Filters: Cutoff, Resonance, Slope and Key Tracking

Subtractive synthesis gets its name from the filter. The oscillator supplies a rich set of harmonics; the filter removes the ones you do not want. Most of a synth's tone, and much of its movement, comes from what the filter is doing.

Filter types

Type	Passes	Removes	Common use
Low-pass (LP)	Below cutoff	Above cutoff	Darkening, basses, pads, most sounds
High-pass (HP)	Above cutoff	Below cutoff	Thinning, removing low end from pads
Band-pass (BP)	Around cutoff	Both sides	Telephone, nasal, vocal-like tones
Notch	Everything else	A narrow band at cutoff	Phaser-like movement

A low-pass filter is the default in almost every classic synth, because most natural sounds lose brightness as they get quieter or fade out.

Cutoff

The cutoff is the frequency where the filter starts acting. On a low-pass filter, lowering it removes more of the upper harmonics and the sound gets darker. A saw bass with the cutoff at 300 Hz sounds round and dull; at 3 kHz it sounds bright and aggressive.

Cutoff is the most-modulated control in synthesis. Sweeping it slowly over 8 bars is a build-up; opening it quickly at the start of each note is a pluck. That movement is what makes a synth sound alive.

Resonance

Resonance (also called Q or emphasis) boosts a narrow band right at the cutoff frequency. A little resonance makes the filter sound more focused and gives sweeps a vocal "wow" quality. A lot makes it whistle at the cutoff frequency, and on many analog-style filters very high resonance makes the filter self-oscillate into a sine tone.

• 0–20%: transparent shaping.
• 20–50%: characterful, audible peak, classic for acid and sweeps.
• 60%+: pronounced whistle; useful for effects, risky for sustained parts.

High resonance also tends to thin out the low end on many filter designs, so a resonant bass may need a separate sub layer.

Slope

The slope is how steeply the filter cuts beyond the cutoff, measured in decibels per octave.

Slope	Also called	Character
6 dB/oct	1-pole	Very gentle, like a tone knob
12 dB/oct	2-pole	Smooth, keeps some brightness; Oberheim-style
18 dB/oct	3-pole	In between; less common
24 dB/oct	4-pole	Steep, thick, dark; Moog-style

As a worked example, set a low-pass at 1 kHz. With a 12 dB/oct slope, content at 2 kHz is about 12 dB quieter and at 4 kHz about 24 dB quieter. With a 24 dB/oct slope the same frequencies are about 24 and 48 dB quieter. Choose 24 dB/oct for basses and dark pads where you want the top truly gone, and 12 dB/oct for brass, strings and leads that should stay open and airy.

Key tracking

Without key tracking, the cutoff stays at the same frequency whatever note you play. A low note keeps many of its harmonics under a 1 kHz cutoff; a high note might have only its fundamental left. High notes sound dull and low notes sound bright.

Key tracking (or keyboard follow) moves the cutoff up as you play higher.

• 0%: cutoff fixed. Fine for basses played in a narrow range.
• 50%: cutoff moves half as fast as the pitch; a common compromise.
• 100%: cutoff moves exactly with the pitch, so every note has the same harmonic balance. Good for leads and pads played across several octaves.

Drive and filter character

Many synths put a drive or saturation stage before or inside the filter. Driving a filter adds harmonics, which the filter then shapes, and it is a large part of why analog-modelled filters sound "fat". Add drive after setting cutoff and resonance, and lower the output to keep the level fair when comparing.

A listening routine

Load a single saw, low-pass at 24 dB/oct. Hold a chord and:

1. Sweep the cutoff from 100 Hz to 10 kHz slowly.
2. Repeat with resonance at 40%.
3. Switch to 12 dB/oct and repeat.
4. Play a two-octave scale with key tracking at 0% and then 100%.

Those four passes teach more than any diagram. In envelopes you will automate the cutoff per note.