test(audio): reproduce aliasing in native voice playback

2026-10-23 17:52:49 +00:00 · 2026-10-18 06:41:39 +02:00
parent cf62b00911
commit 2a8bb00cb5
1 changed files with 37 additions and 0 deletions
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+import { expect, test } from "vitest";
+import { playPcm16 } from "./index";
+
+test("native voice playback suppresses frequencies above the output Nyquist limit", async () => {
+  const inputRate = 24000;
+  const outputRate = 16000;
+  const input = new Uint8Array((inputRate / 10) * 2);
+  const view = new DataView(input.buffer);
+  for (let index = 0; index < input.length / 2; index++) {
+    view.setInt16(
+      index * 2,
+      Math.round(16000 * Math.sin((2 * Math.PI * 10000 * index) / inputRate)),
+      true,
+    );
+  }
+  let rendered: Uint8Array = new Uint8Array();
+  await playPcm16(input, "audio/pcm;rate=24000;bits=16", new AbortController().signal, {
+    resumePlayback() {},
+    playPCMData(bytes) {
+      rendered = bytes;
+    },
+    stopPlayback() {},
+  });
+  const output = new DataView(rendered.buffer, rendered.byteOffset, rendered.byteLength);
+  let real = 0;
+  let imaginary = 0;
+  const count = rendered.length / 2;
+  for (let index = 0; index < count; index++) {
+    const sample = output.getInt16(index * 2, true);
+    const phase = (2 * Math.PI * 6000 * index) / outputRate;
+    real += sample * Math.cos(phase);
+    imaginary += sample * Math.sin(phase);
+  }
+  const aliasAmplitude = (2 * Math.hypot(real, imaginary)) / count;
+  // A 10 kHz source tone must not become an audible 6 kHz tone after downsampling.
+  expect(aliasAmplitude / 16000).toBeLessThan(0.01);
+});