slightly better

public/worklets/svf-processor.js

@@ -0,0 +1,103 @@
+class SVFProcessor extends AudioWorkletProcessor {
+  static get parameterDescriptors() {
+    return [
+      {
+        name: 'frequency',
+        defaultValue: 1000,
+        minValue: 20,
+        maxValue: 20000,
+        automationRate: 'k-rate'
+      },
+      {
+        name: 'resonance',
+        defaultValue: 0.707,
+        minValue: 0.05,
+        maxValue: 10,
+        automationRate: 'k-rate'
+      }
+    ]
+  }
+
+  constructor() {
+    super()
+
+    this.mode = 'lowpass'
+    this.ic1eq = 0
+    this.ic2eq = 0
+
+    this.port.onmessage = (event) => {
+      const { type, value } = event.data
+      if (type === 'mode') {
+        this.mode = value
+      }
+    }
+  }
+
+  process(inputs, outputs, parameters) {
+    const input = inputs[0]
+    const output = outputs[0]
+
+    if (input.length === 0 || output.length === 0) {
+      return true
+    }
+
+    const inputChannel = input[0]
+    const outputChannel = output[0]
+    const frequency = parameters.frequency
+    const resonance = parameters.resonance
+
+    const sampleRate = globalThis.sampleRate || 44100
+    const isFreqArray = frequency.length > 1
+    const isResArray = resonance.length > 1
+
+    for (let i = 0; i < inputChannel.length; i++) {
+      const freq = isFreqArray ? frequency[i] : frequency[0]
+      const res = isResArray ? resonance[i] : resonance[0]
+
+      const g = Math.tan(Math.PI * Math.min(freq, sampleRate * 0.49) / sampleRate)
+      const k = 1 / Math.max(0.05, res)
+
+      const inputSample = inputChannel[i]
+
+      const a1 = 1 / (1 + g * (g + k))
+      const a2 = g * a1
+      const a3 = g * a2
+
+      const v3 = inputSample - this.ic2eq
+      const v1 = a1 * this.ic1eq + a2 * v3
+      const v2 = this.ic2eq + a2 * this.ic1eq + a3 * v3
+
+      this.ic1eq = 2 * v1 - this.ic1eq
+      this.ic2eq = 2 * v2 - this.ic2eq
+
+      const lp = v2
+      const bp = v1
+      const hp = inputSample - k * v1 - v2
+      const notch = inputSample - k * v1
+
+      let outSample
+      switch (this.mode) {
+        case 'lowpass':
+          outSample = lp
+          break
+        case 'highpass':
+          outSample = hp
+          break
+        case 'bandpass':
+          outSample = bp
+          break
+        case 'notch':
+          outSample = notch
+          break
+        default:
+          outSample = lp
+      }
+
+      outputChannel[i] = outSample
+    }
+
+    return true
+  }
+}
+
+registerProcessor('svf-processor', SVFProcessor)