SVF Filter

Module ID filter.svf · Category Filter

The response curve on the node is computed from the filter itself, so it matches what you hear.

The SVF (state variable filter) is the all-purpose filter: one input, four outputs, all running at once. LowPass darkens, HighPass thins, BandPass keeps a band around the cutoff and Notch takes one out. Patch whichever you need, or several at once, without switching modes.

It is a 12 dB/octave filter with an analog-style resonance. Turn the resonance up and the peak grows and saturates rather than getting louder without limit; turn it all the way and the filter sings on its own as a clean sine at the cutoff. Cutoff moves in octaves, so a sweep sounds even from bass to treble, and it stays stable right up to 20 kHz.

The display draws each output's response, the patched ones lit and the rest faint (with nothing patched, LowPass is lit). With CV in Cutoff or Resonance, the curves follow where the CV has taken the filter, a faint copy stays where the knobs put it, and the cutoff's tick on the floor turns orange: an envelope into Cutoff opens and closes the curve with each note. When the resonance is high enough to ring on its own, the peak glows and the display says sings.

Beneath the curves lies the sound itself. What comes into In is a faint blue landscape, and what the lit output lets through is green over it, so the gap between them is what the filter is taking away, and a resonant peak shows where the green rises above the blue. It's scaled to the loudest part of what comes in, so a quiet sound fills the display as well as a loud one, and a polyphonic cable shows all its voices together. When the sound stops, the landscape sinks away and the curve's own glow fills in beneath it.

With CV in Cutoff or Resonance, a thin strip along the floor keeps the last two seconds of what comes out, newest at the top and falling away beneath, brighter where it's louder. As an LFO sweeps the cutoff up and back, each harmonic's column lights as the cutoff passes over it and dims as it moves on, so the sweep leaves its trail through the sound.

For a thicker, steeper lowpass, see the Ladder Filter.

Inputs

Cutoff and Resonance modulate around their knobs. The knob sets the center, and stays live while a cable is patched in.

Outputs

Parameters

The four outputs

LowPass is the classic subtractive synth filter. It rolls off everything above the cutoff at 12 dB per octave, taking the edge off a bright oscillator. Most patches start here.

HighPass is the mirror image: it removes everything below the cutoff. Use it to thin a sound out, clear rumble, or leave room for a bass part.

BandPass keeps a band around the cutoff and rolls off both sides. The band narrows as resonance rises. It gives vocal, nasal and telephone-like tones, and swept by an envelope or LFO it becomes a wah.

Notch removes a band around the cutoff and leaves the rest. The notch is deepest exactly at the cutoff and narrows as resonance rises. It equals LowPass and HighPass added together. Swept slowly, it gives a phaser-like movement that hollows a sound without darkening it.

Resonance and self-oscillation

Resonance feeds part of the filter's output back into itself, which adds a peak at the cutoff. A little (20 to 40%) adds character; a lot (70 to 90%) gives the squelchy, whistling peak of an acid bassline. The feedback runs through a saturator, so a loud input at high resonance pushes back instead of running away.

Above about 97% the filter self-oscillates:

• It produces a sine at the cutoff frequency, with nothing patched into In. The oscillation grows out of a tiny noise floor, like circuit noise in an analog filter.
• The pitch lands within a few cents of the Cutoff knob, so the knob reads as a pitch.
• The saturator holds the level at about -12 dBFS.
• Patch a keyboard's Pitch into Cutoff and it plays in tune.

Feed audio in while it oscillates and the two interact, with the resonance pushing back against loud input.

CV Amt

CV Amt sets how far the Cutoff input reaches: how many octaves one unit of CV moves the cutoff. It runs from -4 to +4 octaves per unit, and starts at 1, the V/Oct scale.

An envelope runs from 0 to 1, so CV Amt is the size of its sweep, the knob a hardware synth calls Env Amount or Contour:

• 1 opens the filter one octave at the envelope's peak. Gentle: a sound that brightens a little at each note.
• 2 to 3 suits a pluck or a brassy swell.
• 3 to 4 is acid: a resonant peak that shoots up from the bass to the treble and falls back.
• Negative values turn the CV upside down. An envelope then closes the filter as the note starts and opens it again as it releases, so set the Cutoff knob high.

With pitch on Cutoff, CV Amt is the keyboard tracking: 1 keeps the filter's brightness the same on every note, 0.5 tracks at half the rate, and 0 ignores the keys.

CV Amt scales whatever is patched into Cutoff before it's added to the Cutoff knob, so the knob still sets where the sweep starts. With a polyphonic envelope, each voice's sweep is scaled the same way.

Drive

Drive raises the level into a soft saturator before the filter. At 1x it is nearly clean. Turning it up thickens the sound and then adds grit, and the louder signal also pushes harder against the resonance.

Patch examples

Basic filtering

[Oscillator Out (Saw)] ──> [SVF Filter In]
[SVF Filter LowPass] ──> [VCA In] ──> [Audio Output]

Start with the cutoff around 1000 Hz and Res at 20 to 40%, then turn the cutoff down for a darker sound.

Filter envelope

[Note In Gate] ──> [ADSR Gate]
[ADSR Out] ──> [SVF Filter Cutoff]

Set the Cutoff knob low (200 to 500 Hz) and CV Amt to 2 or 3. The envelope's 0-to-1 output raises the cutoff by up to CV Amt octaves as it plays, so the knob sets where the sweep starts and CV Amt how far it goes. Add resonance to make the sweep more pronounced. A fast attack and decay gives a pluck; a slow attack, a swell.

Keyboard tracking

[Note In Pitch] ──> [Oscillator V/Oct]
                ──> [SVF Filter Cutoff]

The filter follows the notes you play, so high notes are as bright as low ones. At the default CV Amt of 1 the Cutoff input is on the V/Oct scale, so no scaling is needed. Turn CV Amt down to 0.5 for half tracking, so high notes come out a little darker.

Parallel modes

[Oscillator Out] ──> [SVF Filter In]
[SVF Filter LowPass] ──> [Mix In 1]
[SVF Filter BandPass] ──> [Mix In 2]
[Mix Out] ──> [Audio Output]

Blending lowpass with a little bandpass gives body plus a resonant edge.

Moving notch

[Oscillator Out (Saw)] ──> [SVF Filter In]
[LFO Out (slow)] ──> [SVF Filter Cutoff]
[SVF Filter Notch] ──> [Audio Output]

A slow LFO sweeping the notch gives a phaser-like whoosh.

Polyphony

The SVF Filter is polyphonic. Each voice on a polyphonic cable gets its own filter, with its own resonance, so a chord into In with an envelope on Cutoff opens each note separately. The knobs are shared by every voice. See Polyphony.

Bypass

Click the power switch at the left of the header, choose Bypass from the module's right-click menu, or press Ctrl + B to take the filter out of the signal path. All four outputs then pass In straight through, and the header and controls dim. Switching takes a 20 ms crossfade, so it doesn't click.

Related modules

• Ladder Filter, the steeper, saturating lowpass
• ADSR Envelope to sweep the cutoff with each note
• LFO for wobbles and phaser sweeps
• Attenuverter to offset a cutoff sweep, or to scale one CV for several modules at once