feat(mq_editor): post-synthesis LP/HP filter pair in synthesizeMQ

Add first-order IIR low-pass (options.k1) and high-pass (options.k2)
filters applied globally after the synthesis loop and before normalization.
Both are bypassed when the key is absent; LP is applied before HP
(bandpass configuration when both active).

handoff(Claude): LP/HP filters added to synthesizeMQ, no UI yet.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

1 files changed, 24 insertions, 0 deletions

diff --git a/tools/mq_editor/mq_synth.js b/tools/mq_editor/mq_synth.js
index 2d3111b..f298392 100644
--- a/tools/mq_editor/mq_synth.js
+++ b/tools/mq_editor/mq_synth.js
@@ -26,6 +26,8 @@ function randFloat(seed, min, max) {
 // resonator: {enabled, r, gainComp}  — two-pole resonator mode per partial
 // integratePhase: true = accumulate 2π*f/SR per sample (correct for varying freq)
 //                 false = 2π*f*t (simpler, only correct for constant freq)
+// options.k1: LP coefficient in (0,1] — omit to bypass
+// options.k2: HP coefficient in (0,1] — omit to bypass
 function synthesizeMQ(partials, sampleRate, duration, integratePhase = true, options = {}) {
   const numSamples = Math.floor(sampleRate * duration);
   const pcm = new Float32Array(numSamples);
@@ -137,6 +139,28 @@ function synthesizeMQ(partials, sampleRate, duration, integratePhase = true, opt
     pcm[i] = sample;
   }
 
+  // Post-synthesis filters (applied before normalization)
+  // LP: y[n] = k1*x[n] + (1-k1)*y[n-1]  — options.k1 in (0,1], omit to bypass
+  // HP: y[n] = k2*(y[n-1] + x[n] - x[n-1]) — options.k2 in (0,1], omit to bypass
+  if (options.k1 != null) {
+    const k1 = Math.max(0, Math.min(1, options.k1));
+    let y = 0.0;
+    for (let i = 0; i < numSamples; ++i) {
+      y = k1 * pcm[i] + (1.0 - k1) * y;
+      pcm[i] = y;
+    }
+  }
+  if (options.k2 != null) {
+    const k2 = Math.max(0, Math.min(1, options.k2));
+    let y = 0.0, xp = 0.0;
+    for (let i = 0; i < numSamples; ++i) {
+      const x = pcm[i];
+      y = k2 * (y + x - xp);
+      xp = x;
+      pcm[i] = y;
+    }
+  }
+
   // Normalize
   let maxAbs = 0;
   for (let i = 0; i < numSamples; ++i) maxAbs = Math.max(maxAbs, Math.abs(pcm[i]));