The whole idea is removal
A subtractive synthesiser does not build complexity from nothing. It generates a sound that already contains far more than you want, then subtracts. An oscillator produces a fundamental plus a whole series of harmonics; a filter is what decides which of those survive.
This is worth pausing on, because it inverts a common intuition. The oscillator is not the instrument. The filter is.
The oscillator is raw material
Different waveforms carry different harmonic content. A sawtooth is rich and harmonically complete, which makes it a good starting point for subtractive work. A sine is almost pure, so there is less to remove and less character to find.
Detuned oscillators, which is most of them, produce beating between the slightly different frequencies. That thickness is useful, and it is also why an over-detuned pad turns to mush in a dense mix.
The filter decides the character
A low pass filter closes down brightness and leaves weight. A high pass removes low content and leaves air and definition. Band pass keeps a region and discards the rest. Resonance emphasises the cutoff frequency itself, which is how you get the classic vocal or acid quality.
Resonance is the most abused control on most subtractive synths. Because it boosts as it filters, it eats headroom elsewhere in the spectrum. Use it, but check what it costs you before settling on a setting.
Envelopes turn a sound into an event
A static filtered tone is just a tone. What makes it a note is how the filter moves over time. An envelope on the cutoff, from bright to dark, is the classic way to give a sound a beginning and an end.
Short envelopes produce plucks. Long ones produce swells. Most patches that fail do so because the envelope is wrong for the role, not because the filter is.
Design for the role, not the sound
The most useful question when building a patch is not what it sounds like alone. It is what it is doing in the arrangement. A pad that occupies the same frequency range as the lead is not a pad, it is a fight waiting to happen.
Synth patch design for mixing develops this properly. The short version is to decide the role first and let the frequency space follow from it.
Then decide the reference
However the patch is built, it will be built against whatever reference the session is running at. If the track needs a different reference, the cleanest place to change it is at the end, on the finished mix.
432Hz MASTER moves a mix from 440Hz to 432Hz without touching tempo. RASTA 528Hz shifts to the 528Hz Solfeggio reference. Both leave the internal relationships between your synth parts exactly as you mixed them.
Choosing a waveform for the job
The waveform decides how much material the filter has to work with, and therefore how much character is available and how hard you have to work to find it.
| Waveform | Harmonic content | Subtractive potential |
|---|---|---|
| Sine | Fundamental only | Very little to remove; clean but plain |
| Triangle | Fundamental with a few odd harmonics | Moderate; smooth and less abrasive than a saw |
| Sawtooth | Full harmonic series, all harmonics present | Maximum; the usual starting point |
| Square | Odd harmonics only, hollow | Good; a distinct timbre before filtering |
| Noise | Broadband, no pitch structure | Very high; used for texture rather than tone |
The practical rule is to start with the richest waveform that suits the role and filter down from there. Beginning with a sine and trying to add brightness later is the harder direction to work in, because there is nothing to reveal.
The bottom line
Subtractive synthesis is a simple idea executed well or badly: start with more than you need, and remove until only the part you want is left. The filter is where the character lives, and the oscillator is only the material. Design with a role in the arrangement in mind, because that is what decides how much frequency space the patch should occupy.
Hear the difference in tuning
A filtered patch built at 440Hz still needs a reference decision. 432Hz MASTER and RASTA 528Hz handle it on the master bus.
Frequently asked questions
What does subtractive synthesis mean?
It means building a sound by taking frequencies away from a rich source. Start with an oscillator that generates harmonics, then use a filter to remove the ones you do not want.
What is the difference between subtractive and additive synthesis?
Subtractive removes frequencies from a complex source. Additive builds a sound by stacking individual sine waves at chosen frequencies. Subtractive is more common because one oscillator with a filter can cover a huge range.
What does a low pass filter do?
It passes low frequencies and attenuates high ones, with a cutoff point and a slope. Closing the cutoff removes brightness and opens up darkness.
How do I make a synth sit better in a mix?
Give it a job. Decide whether it is a lead, a pad or a texture, and leave frequency space for everything else. A patch designed to be the loudest thing in the arrangement rarely needs to be.