Filters: carving the sound

If oscillators are the clay, filters are the chisel. Most of the classic synth sounds you know, from squelchy acid bass to warm brass to dark pads, are a bright waveform with part of it carved away. That's called subtractive synthesis: start with too much, then take away.

This guide is about why that works so well, and how to make a filter move.

What a filter does

Remember from the oscillators guide that a bright sound is full of harmonics: quieter copies of the note at 2×, 3×, 4× its frequency and up. A filter turns some of those frequencies down.

The most common kind, and the one you'll use most, is the low-pass filter (LP). It lets the low frequencies pass and cuts the high ones. Everything below its Cutoff gets through; everything above is turned down, more and more the higher you go. Lower the cutoff and the sound gets darker and more muffled, like hearing music through a wall.

The Filter module has four modes:

• LP (low-pass): keeps the lows. Darker. The classic synth filter.
• HP (high-pass): keeps the highs. Thinner, airier.
• BP (band-pass): keeps a band around the cutoff. Nasal, focused, telephone-like.
• NOTCH: removes a band around the cutoff and keeps the rest.

Why filter a sawtooth?

A filter can only remove what's there. A sine wave has no harmonics, so filtering it does almost nothing. A sawtooth has every harmonic, so there's lots to carve away, and every cutoff setting sounds different. That's why the classic starting point is a saw (or a square) into a low-pass filter.

Resonance: the whistle at the edge

The Res knob boosts the frequencies right at the cutoff. A little resonance makes the filter's edge more pronounced, so when the cutoff moves you can really hear it move. A lot of resonance makes the filter whistle or "squelch" at the cutoff frequency. Turn it right up and the filter starts to ring on its own.

One envelope, two jobs

A filter that sits still is fine, but the magic happens when the cutoff moves. And the most useful thing to move it with is the same envelope that shapes the note's volume.

Why does that sound so good? Because real instruments behave that way. Pluck a string, strike a piano key or blow hard into a trumpet and the start of the note is bright, full of harmonics. As the note fades, the high harmonics die away first and it gets mellower. An envelope sweeping a low-pass filter copies exactly that: bright at the start of each note, darker as it fades.

In a modular, any output can go to as many inputs as you like, so one envelope can open the VCA and sweep the filter at the same time. The filter's CV Amt knob sets how far the envelope moves the cutoff.

Keeping high notes bright

There's one more cable in that patch: the keyboard's pitch also goes into the filter's V/OCT input. Without it, a high note sits closer to the cutoff than a low note does, so high notes sound duller. With it, the cutoff follows the keyboard, and every note sounds equally bright. This is called key tracking.

Filtering noise

Filters aren't only for oscillators. Noise contains every frequency at once, so a filter can pick out any part of it. Keep a narrow band with a resonant BP filter and sweep it slowly: wind. Keep only the very top with HP: the sizzle of a hi-hat.

Lesson 11 builds both at once, with a clock turning the hiss into a hi-hat rhythm. It'll make more sense after the rhythm guide.

Formants: filters that talk

Your mouth is a filter too. When you say "aah" or "ooh", your vocal cords make the same buzzy tone, and the shape of your mouth filters it, boosting a few bands called formants. Each vowel has its own pattern of formants. That's how you can hear the difference between "ee" and "oo" sung on the same note.

The Vowel module is a set of filters tuned to those patterns. Feed it a buzzy saw and it starts to sound like a voice.

Lesson 15 lets an LFO move Shift, so the voice keeps changing as you play.

EQ: turning parts up and down

An EQ is a gentler cousin of the filter. Instead of cutting everything past a point, it turns broad areas of the sound up or down: Low (bass), Mid (a middle band you choose with Mid Freq) and High (treble). Filters are for making sounds; EQ is mostly for fitting them together, which is why you'll usually find it at the end of a patch.

What to remember

• A low-pass filter keeps the lows and turns down everything above its Cutoff. Lower cutoff, darker sound.
• Filter something bright (saw, square, noise): you can only remove harmonics that are there.
• Resonance boosts the cutoff frequency, so you can hear the filter move. Lots of it squelches and whistles.
• An envelope on the cutoff makes notes start bright and get darker, like real instruments do.
• Key tracking (pitch into the filter's V/OCT) keeps high notes as bright as low ones.
• Formants are the filter patterns of vowels; EQ turns broad parts of a sound up or down.