Robin Vincent’s Guide to Making a Basic Synth Voice in Eurorack
“Basic” and “Eurorack” are not two terms that are easily associated. Modular synthesis feels complex right down to its core. But within the advanced forms of modulation, experimental noises and randomising patterns can be found the very essence of synthesis. In fact, where better to uncover the basics of a synth voice than within the stripped-down individual module building blocks of Eurorack?
A Brief History
What does a “basic synth voice” contain? That would certainly be up for a bit of discussion, but I’ll take the liberty of suggesting that for the most prevalent form of analogue synthesis that we can call “subtractive” or “East coast” we need five modules. It can snowball from there depending on what you are trying to achieve, and the realities of Eurorack will require us to mention a couple of other modules along the way; but it’s these five that constitute beating heart, breath and cry of the simple synthesizer. We will need an oscillator, a filter, an amplifier, an envelope and a modulator.
In a traditional synthesizer all the connections are made for you behind the scenes, and it’s easy to be familiar with what the turn of a knob does to the sound without any knowledge about how it is patched to the other components. Eurorack, with its absence of premade connections, makes this unmistakably clear. In Eurorack there are two types of signal flowing through the system that utilise precisely the same patch sockets and cables; these are audio and control voltage (CV). Audio is really just continuously variable and fast-moving voltage, but it’s helpful to see these two things as separate signals because we tend to treat them differently.
Control Voltage or CV is the method by which control over parameters is sent from one module to another. In Eurorack the range is usually +-5V and a modulator outputting that range would normally be assured of putting a parameter through its entire turn. Patching the CV output of an LFO (Low Frequency Oscillator) to the frequency cut-off of a filter would result in the frequency being modulated. The audio patched into the audio input of the filter would be the signal being swept by the filter CV. The audio signal is the sound we want to hear; the CV is controlling what’s happening to that sound.
Armed with these basic concepts, let’s focus on the five modules.
Oscillator – VCO
The oscillator is what normally generates the sound. In Eurorack a sound generating oscillator is known as a VCO (voltage controlled oscillator). In the simplest terms, it’s a sine wave running at a frequency we can hear. In an analogue VCO we usually find sawtooth, triangle, square or pulse waves, as well as the sine wave. It doesn’t have to be analogue generated; it can be digital like a wavetable or sample.
The pitch of the VCO is set by the tuning knob on the module. As Eurorack doesn’t conform to the constraints of a piano keyboard then this pitch could be anything. Pitch in Eurorack works on the basis that 1 volt will offset a pitch by 1 octave. For people used to regular keyboard-based synthesizers this can seem very odd, but it makes pitch much more fluid and versatile. You can patch in a sequence of voltages from a sequencer but you can also add to or mix in CV from other modules to add variation or randomisation to the melody.
At a basic level the VCO is our sound source. Sound is generated by the VCO at all times to all outputs and if you were to patch it to your speakers you would hear it as a drone. But we want more than a drone; we want a synthesizer voice that we can play like an instrument.
Filter – VCF
The filter (known as a Voltage Controlled Filter – VCF) is the most common sound shaping device in synthesis. Filtering frequencies and harmonics allows us to add variation to the tone of the sound. There are three common modes: Low-Pass, High-Pass and Band-Pass. Low-Pass Filters allow low frequencies through and block any frequencies above a set level. High-Pass Filters do the opposite. Band=Pass Filters are like a combined Low-Pass and High-Pass Filter and only the frequencies in between are allowed to pass. Most filters have a Resonance control that boosts frequencies on the edge of the cut-off. In Eurorack the cut-off and resonance can usually be CV controlled.
The sound from the VCO will be patched directly into the VCF for our basic synth voice. On some VCOs you can choose a mix of waveforms, on others each waveform has an individual output so we can choose which one we would like to use in our voice. Alternatively, you could patch them all out to a little mixer module and then come back with a mix to the filter – it’s all about versatility. It’s worth noting that as a filter acts on the harmonics and overtones of a sound, the more interesting or complex the source is, the better. A sine wave, for instance, doesn’t have any harmonics and so there are none to apply the filter to – better off with a square or sawtooth wave.
Amplifier – VCA
At the end of our audio signal chain we have the Voltage Controlled Amplifier, or VCA. It acts as a volume control for the sound. As it’s voltage controlled you could also use another module to control the volume of the output which is very useful as we’ll see in a minute. A VCA can also control the level of CV. If it was desired to modulate the filter cutoff of another module only a small amount, a VCA could be used to reduce the amount of CV being sent to the filter cutoff.
VCAs are often grouped together into single modules which can then act as mixers with a single mix output. This is also useful for mixing CV together to create complex and exciting modulation waveforms.
We now have a tonally interesting drone coming out of our VCA. Next, we can add some dynamics.
Envelope – ADSR
Notes need a shape and that’s where the envelope comes in. The envelope dictates the volume of the sound over time and is most commonly split into four sections. “Attack” is how quickly the sound comes in, so slow for a pad and quick for bass or lead sound. “Decay” is how quickly the sound falls away which works with the “Sustain” to define what happens when you hold a note. “Sustain” is the level of the volume when held and the “Decay” is how fast the sound comes down to that level after it has reached the top of the “Attack”. Finally, you get “Release” which is how long it takes to fade to nothing after you’ve released the note. That gives us our “ADSR” envelope shape but you can also get simpler envelopes like an AD or ADR.
One misconception is that an envelope is applied to the audio signal – it’s not. An envelope generates CV which is then applied to the level control on the VCA. This also means that you can use envelopes on any other CV controllable module such as a filter or even the pitch of a VCO.
Modulator – LFO
A less essential but musically interesting part of our voice is the LFO (Low Frequency Oscillator), it’s an oscillator like a VCO but designed to run at much lower, sub-audio frequencies. You’ll probably know it as vibrato, or tremolo or what happens when you push the modulation wheel on a synth. It pumps its waveform into the cutoff or the pitch or the volume to produce interesting effects. Using a sine wave gives those wobbling effects but an LFO can usually produce other waves such as square waves for a more on/off tremolo effect or random voltages for a bit of craziness. LFO’s add movement and liveliness to synthesizers from long slow sweeps to manic pulsating farts.
Those are the elements of the classic synth voice. A sound source, something to filter harmonics, something to shape how it evolves and a way to make it crazy. And with Eurorack all those parts are laid out in front of you and patched by hand as you craft your sound.
Except for one thing. How on earth do you play it? Good question!
Pitch and Gate
In a regular synthesizer the playing aspect focuses on the piano style keyboard. Eurorack doesn’t demand that connection. All it needs is a CV to vary the pitch and another to trigger the envelope to open the gate on the VCA and let the sound through. How those things are achieved is up to you. You could use a keyboard; some have CV/Gate outputs which could be plugged into the VCO and envelope and off you go, or you can go via a MIDI-to-CV module. The pitch could also be controlled from a sequencer, the random output of an LFO, an envelope or a mixture of different CV sources. The Gate that triggers the envelope to start the journey of controlling the volume can be from a keyboard, but it could just as easily be a pulse from trigger module or square wave LFO or gate sequencer. With Eurorack there is no one way of doing something and that versatility and emphasis on experimentation is what makes it so engaging.
If you’ve never considered Eurorack before, you can get your feet wet with the VCV Rack virtual Eurorack environment. It’s free, open source and has an overwhelming amount of modules waiting for you to patch them together on your computer. The default template that loads when you run the software is the exact sort of synth voice that you’ve now learned about. You can see how it’s patched and experiment with what happens when you connect it up differently. They are using a Mixer module instead of a VCA but it works in the same way and they’ve not included an LFO. But you can add one of those yourself now can’t you?
About The Author
Robin Vincent is a veteran of the computer music industry. Back in the 1990’s he ran the PC Product department at Turnkey, released a couple of books on the subject and helped design the Carillon AC-1 audio PC and bring it to market. From 2006 he ran the UK operation of audio PC manufacturer Rain Computers and in 2013 he formed Molten Music Technology to continue building audio PCs under his own steam. In 2016 he joined the team of journalists writing for music technology news website Gearnews.com taking on hardware and software synthesis duties.
Later on that year Robin launched a new blog, Molten Modular, to follow his journey and exploration of Eurorack modular synthesis.