fix(stt): preserve media-clock audio offsets

electron/stt/extractAudio.integration.test.ts

@@ -0,0 +1,88 @@
+import { spawnSync } from "node:child_process";
+import { mkdtempSync, rmSync } from "node:fs";
+import { tmpdir } from "node:os";
+import path from "node:path";
+import { afterAll, beforeAll, describe, expect, it, vi } from "vitest";
+
+// Only the binary lookup is replaced: extraction still spawns and reads real FFmpeg.
+vi.mock("../media/audioPeaks", () => ({ resolveFfmpeg: () => "ffmpeg" }));
+
+import { extractMono16kPcm } from "./extractAudio";
+
+const ffmpegAvailable = spawnSync("ffmpeg", ["-version"], { timeout: 5_000 }).status === 0;
+
+if (!ffmpegAvailable) {
+	console.warn("Skipping audio-origin integration tests: FFmpeg is unavailable on PATH.");
+}
+
+describe.skipIf(!ffmpegAvailable)("extractMono16kPcm with real FFmpeg", () => {
+	let fixtureDir: string;
+	let delayedVideo: string;
+	let standaloneAudio: string;
+
+	function makeFixture(filename: string, args: string[]): string {
+		const filePath = path.join(fixtureDir, filename);
+		const result = spawnSync(
+			"ffmpeg",
+			["-hide_banner", "-loglevel", "error", "-y", ...args, filePath],
+			{ encoding: "utf8", timeout: 10_000 },
+		);
+		if (result.status !== 0) {
+			throw new Error(result.error?.message ?? result.stderr);
+		}
+		return filePath;
+	}
+
+	beforeAll(() => {
+		fixtureDir = mkdtempSync(path.join(tmpdir(), "openscreen-stt-audio-origin-"));
+		const tone = [
+			"-itsoffset",
+			"2",
+			"-f",
+			"lavfi",
+			"-i",
+			"sine=frequency=440:sample_rate=16000:duration=2",
+		];
+		delayedVideo = makeFixture("delayed.mkv", [
+			"-f",
+			"lavfi",
+			"-i",
+			"color=c=black:s=16x16:r=1:d=4",
+			...tone,
+			"-map",
+			"0:v",
+			"-map",
+			"1:a",
+			"-c:v",
+			"ffv1",
+			"-c:a",
+			"pcm_s16le",
+		]);
+		standaloneAudio = makeFixture("standalone.mka", [...tone, "-c:a", "pcm_s16le"]);
+	});
+
+	afterAll(() => {
+		if (fixtureDir) rmSync(fixtureDir, { recursive: true, force: true });
+	});
+
+	it("keeps delayed audio on the video's clock", async () => {
+		// The audio stream starts two seconds after video, without encoded silence.
+		// Dropping that timestamp gap makes every word and text cut two seconds early.
+		const samples = await extractMono16kPcm(delayedVideo);
+		expect(samples).toHaveLength(4 * 16_000);
+		expect(samples.subarray(0, 2 * 16_000).every((sample) => sample === 0)).toBe(true);
+		const firstAudibleSample = samples.findIndex((sample) => Math.abs(sample) > 0.01);
+		expect(firstAudibleSample).toBeGreaterThanOrEqual(2 * 16_000);
+		expect(firstAudibleSample).toBeLessThan(2 * 16_000 + 16);
+	});
+
+	it("keeps standalone audio relative to its own media origin", async () => {
+		// This container starts at two seconds too, but has no earlier video stream.
+		// Its own origin is still zero: padding it would invent two seconds of silence.
+		const samples = await extractMono16kPcm(standaloneAudio);
+		expect(samples).toHaveLength(2 * 16_000);
+		const firstAudibleSample = samples.findIndex((sample) => Math.abs(sample) > 0.01);
+		expect(firstAudibleSample).toBeGreaterThanOrEqual(0);
+		expect(firstAudibleSample).toBeLessThan(16);
+	});
+});

electron/stt/extractAudio.ts

@@ -96,6 +96,9 @@ export async function extractMono16kPcm(
 			"1",
 			"-ar",
 			String(SAMPLE_RATE),
+			// Pad delayed audio to keep sample zero on the media clock.
+			"-af",
+			"aresample=first_pts=0",
 			"-f",
 			"f32le",
 			"-",