diff --git a/dev/convert.html b/dev/convert.html
index 96c013a..9f18bfd 100644
--- a/dev/convert.html
+++ b/dev/convert.html
@@ -26,7 +26,7 @@
 			chunked: true,
 			chunkSize: 2**20
 		});
-		const outputFormat = new Mediabunny.Mp4OutputFormat({});
+		const outputFormat = new Mediabunny.Mp4OutputFormat({fastStart: 'fragmented'});
 
 		const button = document.createElement('button');
 		button.textContent = 'Cancel';
diff --git a/dev/demux.html b/dev/demux.html
index 233aae3..baa5541 100644
--- a/dev/demux.html
+++ b/dev/demux.html
@@ -18,10 +18,11 @@
 		});
 
 		const videoTrack = await input.getPrimaryVideoTrack();
-		const sink = new Mediabunny.VideoSampleSink(videoTrack);
+		const sink = new Mediabunny.EncodedPacketSink(videoTrack);
 
-		const sample = await sink.getSample(await videoTrack.getFirstTimestamp());
-		console.log(sample);
+		for await (const packet of sink.packets()) {
+			console.log(packet.timestamp);
+		}
 
 		/*
 		
diff --git a/src/isobmff/isobmff-muxer.ts b/src/isobmff/isobmff-muxer.ts
index 4256f53..b7a6158 100644
--- a/src/isobmff/isobmff-muxer.ts
+++ b/src/isobmff/isobmff-muxer.ts
@@ -1245,11 +1245,13 @@ export class IsobmffMuxer extends Muxer {
 	override async onTrackClose(track: OutputTrack) {
 		const release = await this.mutex.acquire();
 
-		if (track.type === 'subtitle' && track.source._codec === 'webvtt') {
-			const trackData = this.trackDatas.find(x => x.track === track) as IsobmffSubtitleTrackData;
-			if (trackData) {
+		const trackData = this.trackDatas.find(x => x.track === track);
+		if (trackData) {
+			if (trackData.type === 'subtitle' && track.source._codec === 'webvtt') {
 				await this.processWebVTTCues(trackData, Infinity);
 			}
+
+			this.processTimestamps(trackData);
 		}
 
 		if (this.allTracksAreKnown()) {
@@ -1274,19 +1276,15 @@ export class IsobmffMuxer extends Muxer {
 			if (trackData.type === 'subtitle' && trackData.track.source._codec === 'webvtt') {
 				await this.processWebVTTCues(trackData, Infinity);
 			}
+
+			this.processTimestamps(trackData);
 		}
 
 		if (this.isFragmented) {
 			await this.interleaveSamples(true);
-
-			for (const trackData of this.trackDatas) {
-				this.processTimestamps(trackData);
-			}
-
 			await this.finalizeFragment(false); // Don't flush the last fragment as we will flush it with the mfra box
 		} else {
 			for (const trackData of this.trackDatas) {
-				this.processTimestamps(trackData);
 				await this.finalizeCurrentChunk(trackData);
 			}
 		}
diff --git a/test/node/isobmff-muxer.test.ts b/test/node/isobmff-muxer.test.ts
index 55cee50..83d1d9e 100644
--- a/test/node/isobmff-muxer.test.ts
+++ b/test/node/isobmff-muxer.test.ts
@@ -61,3 +61,46 @@ test('ISOBMFF muxer internally converts ADTS to AAC', async () => {
 
 	expect(count).toBe(4557);
 });
+
+test('Fragmented fMP4 with video+audio preserves B-frame CTS', async () => {
+	using input = new Input({
+		source: new FilePathSource(path.join(__dirname, '../public/video.mp4')),
+		formats: ALL_FORMATS,
+	});
+
+	const videoTrack = await input.getPrimaryVideoTrack();
+	const audioTrack = await input.getPrimaryAudioTrack();
+	assert(videoTrack);
+	assert(audioTrack);
+
+	const originalVideoSink = new EncodedPacketSink(videoTrack);
+	const originalTimestamps: number[] = [];
+	for await (const packet of originalVideoSink.packets()) {
+		originalTimestamps.push(packet.timestamp);
+	}
+
+	const output = new Output({
+		format: new Mp4OutputFormat({ fastStart: 'fragmented' }),
+		target: new BufferTarget(),
+	});
+
+	const conversion = await Conversion.init({ input, output, showWarnings: false });
+	await conversion.execute();
+
+	using outputAsInput = new Input({
+		source: new BufferSource(output.target.buffer!),
+		formats: ALL_FORMATS,
+	});
+
+	const outputVideoTrack = await outputAsInput.getPrimaryVideoTrack();
+	assert(outputVideoTrack);
+
+	const videoSink = new EncodedPacketSink(outputVideoTrack);
+
+	const timestamps: number[] = [];
+	for await (const packet of videoSink.packets()) {
+		timestamps.push(packet.timestamp);
+	}
+
+	expect(timestamps).toEqual(originalTimestamps);
+});