’re referring to the first revision and its Korg 35 IC design. Its 6dB/Oct highpass and 12dB/Oct lowpass were both extremely resonant and noisy. With a serial filter placement, the MS-20 is capable of bandpass filtering. Later versions of the MS-20 featured a discrete OTA design that was much more restrained.
Best for: Noisy and aggressive leads and lines
Here’s an example of both MS-20 filters working together courtesy of Cherry Audio’s PS-20 emulation:
Oberheim SEM Filter
Originally developed for Tom Oberheim’s Synthesizer Expander Module, the design of the SEM filter is unique in that it’s state variable; it offers more than just a single type of filter at the same time. The SEM has the normal knobs for frequency (cutoff) and resonance but adds a third, which allows you to sweep from lowpass through notch and then to highpass, with a separate bandpass setting as well. Smooth with no self oscillation, the 12 dB/Oct SEM filter sounds great on polyphonic sounds like pads.
Best for: Massive poly sounds, unisons, taming overly digital tones
The SEM filter on the bx_oberhausen from Brainworx/Plugin Boutique:
Roland TB-303 Acid Filter
Sometimes called the acid filter, the one in the Roland TB-303 was a 24dB/Oct diode Ladder design. Although it can’t self-oscillate, it has a distinct resonant peak and it’s this squelch that gives the 303 its unique character.
Best for: Anything acid
Native Instruments Massive with its filter set to Acid:
