There are a lot of types of synthesis. Like, a LOT. However, one of the most common types of synthesis is subtractive. It sounds nice, it’s straight forward, and it’s easy to control. It’s also one of the easiest forms of synthesis to patch in Eurorack. But what makes up a subtractive voice? Let’s quickly go over all the stuff you need!

The oscillator: the thing that actually makes the sound

Obviously, we need something to actually generate sound. That will come from our oscillator(s). A simple patch can have a single oscillator (and great results can be made with just one), but many famous subtractive synthesizers like the Moog Model D and the Korg MS-20 use multiple oscillators to create their sounds. The most common waveforms to use for subtractive patches are saws and square or pulse waves, since they’re the most harmonically rich. This will be important when we get to the filtering stage.

The mixer: combining stuff

While not necessary for a subtractive voice, it’s definitely useful to have a mixer. Even if you’re just using one oscillator, many analog oscillators have multiple waveform outputs that can be mixed together at this stage for more options. And obviously, if you’re using multiple oscillators, you’ll need to mix them together here, too.

The filter: the actual subtractive part of this whole thing

Now the really fun part: filters! Filters, in my opinion, are what give subtractive voices their character. There are loads of different filters in Eurorack. Everyone has their favorites, so it’s worth doing some research and listening to a bunch of different ones before you choose one for your system. Generally filters will have at least two controls: frequency and resonance. Frequency is the point that the filter starts cutting out parts of your sound (hence the name subtractive). There are many different types of filters, the most common being lowpass (cutting off high frequencies and letting low frequencies through), highpass (cutting off low frequencies and letting high frequencies through), and bandpass (cutting off low and high frequencies and letting a band through). I love lowpass filters in my patches, but it’s always worth experimenting with whatever tools you have at your disposal.

Resonance emphasises the frequencies around the cutoff. It’s just another way to shape our patch, and often adds a load of character to our sound (especially when modulated by an envelope, which we’ll get to in a bit).

The VCA: dynamics

Oscillators generate a continuous output. Generally, if we’re going to play or sequence our patch, we want to have some control over volume. A VCA, or Voltage-Controlled Amplifier, allows us to control the volume of our patch. Generally, that’s all a VCA does, although there are some that do things like soft clipping to change the tone of our patch a little bit.

The envelope: controlling everything else

Almost done! In a standard subtractive patch, our envelope generators will control our VCA’s level (to actually create those dynamics) and the frequency cutoff of our filter (to make the sound more like an actual instrument). Most subtractive patches use ADSR envelopes, or Attack-Decay-Sustain-Release. Envelopes are triggered by signals called gates, so you’ll need something in your case to trigger them. I say “them” because often, patches will use multiple envelopes: one to control the VCA, and one to control the filter, to create more interesting and evolving tones.

Sequencing: actually making some music

Last step! All we need now is a way to control our patch. Eurorack generally uses control voltage for pitch which will control our oscillators and gates to control envelopes. There are many sequencers that output pitch and gate signals, or you can use a keyboard with CV and gates if keys are more your style.

Let’s hear some stuff

How about we make some noise? Here’s a quick patch. Our sound starts with two Sinc Iters. This is a paraphonic patch: there are multiple oscillators playing different pitches, but they’re mixed together and go through one filter and VCA.

Speaking of, we’re using the IME Polivoks filter (which happens to have a built-in mixer) as our filter in this context. A lot of famous synthesizers are based off of the basic subtractive format we’ve talked about here, so keep an eye out for filters inspired by your favorite old-school synths.

Everything then goes through a 2hp VCA. We’re also using two 2hp ADSRs for our envelopes. Remember, there are lots of different envelopes and ways to modulate your patch, so experiment with what you have in your rack!

Lastly, we have our sequencing: a Mimetic Digitalis sequences pitches for our two oscillators, Bin Seq is on trigger/gate duty, and those are both clocked by Horologic Solum. And that’s all there is to it!

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