Hold the note below and slowly turn Cutoff up from the bottom. Nobody is playing a melody, and the key never changes. But listen closely as the filter opens: you'll hear a little whistled tune climbing out of the buzz, note by note, like a bugle call.
Before you start: will the sweep sound like one smooth "wow", or like separate notes?
Turn Cutoff up slowly while the note holds. Do you hear a smooth glide, or separate notes stepping up?
The whistling sweep
What just happened?
A saw wave contains every harmonic of its note: two times the frequency, three times, four times, and on up. On that low C those harmonics sit at 65, 131, 196, 262 Hz and so on, and in musical terms they spell out the harmonic series: C, C an octave up, G, C, E, G, a slightly flat B flat, C. A bugle has no valves, so this is the only set of notes it can play. That's why the sweep sounds like a bugle call.
Resonance puts a narrow boost just at the cutoff. As you raise the cutoff, that boost slides over the harmonics one at a time and makes each one ring out. Between harmonics there's nothing to boost, so instead of a smooth glide you hear steps. The tune was hiding inside the saw all along; the filter picked it out.
That's the heart of subtractive synthesis: start with a sound rich in harmonics, then carve. The classic signal path has three stages:
The subtractive recipe
This is the recipe Bob Moog built into the Minimoog in 1970, and it's the sound of a huge amount of music since: Bernie Worrell's rubbery Minimoog bass on Parliament's "Flash Light", the squelchy acid lines of a Roland TB-303 on Phuture's "Acid Tracks", the warm pads of 1980s pop. Most of those sounds are a saw or square, one filter and an envelope.
Now try the sweep on a sine. Open the full OSC Mini panel under the lab, set the wave to sine and sweep again. Nothing whistles. A sine has only one harmonic, so the resonant peak has nothing to pick out. Subtractive synthesis needs something to subtract.
Cutoff is a slope, not a wall
A low-pass filter lets low frequencies through and turns down high ones. Its cutoff is where that turning-down begins. It isn't a brick wall: on this synth, harmonics above the cutoff lose 12 dB for every octave you go up, so the ones just above it are still faintly there, and the ones far above it are gone.
Turn resonance down to about 1 and sweep again. Now you hear the classic "wow" of a filter opening: dark and round at the bottom, like a muted horn, then brighter and buzzier as more harmonics pass. With resonance low, the filter sounds polite. With resonance high, it sounds vocal and a bit nasal, and that's the sound people mean when they say a synth "squelches".
Why the same cutoff sounds different on every key
Here's a trap almost everyone falls into. You dial in a lovely bass with the cutoff around 1 kHz. Then you play a melody two octaves higher on the same patch, and it sounds muffled and lifeless. Nothing about the filter changed.
One cutoff, two notes
A low note has lots of harmonics below the cutoff. A high note's harmonics are spread far apart, and most of them land above it. The cutoff number stayed the same; its relationship to the note changed.
Many synths solve this with key tracking, which moves the cutoff up as you play higher. This one doesn't have it, so if a patch needs to work across a wide range, test the lowest and highest notes you'll actually play, and set the cutoff for the compromise. Try it now on the lab above: set Cutoff to about 800 Hz with low resonance, then play a low C and a high C on the keyboard. One is a warm bass; the other is a dull beep.
Let the note open the filter
A filter that stays still sounds like a tone control. A filter that moves sounds like an instrument. The fastest way to make it move is to give it an envelope: a shape that opens the cutoff as the note starts and closes it as the note fades.
On this synth, one envelope does both jobs. Env Amt says how many octaves that same envelope pushes the cutoff up. So the attack, decay, sustain and release you set for the volume also steer the brightness. That's a simplification, and a handy one: the note gets brighter exactly when it gets louder, which is what acoustic instruments tend to do too.
Without touching the wave or the cutoff, can you turn this pluck into a pad, then into a bass?
Bass, pluck and pad from one patch
Try three jobs from one patch:
- Pluck. As loaded: fast attack, short Decay, Sustain at zero. The filter snaps open and falls shut, and you hear a bright "tick" melting into a dark tail. This is the shape of a thousand house and trance stabs.
- Pad. Raise Attack to about a second, Sustain to 0.8, Release to 1.5 s, and lower Env Amt to 1. Now the note fades in and opens slowly, like a string section swelling. Same oscillator, same cutoff, same resonance.
- Bass. Play an octave or two lower, with a short decay and a little sustain. The brief bright start lets you hear each note's attack on small speakers; the dark body keeps it from clashing with everything above.
Nothing about the raw material changed between those three. Only the way the filter and the volume move through time.
Where the recipe breaks
Too much resonance. Push Resonance high on a sustained note and the peak can ring louder than the note itself, especially when it lands right on a harmonic. Turn it down before you turn the whole track down. A whistle that makes an acid line come alive can wreck a background pad.
Too dark. A low cutoff can remove the harmonics that let a bass be heard on phones and laptops. If the bass disappears on small speakers, open the filter a little or let the envelope give each note a brighter front, rather than turning the bass up.
Too long. A long release on a fast line fills every gap. If a bass sounds muddy only in quick passages, check the release before you reach for an EQ cut. The problem is overlap, not tone.
Same idea, different place: you're a subtractive synth. Your vocal folds make a buzzy, harmonic-rich tone, like a saw, and the shape of your mouth filters it into vowels. Say "ooo" then "eee" on one pitch and you're sweeping a filter. A vocoder borrows exactly that changing filter from a voice and puts it on a synth. See Vocoders.
Make it yours
Take the starting bass from the oscillators guide, or the saw above, and make one patch that works as a bass low down and a pluck higher up. Play a low phrase and a fill an octave and a half up. If the fill goes dull, open the cutoff a little and pull Env Amt down to compensate, so the low notes don't get too bright. You're trading cutoff against envelope until both registers work.
When both ends sound right, open OSC Mini from the link at the top of the lab, which carries your settings, and save it as a preset. You've made a sound whose every control you can explain, which means you can fix it in a mix instead of hunting for another preset.
Quick check
- Your bass patch sounds great on the low notes but the high fill sounds muffled. The filter hasn't moved. What's going on, and what would you try? The high notes' harmonics sit mostly above the cutoff, so they lose almost everything. Raise the cutoff a little and reduce the envelope amount, or keep the fill in a lower octave. Without key tracking, every patch has a sweet range.
- You sweep a resonant filter over a sine and over a saw. Only the saw "sings" a little tune. Why? The resonance can only emphasise harmonics that are already there. The saw has a whole ladder of them; the sine has one.
- Your pad is lovely on slow chords but turns to mush when the part speeds up. Do you cut low-mids with EQ, or check something else first? Check the release first. The notes are overlapping. Shortening the release often fixes it while keeping the warmth that an EQ cut would take away.
