@synthlet/state-variable-filter

A zero-delay-feedback state variable filter with seven responses

Part of Synthlet.

Seven filter responses out of one topology, switchable by a parameter, with a cutoff built to be swept. BiquadFilterNode covers the same responses but goes unstable when you move its coefficients fast; this uses trapezoidal integration with the feedback solved rather than delayed, so an envelope slammed onto the cutoff stays stable and keeps its resonance.

This is the filter MonoSynth sweeps.

Install

npm i @synthlet/state-variable-filter

Or npm i synthlet for every module at once.

Usage

import {
  registerSvfWorklet,
  Svf,
  SvfType,
} from "@synthlet/state-variable-filter";

const ac = new AudioContext();
await registerSvfWorklet(ac);

const filter = Svf(ac, { type: SvfType.LowPass, frequency: 800, Q: 6 });
osc.connect(filter).connect(ac.destination);

// The cutoff is a-rate: this is a smooth sweep, not a staircase
const env = AdsrEnv(ac, { attack: 0.01, decay: 0.4, gain: 4000, offset: 200 });
env.connect(filter.frequency);

Parameters

| Param | Default | Range | Rate | Meaning | | ----------- | ------- | ---------- | ------ | ------------------------ | | type | 1 | 0 … 6 | k-rate | Response — see SvfType | | frequency | 1000 | 20 … 20000 | a-rate | Cutoff in Hz | | Q | 0.5 | 0.025 … 40 | k-rate | Resonance |

SvfType is ByPass (0), LowPass (1), BandPass (2), HighPass (3), Notch (4), Peak (5) and AllPass (6).

frequency is a-rate, and this package is where that idiom in the library started: filter cutoff is the modulation target in subtractive synthesis, and at k-rate an envelope sweep is a 344 Hz staircase. The coefficients are recomputed per sample when the cutoff is automated and once per block when it is not, so an unmodulated filter costs what it always did.

type is structural — crossing from a lowpass to a highpass mid-block is a discontinuity, not a sweep. Q is k-rate today, and that is an acknowledged gap rather than a principle: Q opening with an envelope is as ordinary a patch as cutoff sweeping with one, and @synthlet/virtual-analog-filter already declares its resonance a-rate.

Every channel of the input is filtered, so a stereo signal stays stereo.

Credits

Ported from SvfLinearTrapOptimised2.hpp in FredAntonCorvest/Common-DSP, MIT licensed, Copyright (c) 2016 Fred Anton Corvest (FAC).

The algorithm is Andrew Simper (Cytomic), Solving the continuous SVF equations using trapezoidal integration and equivalent currents.

Full notice in LICENSE.md and in the repository's THIRD-PARTY-LICENSES.md.

License

MIT © danigb