From 176d21cf50a8af3080a10bcec1cc27ef9cb42540 Mon Sep 17 00:00:00 2001
From: Vanilagy <1696106+Vanilagy@users.noreply.github.com>
Date: Fri, 30 May 2025 20:12:00 +0200
Subject: [PATCH] Make media player AudioContext match audio track sample rate

---
 dev/convert.html                      |  7 +++---
 examples/media-player/media-player.ts | 33 ++++++++++++++-------------
 2 files changed, 21 insertions(+), 19 deletions(-)

diff --git a/dev/convert.html b/dev/convert.html
index e372bd0..80190bb 100644
--- a/dev/convert.html
+++ b/dev/convert.html
@@ -21,7 +21,7 @@
 			chunked: true,
 			chunkSize: 2**20
 		});
-		const outputFormat = new Metamuxer.Mp4OutputFormat();
+		const outputFormat = new Metamuxer.WavOutputFormat();
 
 		const button = document.createElement('button');
 		button.textContent = 'Cancel';
@@ -38,8 +38,8 @@
 				target
 			}),
 			audio: {
-				numberOfChannels: 2,
-				//sampleRate: 8000
+				numberOfChannels: 1,
+				sampleRate: 4000
 				//discard: true
 				//forceReencode: true,
 			},
@@ -68,6 +68,7 @@
 			},
 			*/
 			video: {
+				discard: true,
 				width: 1280,
 				//discard: true,
 				//width: 640
diff --git a/examples/media-player/media-player.ts b/examples/media-player/media-player.ts
index 625060a..4281440 100644
--- a/examples/media-player/media-player.ts
+++ b/examples/media-player/media-player.ts
@@ -29,9 +29,9 @@ const fullscreenButton = document.querySelector('#fullscreen-button') as HTMLBut
 const errorElement = document.querySelector('#error-element') as HTMLDivElement;
 
 const context = canvas.getContext('2d')!;
-const audioContext = new AudioContext();
-const gainNode = audioContext.createGain();
-gainNode.connect(audioContext.destination);
+
+let audioContext: AudioContext | null = null;
+let gainNode: GainNode | null = null;
 
 let fileLoaded = false;
 let videoSink: CanvasSink | null = null;
@@ -79,11 +79,6 @@ const initMediaPlayer = async (file: File) => {
 		horizontalRule.style.display = '';
 		playerContainer.style.display = 'none';
 
-		if (audioContext.state === 'suspended') {
-			// Resume the audio context now that a user gesture has happened
-			await audioContext.resume();
-		}
-
 		// Create an Input from the file
 		const input = new Input({
 			source: new BlobSource(file),
@@ -110,6 +105,13 @@ const initMediaPlayer = async (file: File) => {
 			throw new Error('Media file has no playable video or audio track.');
 		}
 
+		// We must create the audio context with the matching sample rate for correct acoustic results
+		// (especially for low-sample rate files)
+		audioContext = new AudioContext({ sampleRate: audioTrack?.sampleRate });
+		gainNode = audioContext.createGain();
+		gainNode.connect(audioContext.destination);
+		updateVolume();
+
 		// For video, let's use a CanvasSink as it handles rotation and closing video samples for us.
 		// Pool size of 2: We'll only ever have the current and the next frame around, so we only need two canvases.
 		videoSink = videoTrack && new CanvasSink(videoTrack, { poolSize: 2 });
@@ -248,19 +250,19 @@ const runAudioIterator = async () => {
 	}
 
 	for await (const { buffer, timestamp } of audioBufferIterator!) {
-		const node = audioContext.createBufferSource();
+		const node = audioContext!.createBufferSource();
 		node.buffer = buffer;
-		node.connect(gainNode);
+		node.connect(gainNode!);
 
 		const startTimestamp = audioContextStartTime! + timestamp - playbackTimeAtStart;
 
 		// Two cases: Either, the audio starts in the future or in the past
-		if (startTimestamp >= audioContext.currentTime) {
+		if (startTimestamp >= audioContext!.currentTime) {
 			// If the audio starts in the future, easy, we just schedule it
 			node.start(startTimestamp);
 		} else {
 			// If it starts in the past, then let's only play the audible section that remains from here on out
-			node.start(audioContext.currentTime, audioContext.currentTime - startTimestamp);
+			node.start(audioContext!.currentTime, audioContext!.currentTime - startTimestamp);
 		}
 
 		queuedAudioNodes.add(node);
@@ -290,7 +292,7 @@ const getPlaybackTime = () => {
 	if (playing) {
 		// To ensure perfect audio-video sync, we always use the audio context's clock to determine playback time, even
 		// when there is no audio track.
-		return audioContext.currentTime - audioContextStartTime! + playbackTimeAtStart;
+		return audioContext!.currentTime - audioContextStartTime! + playbackTimeAtStart;
 	} else {
 		return playbackTimeAtStart;
 	}
@@ -303,7 +305,7 @@ const play = async () => {
 		await startVideoIterator();
 	}
 
-	audioContextStartTime = audioContext.currentTime;
+	audioContextStartTime = audioContext!.currentTime;
 	playing = true;
 
 	if (audioSink) {
@@ -398,7 +400,7 @@ const updateVolume = () => {
 	const actualVolume = volumeMuted ? 0 : volume;
 
 	volumeBar.style.width = `${actualVolume * 100}%`;
-	gainNode.gain.value = actualVolume ** 2; // Quadratic for more fine-grained control
+	gainNode!.gain.value = actualVolume ** 2; // Quadratic for more fine-grained control
 
 	const iconNumber = volumeMuted ? 0 : Math.ceil(1 + 3 * volume);
 	for (let i = 0; i < volumeIconWrapper.children.length; i++) {
@@ -406,7 +408,6 @@ const updateVolume = () => {
 		icon.style.display = i === iconNumber ? '' : 'none';
 	}
 };
-updateVolume();
 
 volumeBarContainer.addEventListener('pointerdown', (event) => {
 	draggingVolumeBar = true;