THE ANATOMY OF A SUBTRACTIVE SYNTH
In modern synths, particularly plugins, the connections between parts of synthesizers are hidden. In a desire to make workflow smooth and straightforward, synth designers do the patching for you, forming instant connections between the components. In the modular synth systems that prompted the synth revolution, these weren’t as immediate, which forced users to carefully learn the inner workings of a synth to wrest the desired sounds from it. Understanding the components of a synth is essential.
Many synth designs and approaches have existed through the years, but the subtractive synth model pioneered by Dr Robert Moog is perhaps the most enduring. In subtractive synthesizers, sound starts in the VCO (Voltage-controlled Oscillator), which produces a waveform. This is a collection of musical harmonics, which sound in different combinations as you explore the waveforms available in your instrument. Some synths provide many waveforms; others offer a more streamlined set. At the Oscillator stage, the waveform is at its ‘fullest’. The reason this synthesizer model is referred to as ‘subtractive’ is due to what happens next…
The output of the Oscillator stage is connected to a Filter, whose job it is to select which of those harmonics are heard and which ones are reduced in volume, or indeed, omitted from the sound entirely. So the Filter attenuates, or cuts, volume, either leaving in the low harmonics and cutting out the top ones (a low-pass filter), letting through the highs and cutting the lows (a high-pass filter), or letting ‘middle’ frequencies through, cutting out those on either side (a band-pass filter). So far, our nascent analogue synth sound exists only in ‘potential’ form – to actually get it to make a noise, we’ll need an Amplifier stage. Even with a connection to the Amplifier established, it’s probably fair to say that our sound is flat and predictable, as there’s nothing to get it moving, or control its behaviour over time. This where modulators come into play.
The two modulators most commonly found in analogue synths are LFOs (Low Frequency Oscillators) and Envelopes. LFOs use a waveform, just like a ‘regular’ oscillator but far slower. LFOs use their output signals as controller waves. We can use an LFO to ‘interrupt’ the Oscillator stage, causing the pitch will ‘wobble’, and to do so at a rate we can notice. A lot of wobble produces pitch variations like a police car siren, with a broad sweep from a high pitch to a low pitch and back again. A much narrower LFO assignment will lead to more subtle pitch wobble, more like the vibrato you’ll hear singers and players of acoustic instruments using, where notes are bent a little sharp and a little flat, to widen their pitch centre (like a constantly fluctuating pitch detune).
Similarly, connecting an LFO to the Filter or Amplifier will produce corresponding tone or volume variations. Meanwhile, Envelopes control sound through time, starting from the onset of every note pressed. Does your sound ‘speak’ immediately, with a heavy attack, before stopping quickly too, like a snare drum? Or is it more constant, with a volume which lasts for as long as you hold a note down? Described here are the stages of a volume Envelope where Attack, Decay, Sustain and Release controls let you determine volume behaviour through a note. Just like LFOs, Envelopes can be applied to Oscillator and Filter stages to control pitch and tone over time too. Modern synths provide plenty more options, but if you understand the fundamentals of Oscillators, Filters, Amplifiers, LFOs and Envelopes, you’re on the road to having the knowledge you need to build killer synth sounds.