diff --git a/dev/demux.html b/dev/demux.html
index 7d7796e..8057547 100644
--- a/dev/demux.html
+++ b/dev/demux.html
@@ -32,7 +32,7 @@
 
 	fileInput.addEventListener('change', async () => {
 		const file = fileInput.files[0];
-		const source = new Metamuxer.BlobSource(file);
+		const source = new Metamuxer.BufferSource(await file.arrayBuffer()) ?? new Metamuxer.BlobSource(file);
 
 		const start = performance.now();	
 		const input = new Metamuxer.Input({
@@ -40,7 +40,27 @@
 			source
 		});
 
-		const videoTrack = await input.getPrimaryVideoTrack();
+		const output = new Metamuxer.Output({
+			format: new Metamuxer.WaveOutputFormat(),
+			target: new Metamuxer.BufferTarget()
+		});
+		output.start();
+
+		/*
+		const audioTrack = await input.getPrimaryAudioTrack();
+		const sink = new Metamuxer.EncodedAudioSampleSink(audioTrack);
+
+		let total = 0;
+		for await (const sample of sink.samples(undefined, undefined, { metadataOnly: true })) {
+			total++;
+			if (performance.now() > 10000) {
+				break;
+			}
+		}
+		console.log("Done", total);
+		*/
+
+		/*
 		const sink = new Metamuxer.VideoFrameSink(videoTrack);
 
 		console.log(await videoTrack.getLanguageCode());
@@ -49,6 +69,7 @@
 			console.log(thing)
 		}
 		console.log("Done")
+		*/
 
 		/*
 		//const videoTrack = await input.getPrimaryVideoTrack();
diff --git a/src/index.ts b/src/index.ts
index 1e4c1c4..40b82e5 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -5,6 +5,8 @@ export {
 	VideoTrackMetadata,
 	AudioTrackMetadata,
 	SubtitleTrackMetadata,
+	TrackType,
+	ALL_TRACK_TYPES,
 } from './output';
 export {
 	OutputFormat,
@@ -17,6 +19,8 @@ export {
 	WebMOutputFormat,
 	WebMOutputFormatOptions,
 	WaveOutputFormat,
+	TrackCountLimits,
+	InclusiveRange,
 } from './output-format';
 export {
 	VideoEncodingConfig,
diff --git a/src/output-format.ts b/src/output-format.ts
index bbed1f4..0a426b9 100644
--- a/src/output-format.ts
+++ b/src/output-format.ts
@@ -2,18 +2,28 @@ import { AUDIO_CODECS, MediaCodec, PCM_CODECS, SUBTITLE_CODECS, VIDEO_CODECS } f
 import { IsobmffMuxer } from './isobmff/isobmff-muxer';
 import { MatroskaMuxer } from './matroska/matroska-muxer';
 import { Muxer } from './muxer';
-import { Output } from './output';
+import { Output, TrackType } from './output';
 import { WaveMuxer } from './wave/wave-muxer';
 
+/** @public */
+export type InclusiveRange = { min: number; max: number };
+/** @public */
+export type TrackCountLimits = {
+	[K in TrackType]: InclusiveRange;
+} & {
+	total: InclusiveRange;
+};
+
 /** @public */
 export abstract class OutputFormat {
 	/** @internal */
 	abstract _createMuxer(output: Output): Muxer;
-
 	/** @internal */
 	abstract _getName(): string;
+
 	abstract getFileExtension(): string;
 	abstract getSupportedCodecs(): MediaCodec[];
+	abstract getSupportedTrackCounts(): TrackCountLimits;
 
 	getSupportedVideoCodecs() {
 		return this.getSupportedCodecs().filter(codec => (VIDEO_CODECS as readonly string[]).includes(codec));
@@ -57,6 +67,15 @@ export abstract class IsobmffOutputFormat extends OutputFormat {
 		this._options = options;
 	}
 
+	getSupportedTrackCounts(): TrackCountLimits {
+		return {
+			video: { min: 0, max: Infinity },
+			audio: { min: 0, max: Infinity },
+			subtitle: { min: 0, max: Infinity },
+			total: { min: 1, max: 2 ** 32 - 1 }, // Have fun reaching this one
+		};
+	}
+
 	/** @internal */
 	_createMuxer(output: Output) {
 		return new IsobmffMuxer(output, this);
@@ -161,6 +180,15 @@ export class MkvOutputFormat extends OutputFormat {
 		return 'Matroska';
 	}
 
+	getSupportedTrackCounts(): TrackCountLimits {
+		return {
+			video: { min: 0, max: Infinity },
+			audio: { min: 0, max: Infinity },
+			subtitle: { min: 0, max: Infinity },
+			total: { min: 1, max: 127 },
+		};
+	}
+
 	getFileExtension() {
 		return '.mkv';
 	}
@@ -228,6 +256,15 @@ export class WaveOutputFormat extends OutputFormat {
 		return 'WAVE';
 	}
 
+	getSupportedTrackCounts(): TrackCountLimits {
+		return {
+			video: { min: 0, max: 0 },
+			audio: { min: 1, max: 1 },
+			subtitle: { min: 0, max: 0 },
+			total: { min: 1, max: 1 },
+		};
+	}
+
 	getFileExtension() {
 		return '.wav';
 	}
diff --git a/src/output.ts b/src/output.ts
index dd648df..7e31b57 100644
--- a/src/output.ts
+++ b/src/output.ts
@@ -11,9 +11,15 @@ export type OutputOptions = {
 	target: Target;
 };
 
+/** @public */
+export const ALL_TRACK_TYPES = ['video', 'audio', 'subtitle'] as const;
+/** @public */
+export type TrackType = typeof ALL_TRACK_TYPES[number];
+
 export type OutputTrack = {
 	id: number;
 	output: Output;
+	type: TrackType;
 } & ({
 	type: 'video';
 	source: VideoSource;
@@ -152,6 +158,29 @@ export class Output {
 			throw new Error('Source is already used for a track.');
 		}
 
+		// Verify maximum track count constraints
+		const supportedTrackCounts = this._format.getSupportedTrackCounts();
+		const presentTracksOfThisType = this._tracks.reduce(
+			(count, track) => count + (track.type === type ? 1 : 0),
+			0,
+		);
+		const maxCount = supportedTrackCounts[type].max;
+		if (presentTracksOfThisType === maxCount) {
+			throw new Error(
+				maxCount === 0
+					? `${this._format._getName()} does not support ${type} tracks.`
+					: (`${this._format._getName()} does not support more than ${maxCount} ${type} track`
+						+ `${maxCount === 1 ? '' : 's'}.`),
+			);
+		}
+		const maxTotalCount = supportedTrackCounts.total.max;
+		if (this._tracks.length === maxTotalCount) {
+			throw new Error(
+				`${this._format._getName()} does not support more than ${maxTotalCount} tracks`
+				+ `${maxTotalCount === 1 ? '' : 's'} in total.`,
+			);
+		}
+
 		const track = {
 			id: this._tracks.length + 1,
 			output: this,
@@ -219,6 +248,35 @@ export class Output {
 			throw new Error('Output already started.');
 		}
 
+		// Verify minimum track count constraints
+		const supportedTrackCounts = this._format.getSupportedTrackCounts();
+		for (const trackType of ALL_TRACK_TYPES) {
+			const presentTracksOfThisType = this._tracks.reduce(
+				(count, track) => count + (track.type === trackType ? 1 : 0),
+				0,
+			);
+			const minCount = supportedTrackCounts[trackType].min;
+			if (presentTracksOfThisType < minCount) {
+				throw new Error(
+					minCount === supportedTrackCounts[trackType].max
+						? (`${this._format._getName()} requires exactly ${minCount} ${trackType}`
+							+ ` track${minCount === 1 ? '' : 's'}.`)
+						: (`${this._format._getName()} requires at least ${minCount} ${trackType}`
+							+ ` track${minCount === 1 ? '' : 's'}.`),
+				);
+			}
+		}
+		const totalMinCount = supportedTrackCounts.total.min;
+		if (this._tracks.length < totalMinCount) {
+			throw new Error(
+				totalMinCount === supportedTrackCounts.total.max
+					? (`${this._format._getName()} requires exactly ${totalMinCount} track`
+						+ `${totalMinCount === 1 ? '' : 's'}.`)
+					: (`${this._format._getName()} requires at least ${totalMinCount} track`
+						+ `${totalMinCount === 1 ? '' : 's'}.`),
+			);
+		}
+
 		this._started = true;
 		this._writer.start();