fix(core): reverb level, phaser wiring, per-channel dynamics, one-pole rebuild

Four defects in the graph builders, all found by review rather than by ear.

**Reverb was unusable at its own defaults.** A ConvolverNode applies the
impulse's gain whole — the graph sets `normalize = false` so a room is
deterministic rather than browser-defined — but the impulse was raw decaying
noise. Measured L2 at the registry default (size 0.7 / damping 0.5): 46.4, or
+33.3 dB, putting the wet path ~24 dB over dry at the default `wet: 0.35`. It is
now normalised to unit energy, so the wet knob means what it says. Preview and
render both convolve this buffer, so they stayed identical throughout — equally
deafening before, equally correct now.

**Phaser in_gain/out_gain trim the signal entering and leaving the effect**, not
a wet/dry pair. Wired to the wet and dry legs, "Input" muted the dry path and
the two defaults summed to 1.14, so inserting a phaser raised the track level.
They are now input and output trims with the legs summed at unity. Its declared
waveform is also honoured: `lfo.type` was never assigned, so the default
"Triangular" was silently a sine.

**The dynamics worklets held one envelope across a channel-major loop.** The
followers advance per sample, so on stereo a 20 ms attack behaved as 10 ms, and
the right channel's gain came from an envelope that had already traversed the
left — the two ducked differently from the same input and the image pumped.
State is now per channel, as is the gate's smoothed gain and bitcrush's
sample-hold counter, which previously advanced only on the last channel and left
every earlier one frozen for a whole quantum. The gate also honours the knee it
declares instead of chattering on material sitting at the threshold.

**A one-pole filter's cutoff was swallowed in preview.** Its coefficients are
fixed at construction, so `update` cannot push a new frequency — but the shape
signature carried only type and pole count, so a cutoff change looked like a
values-only edit and went into a no-op updater. Preview kept filtering at the
old frequency while the render used the new one: a preview/render divergence in
exactly the two effects that do not use a BiquadFilterNode.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

.fallowrc.jsonc

@@ -146,6 +146,17 @@
       "file": "packages/studio/src/utils/studioHelpers.ts",
       "exports": ["resolveDroppedAssetDimensions"],
     },
+    // Audio FX worklets sit near the bottom of the audio stack: the worklet
+    // source and its test reset are consumed by the runtime and engine PRs
+    // upstack, so a per-PR audit against the merge base sees them as unused.
+    {
+      "file": "packages/core/src/audio/audioFxWorklets.ts",
+      "exports": ["AUDIO_FX_WORKLET_SOURCE", "__resetAudioFxWorkletsForTests"],
+    },
+    {
+      "file": "packages/core/src/audio/audioFxGraph.ts",
+      "exports": ["ensureAudioFxWorklets"],
+    },
     // drawElementService is the bottom of the fast-capture Graphite stack
     // (#1917): its consumers (frameCapture in #1919) land two PRs upstack, so
     // a per-PR audit diffing against the merge base sees these exports as

packages/core/src/audio/audioFxGraph.test.ts

@@ -274,3 +274,70 @@ describe("synthesizeReverbImpulse", () => {
     expect(Math.max(...Array.from(tail, Math.abs))).toBeLessThan(0.01);
   });
 });
+
+describe("levels and per-channel state", () => {
+  const energy = (ir: Float32Array): number => {
+    let e = 0;
+    for (const v of ir) e += v * v;
+    return Math.sqrt(e);
+  };
+
+  it("normalises the reverb impulse, so adding one does not clip the mix", () => {
+    // A ConvolverNode applies the impulse's gain whole (normalize is off so the
+    // room is deterministic). Raw decaying noise ran to +33 dB at the registry
+    // default, which put the wet path ~24 dB over dry at wet: 0.35.
+    for (const [size, damping] of [
+      [0.7, 0.5],
+      [1, 0],
+      [0, 1],
+      [0.5, 0.5],
+    ]) {
+      expect(energy(synthesizeReverbImpulse(48000, size, damping))).toBeCloseTo(1, 5);
+    }
+  });
+
+  it("keeps a phaser at unity, rather than summing its two gains above it", () => {
+    // in_gain/out_gain trim what enters and leaves the effect; wired as a wet/dry
+    // pair their defaults summed to 1.14, so inserting a phaser raised the level.
+    const ctx = new FakeCtx();
+    buildFxNode(ctx as unknown as BaseAudioContext, "phaser", defaultAudioFxParams("phaser"));
+    const gains = ctx.created.filter((n) => n.kind === "gain").map((n) => n.gain.value);
+    // The wet and dry legs are summed at unity; the trims carry the knob values.
+    expect(gains.filter((g) => g === 1).length).toBeGreaterThanOrEqual(2);
+  });
+
+  it("sets the phaser LFO waveform it declares", () => {
+    const ctx = new FakeCtx();
+    buildFxNode(ctx as unknown as BaseAudioContext, "phaser", {
+      ...defaultAudioFxParams("phaser"),
+      type: "0",
+    });
+    const osc = ctx.created.find((n) => n.kind === "osc");
+    expect(osc?.type).toBe("triangle");
+    const ctx2 = new FakeCtx();
+    buildFxNode(ctx2 as unknown as BaseAudioContext, "phaser", {
+      ...defaultAudioFxParams("phaser"),
+      type: "1",
+    });
+    expect(ctx2.created.find((n) => n.kind === "osc")?.type).toBe("sine");
+  });
+
+  it("rebuilds a one-pole filter when its cutoff moves", () => {
+    // Its coefficients are fixed at construction, so a cutoff change cannot be
+    // pushed into the running graph — preview kept filtering at the old
+    // frequency while the render used the new one.
+    const ctx = new FakeCtx() as unknown as BaseAudioContext;
+    const chain = (frequency: number): HfAudioFxChain => ({
+      version: 1,
+      nodes: [{ type: "highpass", enabled: true, params: { frequency, q: 0.707, poles: "1" } }],
+    });
+    const built = buildFxChain(ctx, chain(300));
+    expect(built.update(chain(2000))).toBe(false);
+    // A two-pole filter is a BiquadFilterNode and still updates in place.
+    const twoPole = (frequency: number): HfAudioFxChain => ({
+      version: 1,
+      nodes: [{ type: "highpass", enabled: true, params: { frequency, q: 0.707, poles: "2" } }],
+    });
+    expect(buildFxChain(ctx, twoPole(300)).update(twoPole(2000))).toBe(true);
+  });
+});

packages/core/src/audio/audioFxGraph.ts

@@ -38,9 +38,22 @@ export function synthesizeReverbImpulse(
   };
   const cutoff = Math.max(0.001, 1 - Math.max(0, Math.min(1, damping)));
   let lp = 0;
+  let energy = 0;
   for (let i = 0; i < length; i++) {
     lp += cutoff * (rand() - lp);
-    out[i] = lp * Math.pow(1 - i / length, 2.5);
+    const sample = lp * Math.pow(1 - i / length, 2.5);
+    out[i] = sample;
+    energy += sample * sample;
+  }
+  // Scale to unit energy. A ConvolverNode applies the impulse's gain whole (the
+  // graph sets `normalize = false` so the room is deterministic rather than
+  // browser-defined), and this impulse is decaying noise whose raw energy runs
+  // to +33 dB at the default size — loud enough that simply adding a Reverb
+  // clipped the mix. Normalising here keeps the wet knob meaning what it says
+  // and keeps preview and render identical, since both convolve this buffer.
+  const norm = Math.sqrt(energy);
+  if (norm > 0) {
+    for (let i = 0; i < length; i++) out[i] = (out[i] ?? 0) / norm;
   }
   return out;
 }
@@ -216,12 +229,19 @@ const PHASER_STAGES = 6;
 const allpassPhaser: Builder = (ctx, p) => {
   const input = ctx.createGain();
   const out = ctx.createGain();
+  // aphaser's in_gain/out_gain trim the signal entering and leaving the effect.
+  // Wiring them to the wet and dry legs instead made "Input" mute the dry path
+  // and let the two defaults sum above unity, so inserting a phaser raised the
+  // track level.
+  const inTrim = ctx.createGain();
+  const outTrim = ctx.createGain();
   const lfo = ctx.createOscillator();
   const depth = ctx.createGain();
   const wet = ctx.createGain();
   const dry = ctx.createGain();
   const stages: BiquadFilterNode[] = [];
-  let node: AudioNode = input;
+  input.connect(inTrim);
+  let node: AudioNode = inTrim;
   for (let i = 0; i < PHASER_STAGES; i++) {
     const ap = ctx.createBiquadFilter();
     ap.type = "allpass";
@@ -232,9 +252,14 @@ const allpassPhaser: Builder = (ctx, p) => {
     stages.push(ap);
   }
   lfo.connect(depth);
+  // aphaser's type 0 is triangular, 1 sinusoidal. The builder never set this, so
+  // the declared default ("Triangular") was silently a sine. An OscillatorNode
+  // has no triangle-with-the-same-phase primitive to switch to, so triangle is
+  // the node's own "triangle" type.
   lfo.start();
-  node.connect(wet).connect(out);
+  node.connect(wet).connect(outTrim);
-  input.connect(dry).connect(out);
+  inTrim.connect(dry).connect(outTrim);
+  outTrim.connect(out);
   const apply = (v: HfAudioFxParamValues): void => {
     // aphaser sweeps around a centre derived from its delay; mirror the range
     // rather than the exact curve, and let the parity harness score it.
@@ -242,8 +267,12 @@ const allpassPhaser: Builder = (ctx, p) => {
     for (const ap of stages) ap.frequency.value = centre;
     depth.gain.value = centre * n(v.decay);
     lfo.frequency.value = n(v.speed);
-    wet.gain.value = n(v.out_gain);
+    lfo.type = String(v.type) === "1" ? "sine" : "triangle";
-    dry.gain.value = n(v.in_gain);
+    inTrim.gain.value = n(v.in_gain);
+    outTrim.gain.value = n(v.out_gain);
+    // Summed at unity: the sweep is the effect, not a blend control.
+    wet.gain.value = 1;
+    dry.gain.value = 1;
   };
   apply(p);
   return {
@@ -256,7 +285,7 @@ const allpassPhaser: Builder = (ctx, p) => {
       } catch {
         /* already stopped */
       }
-      [input, out, depth, wet, dry, ...stages].forEach((x) => x.disconnect());
+      [input, out, inTrim, outTrim, depth, wet, dry, ...stages].forEach((x) => x.disconnect());
     },
   };
 };
@@ -351,7 +380,13 @@ function shapeOf(chain: HfAudioFxChain): string {
     .map((node) => {
       const p = normalizeAudioFxParams(node.type, node.params);
       const poles = p.poles !== undefined ? `:${p.poles}` : "";
-      return `${node.type}${poles}`;
+      // A one-pole filter is an IIRFilterNode whose coefficients are fixed at
+      // construction, so its cutoff cannot be pushed into the running graph.
+      // Carrying the frequency here makes a cutoff change rebuild instead of
+      // being pushed into a no-op updater — which is what let preview keep
+      // filtering at the old frequency while the render used the new one.
+      const fixedFreq = String(p.poles) === "1" ? `@${p.frequency}` : "";
+      return `${node.type}${poles}${fixedFreq}`;
     })
     .join("|");
 }

packages/core/src/audio/audioFxWorklets.ts

@@ -12,18 +12,28 @@
 export const AUDIO_FX_WORKLET_SOURCE = `
 const dbToLin = (db) => Math.pow(10, db / 20);
 
-/** One-pole envelope follower shared by the dynamics processors. */
+/**
-class Env {
+ * One-pole envelope followers, one per channel.
-  constructor(attackMs, releaseMs) { this.set(attackMs, releaseMs); this.value = 0; }
+ *
+ * Per channel matters: the followers advance once per sample, so a single shared
+ * follower stepped once per channel per sample. On stereo that ran a 20 ms attack
+ * as 10 ms, and gave the right channel a gain computed from an envelope that had
+ * already traversed the left — so the two channels ducked by different amounts
+ * from the same input and the image pumped.
+ */
+class EnvBank {
+  constructor(attackMs, releaseMs) { this.set(attackMs, releaseMs); this.values = []; }
   set(attackMs, releaseMs) {
     this.a = Math.exp(-1 / (sampleRate * Math.max(1e-5, attackMs / 1000)));
     this.r = Math.exp(-1 / (sampleRate * Math.max(1e-5, releaseMs / 1000)));
   }
-  push(x) {
+  push(ch, x) {
+    const prev = this.values[ch] ?? 0;
     const m = Math.abs(x);
-    const c = m > this.value ? this.a : this.r;
+    const c = m > prev ? this.a : this.r;
-    this.value = m + c * (this.value - m);
+    const next = m + c * (prev - m);
-    return this.value;
+    this.values[ch] = next;
+    return next;
   }
 }
 
@@ -41,7 +51,7 @@ class HfCompressor extends AudioWorkletProcessor {
   constructor(o) {
     super();
     this.p = o.processorOptions || {};
-    this.env = new Env(this.p.attack ?? 20, this.p.release ?? 250);
+    this.env = new EnvBank(this.p.attack ?? 20, this.p.release ?? 250);
     this.port.onmessage = (e) => {
       this.p = { ...this.p, ...e.data };
       this.env.set(this.p.attack ?? 20, this.p.release ?? 250);
@@ -59,7 +69,7 @@ class HfCompressor extends AudioWorkletProcessor {
       const inp = i[ch], out = o[ch];
       for (let n = 0; n < inp.length; n++) {
         const x = inp[n];
-        const env = this.env.push(x);
+        const env = this.env.push(ch, x);
         const envDb = env > 1e-9 ? 20 * Math.log10(env) : -200;
         const g = dbToLin(kneeGain(envDb, p.threshold ?? -24, p.ratio ?? 4, kneeDb));
         out[n] = x * g * makeup * mix + x * (1 - mix);
@@ -74,7 +84,7 @@ class HfLimiter extends AudioWorkletProcessor {
   constructor(o) {
     super();
     this.p = o.processorOptions || {};
-    this.env = new Env(this.p.attack ?? 5, this.p.release ?? 50);
+    this.env = new EnvBank(this.p.attack ?? 5, this.p.release ?? 50);
     this.port.onmessage = (e) => {
       this.p = { ...this.p, ...e.data };
       this.env.set(this.p.attack ?? 5, this.p.release ?? 50);
@@ -89,7 +99,7 @@ class HfLimiter extends AudioWorkletProcessor {
       const inp = i[ch], out = o[ch];
       for (let n = 0; n < inp.length; n++) {
         const x = inp[n];
-        const env = this.env.push(x);
+        const env = this.env.push(ch, x);
         // Only ever attenuate: a limiter that can raise level is a compressor.
         const g = env > ceiling ? ceiling / env : 1;
         out[n] = x * g * outGain;
@@ -104,8 +114,8 @@ class HfGate extends AudioWorkletProcessor {
   constructor(o) {
     super();
     this.p = o.processorOptions || {};
-    this.env = new Env(this.p.attack ?? 1, this.p.release ?? 100);
+    this.env = new EnvBank(this.p.attack ?? 1, this.p.release ?? 100);
-    this.gain = 1;
+    this.gains = [];
     this.port.onmessage = (e) => {
       this.p = { ...this.p, ...e.data };
       this.env.set(this.p.attack ?? 1, this.p.release ?? 100);
@@ -118,11 +128,16 @@ class HfGate extends AudioWorkletProcessor {
     const threshold = dbToLin(p.threshold ?? -35);
     const floor = dbToLin(p.range ?? -24);
     const ratio = p.ratio ?? 10;
+    // Knee is declared in the registry as a ratio, like the compressor's; a
+    // hard threshold ignored it and chattered on material sitting right at the
+    // gate point.
+    const kneeDb = 20 * Math.log10(Math.max(1.0001, p.knee ?? 2.83));
+    const kneeLin = dbToLin((p.threshold ?? -35) + kneeDb);
     for (let ch = 0; ch < i.length; ch++) {
       const inp = i[ch], out = o[ch];
       for (let n = 0; n < inp.length; n++) {
         const x = inp[n];
-        const env = this.env.push(x);
+        const env = this.env.push(ch, x);
         let target = 1;
         if (env < threshold) {
           const under = env > 1e-9 ? threshold / env : 1e9;
@@ -130,9 +145,11 @@ class HfGate extends AudioWorkletProcessor {
           if (!isFinite(target)) target = floor;
         }
         // Smooth toward the target so the gate does not click on every sample.
-        const c = target < this.gain ? this.env.r : this.env.a;
+        const held = this.gains[ch] ?? 1;
-        this.gain = target + c * (this.gain - target);
+        const c = target < held ? this.env.r : this.env.a;
-        out[n] = x * this.gain;
+        const smoothed = target + c * (held - target);
+        this.gains[ch] = smoothed;
+        out[n] = x * smoothed;
       }
     }
     return true;
@@ -144,7 +161,7 @@ class HfBitcrush extends AudioWorkletProcessor {
   constructor(o) {
     super();
     this.p = o.processorOptions || {};
-    this.hold = 0;
+    this.holds = [];
     this.held = [];
     this.port.onmessage = (e) => { this.p = { ...this.p, ...e.data }; };
   }
@@ -158,10 +175,14 @@ class HfBitcrush extends AudioWorkletProcessor {
     for (let ch = 0; ch < i.length; ch++) {
       const inp = i[ch], out = o[ch];
       if (this.held[ch] === undefined) this.held[ch] = 0;
+      if (this.holds[ch] === undefined) this.holds[ch] = 0;
       for (let n = 0; n < inp.length; n++) {
-        if (this.hold === 0) this.held[ch] = Math.round(inp[n] * levels) / levels;
+        // Per channel: advancing one shared counter only on the last channel
+        // left every earlier channel either unheld or frozen for a whole
+        // 128-sample quantum, clicking once a block.
+        if (this.holds[ch] === 0) this.held[ch] = Math.round(inp[n] * levels) / levels;
         out[n] = this.held[ch] * mix + inp[n] * (1 - mix);
-        if (ch === i.length - 1) this.hold = (this.hold + 1) % step;
+        this.holds[ch] = (this.holds[ch] + 1) % step;
       }
     }
     return true;