refactor: extract pcm audio helpers

src/audio/pcm.ts

@@ -0,0 +1,28 @@
+export function upsample24kMonoTo48kStereo(mono24k: Buffer): Buffer {
+  const numSamples = mono24k.length / 2;
+  const out = Buffer.alloc(numSamples * 8);
+
+  for (let i = 0; i < numSamples; i++) {
+    const sample = mono24k.readInt16LE(i * 2);
+    const base = i * 8;
+    out.writeInt16LE(sample, base);
+    out.writeInt16LE(sample, base + 2);
+    out.writeInt16LE(sample, base + 4);
+    out.writeInt16LE(sample, base + 6);
+  }
+
+  return out;
+}
+
+export function rmsDb(pcm: Buffer): number {
+  let sum = 0;
+  const samples = pcm.length / 2;
+
+  for (let i = 0; i < samples; i++) {
+    const sample = pcm.readInt16LE(i * 2) / 32768;
+    sum += sample * sample;
+  }
+
+  const rms = Math.sqrt(sum / samples);
+  return 20 * Math.log10(rms);
+}