fix: always apply aresample sync filter and tighten audio sync tolerances

- Always route audio through aresample=async=1:first_pts=0 filter during
  muxing on both macOS and Windows, even when duration delta is small.
  Previously skipped when delta ≤50ms, which left progressive clock drift
  (from CPU load) completely uncorrected.
- Lower sync detection threshold from 50ms to 20ms so tempo correction
  kicks in earlier.
- Use explicit 48kHz AudioContext sample rate in both browser recording
  and export rendering to prevent sample rate mismatch drift.
- Reduce MediaRecorder timeslice from 1000ms to 250ms to reduce chunk
  loss under CPU pressure.
- Tighten export audio sync: seek threshold 300ms→150ms, tolerance
  15ms→8ms, correction window 2s→0.5s, max adjustment ±8%→±12%.
- Lower companion audio start delay threshold from 50ms to 25ms.

electron/ipc/ffmpeg/filters.ts

@@ -40,7 +40,7 @@ export function getAudioSyncAdjustment(
 
 	const durationDeltaMs = Math.round((videoDuration - audioDuration) * 1000);
 	const absDeltaMs = Math.abs(durationDeltaMs);
-	if (absDeltaMs <= 50) {
+	if (absDeltaMs <= 20) {
 		return { mode: "none", delayMs: 0, tempoRatio: 1, durationDeltaMs };
 	}
 

electron/ipc/recording/mac.ts

@@ -194,53 +194,35 @@ export async function muxNativeMacRecordingWithAudio(
 		tempoRatio: 1,
 		durationDeltaMs: 0,
 	};
-	const needsFilter = systemAdjustment.mode !== "none" || micAdjustment.mode !== "none";
 
+	// Always route through the filter graph so that aresample=async=1 is
+	// applied to every audio stream.  This corrects progressive clock drift
+	// between the video and audio tracks that a simple duration comparison
+	// cannot detect (e.g. audio gradually falling behind under CPU load).
 	let args: string[];
 	if (availableAudioInputs.length === 2) {
-		if (needsFilter) {
-			const filterParts: string[] = [];
-			appendSyncedAudioFilter(filterParts, "[1:a]", "s", systemAdjustment);
-			appendSyncedAudioFilter(filterParts, "[2:a]", "m", micAdjustment);
-			filterParts.push("[s][m]amix=inputs=2:duration=longest:normalize=0[aout]");
-			args = [
-				"-y",
-				...inputs,
-				"-filter_complex",
-				filterParts.join(";"),
-				"-map",
-				"0:v:0",
-				"-map",
-				"[aout]",
-				"-c:v",
-				"copy",
-				"-c:a",
-				"aac",
-				"-b:a",
-				"192k",
-				"-shortest",
-				mixedOutputPath,
-			];
-		} else {
-			args = [
-				"-y",
-				...inputs,
-				"-filter_complex",
-				"[1:a][2:a]amix=inputs=2:duration=longest:normalize=0[aout]",
-				"-map",
-				"0:v:0",
-				"-map",
-				"[aout]",
-				"-c:v",
-				"copy",
-				"-c:a",
-				"aac",
-				"-b:a",
-				"192k",
-				"-shortest",
-				mixedOutputPath,
-			];
-		}
+		const filterParts: string[] = [];
+		appendSyncedAudioFilter(filterParts, "[1:a]", "s", systemAdjustment);
+		appendSyncedAudioFilter(filterParts, "[2:a]", "m", micAdjustment);
+		filterParts.push("[s][m]amix=inputs=2:duration=longest:normalize=0[aout]");
+		args = [
+			"-y",
+			...inputs,
+			"-filter_complex",
+			filterParts.join(";"),
+			"-map",
+			"0:v:0",
+			"-map",
+			"[aout]",
+			"-c:v",
+			"copy",
+			"-c:a",
+			"aac",
+			"-b:a",
+			"192k",
+			"-shortest",
+			mixedOutputPath,
+		];
 	} else {
 		const singleAdjustment = audioAdjustments.get(availableAudioInputs[0]) ?? {
 			mode: "none",
@@ -248,45 +230,26 @@ export async function muxNativeMacRecordingWithAudio(
 			tempoRatio: 1,
 			durationDeltaMs: 0,
 		};
-		if (singleAdjustment.mode !== "none") {
-			const filterParts: string[] = [];
-			appendSyncedAudioFilter(filterParts, "[1:a]", "aout", singleAdjustment);
-			args = [
-				"-y",
-				...inputs,
-				"-filter_complex",
-				filterParts.join(";"),
-				"-map",
-				"0:v:0",
-				"-map",
-				"[aout]",
-				"-c:v",
-				"copy",
-				"-c:a",
-				"aac",
-				"-b:a",
-				"192k",
-				"-shortest",
-				mixedOutputPath,
-			];
-		} else {
-			args = [
-				"-y",
-				...inputs,
-				"-map",
-				"0:v:0",
-				"-map",
-				"1:a:0",
-				"-c:v",
-				"copy",
-				"-c:a",
-				"aac",
-				"-b:a",
-				"192k",
-				"-shortest",
-				mixedOutputPath,
-			];
-		}
+		const filterParts: string[] = [];
+		appendSyncedAudioFilter(filterParts, "[1:a]", "aout", singleAdjustment);
+		args = [
+			"-y",
+			...inputs,
+			"-filter_complex",
+			filterParts.join(";"),
+			"-map",
+			"0:v:0",
+			"-map",
+			"[aout]",
+			"-c:v",
+			"copy",
+			"-c:a",
+			"aac",
+			"-b:a",
+			"192k",
+			"-shortest",
+			mixedOutputPath,
+		];
 	}
 
 	console.log("[mux] Running ffmpeg:", ffmpegPath, args.join(" "));

electron/ipc/recording/windows.ts

@@ -278,58 +278,38 @@ export async function muxNativeWindowsVideoWithAudio(
 			durationDeltaMs: 0,
 		};
 
-		if (pauseFilter || singleAdjustment.mode !== "none") {
-			const filterParts: string[] = [];
-			if (pauseFilter) {
-				filterParts.push(pauseFilter);
-			}
-			const srcLabel = pauseFilter ? "[trimmed_audio]" : "[1:a]";
-			appendSyncedAudioFilter(filterParts, srcLabel, "aout", singleAdjustment);
-
-			await execFileAsync(
-				ffmpegPath,
-				[
-					"-y",
-					...inputs,
-					"-filter_complex",
-					filterParts.join(";"),
-					"-map",
-					"0:v:0",
-					"-map",
-					"[aout]",
-					"-c:v",
-					"copy",
-					"-c:a",
-					"aac",
-					"-b:a",
-					"192k",
-					"-shortest",
-					mixedOutputPath,
-				],
-				{ timeout: 120000, maxBuffer: 10 * 1024 * 1024 },
-			);
-		} else {
-			await execFileAsync(
-				ffmpegPath,
-				[
-					"-y",
-					...inputs,
-					"-map",
-					"0:v:0",
-					"-map",
-					"1:a:0",
-					"-c:v",
-					"copy",
-					"-c:a",
-					"aac",
-					"-b:a",
-					"192k",
-					"-shortest",
-					mixedOutputPath,
-				],
-				{ timeout: 120000, maxBuffer: 10 * 1024 * 1024 },
-			);
+		// Always route through the filter graph so that aresample=async=1 is
+		// applied.  This corrects progressive clock drift between video and
+		// audio tracks that a simple duration comparison cannot detect.
+		const filterParts: string[] = [];
+		if (pauseFilter) {
+			filterParts.push(pauseFilter);
 		}
+		const srcLabel = pauseFilter ? "[trimmed_audio]" : "[1:a]";
+		appendSyncedAudioFilter(filterParts, srcLabel, "aout", singleAdjustment);
+
+		await execFileAsync(
+			ffmpegPath,
+			[
+				"-y",
+				...inputs,
+				"-filter_complex",
+				filterParts.join(";"),
+				"-map",
+				"0:v:0",
+				"-map",
+				"[aout]",
+				"-c:v",
+				"copy",
+				"-c:a",
+				"aac",
+				"-b:a",
+				"192k",
+				"-shortest",
+				mixedOutputPath,
+			],
+			{ timeout: 120000, maxBuffer: 10 * 1024 * 1024 },
+		);
 	}
 
 	await moveFileWithOverwrite(mixedOutputPath, videoPath);

src/hooks/useScreenRecorder.ts

@@ -18,7 +18,7 @@ const HIGH_FRAME_RATE_BOOST = 1.7;
 const DEFAULT_WIDTH = 1920;
 const DEFAULT_HEIGHT = 1080;
 const CODEC_ALIGNMENT = 2;
-const RECORDER_TIMESLICE_MS = 1000;
+const RECORDER_TIMESLICE_MS = 250;
 const BITS_PER_MEGABIT = 1_000_000;
 const MIN_FRAME_RATE = 30;
 const CHROME_MEDIA_SOURCE = "desktop";
@@ -1090,7 +1090,7 @@ export function useScreenRecorder(): UseScreenRecorderReturn {
 				const micAudioTrack = microphoneStream.current?.getAudioTracks()[0];
 
 				if (systemAudioTrack && micAudioTrack) {
-					const context = new AudioContext();
+					const context = new AudioContext({ sampleRate: 48000 });
 					mixingContext.current = context;
 					const systemSource = context.createMediaStreamSource(
 						new MediaStream([systemAudioTrack]),

src/lib/exporter/audioEncoder.ts

@@ -555,7 +555,7 @@ export class AudioProcessor {
 				throw new Error("Export cancelled");
 			}
 
-			audioContext = new AudioContext();
+			audioContext = new AudioContext({ sampleRate: 48000 });
 			const currentDestinationNode = audioContext.createMediaStreamDestination();
 			destinationNode = currentDestinationNode;
 
@@ -721,7 +721,7 @@ export class AudioProcessor {
 							continue;
 						}
 
-						if (Math.abs(audioEl.currentTime - targetTimeSec) > 0.3) {
+						if (Math.abs(audioEl.currentTime - targetTimeSec) > 0.15) {
 							audioEl.currentTime = targetTimeSec;
 						}
 
@@ -729,6 +729,9 @@ export class AudioProcessor {
 							basePlaybackRate: playbackRate,
 							currentTime: audioEl.currentTime,
 							targetTime: targetTimeSec,
+							toleranceSeconds: 0.008,
+							correctionWindowSeconds: 0.5,
+							maxAdjustment: 0.12,
 						});
 						if (Math.abs(audioEl.playbackRate - syncedPlaybackRate) > 0.0001) {
 							audioEl.playbackRate = syncedPlaybackRate;
@@ -748,7 +751,7 @@ export class AudioProcessor {
 
 						if (isInRegion) {
 							const audioOffset = (currentTimeMs - region.startMs) / 1000;
-							if (Math.abs(audioEl.currentTime - audioOffset) > 0.3) {
+							if (Math.abs(audioEl.currentTime - audioOffset) > 0.15) {
 								audioEl.currentTime = audioOffset;
 							}
 
@@ -756,6 +759,9 @@ export class AudioProcessor {
 								basePlaybackRate: playbackRate,
 								currentTime: audioEl.currentTime,
 								targetTime: audioOffset,
+								toleranceSeconds: 0.008,
+								correctionWindowSeconds: 0.5,
+								maxAdjustment: 0.12,
 							});
 							if (Math.abs(audioEl.playbackRate - syncedPlaybackRate) > 0.0001) {
 								audioEl.playbackRate = syncedPlaybackRate;

src/lib/mediaTiming.test.ts

@@ -23,10 +23,11 @@ describe("clampMediaTimeToDuration", () => {
 describe("estimateCompanionAudioStartDelaySeconds", () => {
 	it("returns the positive tail gap when companion audio is shorter", () => {
 		expect(estimateCompanionAudioStartDelaySeconds(10, 9.6)).toBeCloseTo(0.4);
+		expect(estimateCompanionAudioStartDelaySeconds(10, 9.97)).toBeCloseTo(0.03);
 	});
 
 	it("ignores tiny or negative differences", () => {
-		expect(estimateCompanionAudioStartDelaySeconds(10, 9.97)).toBe(0);
+		expect(estimateCompanionAudioStartDelaySeconds(10, 9.99)).toBe(0);
 		expect(estimateCompanionAudioStartDelaySeconds(10, 10.5)).toBe(0);
 	});
 });

src/lib/mediaTiming.ts

@@ -19,7 +19,7 @@ export function estimateCompanionAudioStartDelaySeconds(
 	const safeAudioDuration = Math.max(0, audioDuration ?? 0);
 	const estimatedDelaySeconds = safeTimelineDuration - safeAudioDuration;
 
-	return estimatedDelaySeconds > 0.05 ? estimatedDelaySeconds : 0;
+	return estimatedDelaySeconds > 0.025 ? estimatedDelaySeconds : 0;
 }
 
 export function getMediaSyncPlaybackRate({