Filters

Filters shape the harmonic content of sound by attenuating certain frequencies while passing others.

SVF (State-Variable Filter)

A versatile 12dB/octave TPT / zero-delay-feedback (ZDF) filter with four simultaneous outputs and stable self-oscillation. type_id: svf.

let vcf = patch.add("vcf", Svf::new(44100.0));

Inputs

Outputs

Transfer Functions

Lowpass: $$H_{LP}(s) = \frac{\omega_c^2}{s^2 + \frac{\omega_c}{Q}s + \omega_c^2}$$

Highpass: $$H_{HP}(s) = \frac{s^2}{s^2 + \frac{\omega_c}{Q}s + \omega_c^2}$$

Bandpass: $$H_{BP}(s) = \frac{\frac{\omega_c}{Q}s}{s^2 + \frac{\omega_c}{Q}s + \omega_c^2}$$

Cutoff Mapping

Resonance Behavior

Resonance sets the ZDF damping factor k = 2 − 2·res: res = 0 gives k = 2 (Q ≈ 0.5), and res → 1 drives k → 0 (near-infinite Q). Integrator states are soft-clipped, so high resonance self-oscillates as a slow-decay sine ring at the cutoff frequency rather than blowing up.

DiodeLadderFilter

Classic 24dB/octave (4-pole) TB-303/Moog-style ladder filter with diode saturation. type_id: diode_ladder.

let ladder = patch.add("filter", DiodeLadderFilter::new(44100.0));

Inputs

Outputs

Characteristics

• 24dB/octave slope (4-pole)
• Diode saturation per stage
• Warm, slightly dirty character
• Resonance with bass loss (like original Moog)

The Ladder Topology

flowchart LR
    IN[Input] --> S1[Stage 1<br/>-6dB/oct]
    S1 --> S2[Stage 2<br/>-6dB/oct]
    S2 --> S3[Stage 3<br/>-6dB/oct]
    S3 --> S4[Stage 4<br/>-6dB/oct]
    S4 --> OUT[Output<br/>-24dB/oct]
    S4 -->|Resonance| IN

ParametricEq

Three-band equalizer — low shelf, parametric mid (with Q), high shelf — using cached biquads. Each band spans ±12 dB. type_id: parametric_eq.

let eq = patch.add("eq", ParametricEq::new(44100.0));

Inputs

Outputs

Filter Modulation Techniques

Envelope → Filter

Classic brightness sweep:

patch.connect(env.out("env"), vcf.in_("cutoff"))?;
// Fast decay = plucky, slow decay = pad

LFO → Filter

Rhythmic movement:

patch.connect(lfo.out("sin"), vcf.in_("fm"))?;

Keyboard Tracking

Higher notes = higher cutoff:

patch.connect(pitch.out("out"), vcf.in_("keytrack"))?;
// Set the amount (0-1) via the `keytrack_amt` input; 1.0 = cutoff follows pitch

Audio-Rate FM

Metallic/vocal effects:

// Use oscillator as modulation source
patch.connect(vco2.out("sin"), vcf.in_("fm"))?;

Response Curves

dB
 0 ├──────────────┐
   │               ╲
-6 ├                ╲
   │                 ╲ LP
-12├                  ╲
   │                   ╲
-24├                    ╲
   └────────────────────────
           fc          Frequency

Common Settings