Subtractive synthesis is all about the filter – that’s the bit that “subtracts,” after all. In essence, a filter is a tool for removing frequencies from a signal. You can think of it like the chisel that a sculptor uses to remove all of the stone except what they wish to use. Another way to look at it is as a magnifying glass that focuses on one part of the sound and highlights it. Or both – as a chisel with a magnifying glass, removing big sections of sound and focusing attention on what remains!
Filters are typically defined by the set of frequencies they remove, and how much of them they remove. For example, the most common type of filter is the lowpass filter – so named because it passes the lower frequencies, while removing the high ones; highpass and bandpass are other common types. A filter is further defined by its “slope” – its strength, essentially. Slope is usually expressed in multiples of 6 dB/octave, with each such unit sometimes referred to as a “pole” – typically 2 pole (12 dB/octave) or 4 pole (24 dB/octave).
Slope
A filter is further defined by its “slope” – its strength, essentially. Slope is usually expressed in multiples of 6 dB/octave, with each such unit sometimes referred to as a “pole” – typically 2 pole (12 dB/octave) or 4 pole (24 dB/octave). What that means is for each octave above the cutoff frequency, the signal will be reduced by the specified amount. So, a 12 dB/octave filter will remove 12 dB of signal one octave above the cutoff point, 24 dB of signal two octaves above cutoff, and so on. For example, if you have a signal going thru a 12 dB/octave lowpass filter set to 800 hertz (Hz), then the frequencies at 1600 Hz will be attenuated by 12 dB; at 3200 Hz by 24 dB: and so on.
For what it’s worth, you don’t need to think much about this – adjusting a filter to find your desired tone is about the most intuitive musical gesture you can make. What you should know is a filter with a steeper slope – a higher dB/octave number – will attenuate more frequencies than one with a milder slope, offering a more dramatic effect. Some filters offer a variable slope, frequently switchable between 12dB and 24 dB per octave, tho they may offer additional options, both steeper or more shallow. The slope used can vary by musical application, or can simply be a matter of preference.
Mode
As mentioned, filter modes include the near ubiquitous lowpass filter, as well as highpass (attenuates frequencies below the cutoff point), bandpass (attenuates frequencies above and below the cutoff point) and notch (aka band reject; attenuates a narrow band of frequencies centered on the cutoff point). There are additional, less common filter approaches as well, including formant filters (multiple peaks, usually locked in a set relationship to each other), allpass filters (which shift the phase of signals passing thru them but don’t cut specific frequencies), lowpass gates (a kind of combo filter and amp used extensively in so-called “West Coast” Synthesis) and more – those will have to wait for the next filter lesson, however.
Lowpass filters are the most commonly used types. These attenuate signals in a very familiar and musical way – imagine a curtain slowly closing in front of a band playing and how that would make the music quieter and damp the higher frequencies as the curtain dropped – that’s what a lowpass filter does. A highpass filter attenuates the sound from the bottom up, usually muting the fundamental relatively early, resulting in the sound seeming to “evaporate” as the cutoff rises. Bandpass filters can result in both very natural sounding sounds, and very weird ones, depending on the application. Notch filters are great for adding slight timbral shifts or movement without affecting the core sound too much. When you’re first exploring the use of filters in your synth programming and not sure which option to reach for, start with the lowpass – it’ll be right more often than not. Then, you can reach for the other options if that isn’t doing what you think it should or you need a different type of sound than you’re getting.
Some filters offer multiple configurations (lowpass, highpass, bandpass) and slopes (12 dB or 24 dB /octave) in one package, with the option to switch between them internally. Others, especially in modular, offer multiple, simultaneous outputs for each option (e.g. lowpass, bandpass, and highpass outputs that can be used simultaneously). Similarly, some filters’ output can be varied constantly from lowpass thru bandpass to highpass, and that change can even be controlled with envelopes, LFOs and other control signals.
Design
The final consideration for any given filter, and arguably the most important, is the specific filter design used. Whether analog or digital, it turns out there are lots of approaches to filter design, and the specific approach used can, and does, greatly affect the sound of the filter, and thus the sound of the synth. Many sound designers and synth enthusiasts will argue that the filter is the most important component in determining the sound of the synth – and even those that disagree will concur that the filter is a key component of the synth’s sound.
This is one of those areas that’s difficult, even impossible, to summarize. Sure, I can tell you Moog-style filters are considered smooth, rich and even “buttery” by enthusiasts, or the the Korg MS-20 has a filter that’s considered rough, wild and gritty, but until you play with one you aren’t going to have much of an idea of what to expect. The shortest version is to simply point out that the circuit (or software, in digital) design of a filter can and does have a profound effect on the sound of the filter, and thus the synth that features that filter, and that finding a filter that “sounds good” to you is a key step in finding a synth that you jibe with.
It’s common for digital synths, in particular, to offer multiple different “emulated” filter types, allowing you to compare them side-by-side to hear and experience the differences for yourself. If you have one of these synths, take advantage of it! Don’t worry too much about whether or not the emulations are “accurate” emulations; just use the variety to get a sense of the varying sounds of different filters.
Filter Controls & Modulation
A typical filter will offer controls for cutoff frequency and resonance (also called Q, feedback or emphasis in certain contexts). It might also offer switches (or some kind of continuous control, for filters that can pull that off) to change its mode (lowpass, highpass, etc.) or slope (dB/octave). It could also have settings for options such as keyboard tracking (opens the filter more for higher/brighter notes), input level/drive (for filters that can be overdriven) or other, more exotic, options. Often these controls can be modulated by envelopes, LFOs, and other modulation sources in addition to being manually controllable.
Using these controls to modulate filters can create a number of useful effects – modulating cutoff with an LFO gives you a “wah wah” style effect, for example (use a bandpass filter or the classic wah effect). Controlling it with its own envelope allows you to have the brightness/tone change independently of the loudness – for example, you might want the filter to close faster than the amp on a pluck sound, so you perceive it getting less bright before it fades out completely.
Exercises
- Look at the filter or filters available on your synth of choice. Identify each type, making note of what is available and what isn’t, and any “special” modes or types of filters that don’t fit within the core filter types outlined above.
- Pick a single basic waveshape you like and then put it thru each filter in turn (or at least a representative sampling, if your synth offers too many options to reasonably explore in a sitting). See if you can identify noteworthy differences in each filter type, slope, and mode. If some sound extremely similar, or extremely different, take note – both can be useful!
- Create a basic sound (or modify one used in a previous patch) and then use this sound to explore the filter options of your synth. Swap out the filter you used to begin with, for each other type/configuration available – so each slope setting, each mode, each emulation. Make note of the similarities and differences, and perhaps of which types are worthy of future attention. And save any cool variations you came up with along the way, of course!
- Create a different patch using each filter typer/mode/slope available to you (or as much as possible within reason – if you’re on a soft synth with hundreds of emulated and exotic filters, this could be a life’s work!) but at least create a patch using for each of the basic four modes (lowpass, bandpass, highpass, notch) and all slopes, supposing those are available to you.
- Experiment with different filter modulations to create different effects. What happens if you invert your envelope, so it closes the filter instead of opens it? What if two different envelopes modify a filter simultaneously? What if one envelope controls resonance, and another cutoff? What you can do will vary from synth to synth, but try to cover the basics and maybe a few options that feel weird, or pointless, or stupid, because you might be surprised.