@synthlet/virtual-analog-filter

Nine classic analog filter circuits, modelled and switchable

Part of Synthlet.

Where @synthlet/state-variable-filter gives you clean textbook responses, this gives you circuits: the Moog ladder, the Korg 35, the diode ladder and the Oberheim SEM, each with the nonlinearity and the resonance behaviour that make it recognisable. Nine models behind one type parameter.

Install

npm i @synthlet/virtual-analog-filter

Or npm i synthlet for every module at once.

Usage

import {
  registerVirtualAnalogFilterWorklet,
  VirtualAnalogFilter,
} from "@synthlet/virtual-analog-filter";

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

const filter = VirtualAnalogFilter(ac, {
  type: VirtualAnalogFilter.MOOG_LADDER,
  frequency: 900,
  resonance: 0.9,
});

osc.connect(filter).connect(ac.destination);

// a-rate: a smooth sweep, and a 200 Hz modulator is filter FM
lfo.connect(filter.frequency);

Parameters

| Param | Default | Range | Rate | Meaning | | ----------- | ------- | ---------- | ------ | ------------------------------- | | type | 0 | 0 … 8 | k-rate | Which circuit — see below | | frequency | 1000 | 20 … 20000 | a-rate | Cutoff in Hz | | detune | 0 | −127 … 127 | a-rate | Semitones on top of frequency | | resonance | 0.8 | 0 … 1 | a-rate | Resonance |

The nine models are on the factory as constants: VirtualAnalogFilter.MOOG_LADDER (0), MOOG_HALF_LADDER (1), KORG35_LPF (2), KORG35_HPF (3), DIODE_LADDER (4), OBERHEIM_LPF (5), OBERHEIM_HPF (6), OBERHEIM_BPF (7) and OBERHEIM_BSF (8).

frequency, detune and resonance are a-rate, so an envelope or an LFO on the cutoff is a smooth sweep. At one value per render quantum the cutoff was sampled at 344.5 Hz, so anything above 172 Hz folded — a 200 Hz modulator arrived as 144 Hz.

An unmodulated patch does not pay for it. Each model computes its coefficients — a Math.tan, and a Math.pow too in the Korg 35 — at the top of the block, and the filter renders in runs of constant coefficients: one run when nothing is moving, which is exactly what it did before, and 128 when a genuine per-sample sweep needs them. Each channel keeps its own state, so a stereo sweep updates both.

type stays k-rate: it is an index into a bank of nine circuits, and switching it mid-block is a discontinuity rather than a feature.

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

Credits

Generated from the Faust virtual analog filter library (vaeffects.lib): ve.moogLadder by Dario Sanfilippo, and ve.moogHalfLadder, ve.korg35LPF, ve.korg35HPF, ve.diodeLadder and ve.oberheim by Eric Tarr. All are declared under the STK-4.3 licence.

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

License

MIT © danigb