Implement MP3 muxer

dev/demux.html

@@ -16,11 +16,45 @@
 			source
 		});
 
+		const output = new Metamuxer.Output({
+			format: new Metamuxer.Mp3OutputFormat(),
+			target: new Metamuxer.BufferTarget()
+		});
+
+		const audioTrack = await input.getPrimaryAudioTrack();
+		const sink = new Metamuxer.EncodedAudioSampleSink(audioTrack);
+
+		const mediaSource = new Metamuxer.EncodedAudioSampleSource(await audioTrack.getCodec());
+		output.addAudioTrack(mediaSource);
+
+		output.start();
+
+		for await (const sample of sink.samples()) {
+			await mediaSource.digest(sample);
+		}
+
+		await output.finalize();
+
+		function download(blob, filename) {
+			const url = URL.createObjectURL(blob);
+			const a = document.createElement('a');
+			a.href = url;
+			a.download = filename;
+			a.click();
+			URL.revokeObjectURL(url);
+		}	
+		
+		console.log(output.target.buffer)
+		download(new Blob([output.target.buffer]), 'converted.mp3');	
+	
+
+		/*
 		const videoTrack = await input.getPrimaryVideoTrack();
 		const sink = new Metamuxer.EncodedVideoSampleSink(videoTrack);
 
 		console.log(await sink.getSample(16));
 		console.log(await sink.getFirstSample());
+		*/
 
 		/*
 		const source = new Metamuxer.BufferSource(await file.arrayBuffer()) ?? new Metamuxer.BlobSource(file);

src/index.ts

@@ -18,6 +18,7 @@ export {
 	MkvOutputFormatOptions,
 	WebMOutputFormat,
 	WebMOutputFormatOptions,
+	Mp3OutputFormat,
 	WaveOutputFormat,
 	TrackCountLimits,
 	InclusiveRange,

src/input-format.ts

@@ -179,39 +179,6 @@ export class WebMInputFormat extends MatroskaInputFormat {
 	}
 }
 
-/** @public */
-export class WaveInputFormat extends InputFormat {
-	async _canReadInput(input: Input) {
-		const sourceSize = await input._mainReader.source._getSize();
-		if (sourceSize < 12) {
-			return false;
-		}
-
-		const riffReader = new RiffReader(input._mainReader);
-		const riffType = riffReader.readAscii(4);
-		if (riffType !== 'RIFF' && riffType !== 'RIFX') {
-			return false;
-		}
-
-		riffReader.pos = 8;
-		const format = riffReader.readAscii(4);
-		return format === 'WAVE';
-	}
-
-	/** @internal */
-	_createDemuxer(input: Input) {
-		return new WaveDemuxer(input);
-	}
-
-	getName() {
-		return 'WAVE';
-	}
-
-	getMimeType() {
-		return 'audio/wav';
-	}
-}
-
 /** @public */
 export class Mp3InputFormat extends InputFormat {
 	async _canReadInput(input: Input) {
@@ -248,6 +215,39 @@ export class Mp3InputFormat extends InputFormat {
 	}
 }
 
+/** @public */
+export class WaveInputFormat extends InputFormat {
+	async _canReadInput(input: Input) {
+		const sourceSize = await input._mainReader.source._getSize();
+		if (sourceSize < 12) {
+			return false;
+		}
+
+		const riffReader = new RiffReader(input._mainReader);
+		const riffType = riffReader.readAscii(4);
+		if (riffType !== 'RIFF' && riffType !== 'RIFX') {
+			return false;
+		}
+
+		riffReader.pos = 8;
+		const format = riffReader.readAscii(4);
+		return format === 'WAVE';
+	}
+
+	/** @internal */
+	_createDemuxer(input: Input) {
+		return new WaveDemuxer(input);
+	}
+
+	getName() {
+		return 'WAVE';
+	}
+
+	getMimeType() {
+		return 'audio/wav';
+	}
+}
+
 /** @public */
 export const MP4 = new Mp4InputFormat();
 /** @public */

src/mp3/mp3-demuxer.ts

@@ -5,7 +5,8 @@ import { InputAudioTrack, InputAudioTrackBacking } from '../input-track';
 import { SampleRetrievalOptions } from '../media-sink';
 import { assert, binarySearchExact, binarySearchLessOrEqual, last, UNDETERMINED_LANGUAGE } from '../misc';
 import { EncodedAudioSample, PLACEHOLDER_DATA } from '../sample';
-import { FrameHeader, Mp3Reader } from './mp3-reader';
+import { FrameHeader, getXingOffset, INFO, XING } from './mp3-misc';
+import { Mp3Reader } from './mp3-reader';
 
 const AUDIO_SAMPLES_PER_FRAME = 1152;
 
@@ -54,13 +55,10 @@ export class Mp3Demuxer extends Demuxer {
 					break;
 				}
 
-				const xingOffset = header.mpegVersionId === 3
-					? (header.channelCount === 1 ? 21 : 36)
-					: (header.channelCount === 1 ? 13 : 21);
+				const xingOffset = getXingOffset(header.mpegVersionId, header.channel);
 				this.reader.pos = header.startPos + xingOffset;
 				const word = this.reader.readU32();
-				const isXing = word === 0x58696e67 // 'Xing'
-					|| word === 0x496e666f; // 'Info'
+				const isXing = word === XING || word === INFO;
 
 				this.reader.pos = header.startPos + header.totalSize - 1; // -1 in case the frame is 1 byte too short
 
@@ -137,7 +135,7 @@ class Mp3AudioTrackBacking implements InputAudioTrackBacking {
 
 	async getNumberOfChannels() {
 		assert(this.demuxer.firstFrameHeader);
-		return this.demuxer.firstFrameHeader.channelCount;
+		return this.demuxer.firstFrameHeader.channel === 3 ? 1 : 2;
 	}
 
 	async getSampleRate() {
@@ -150,7 +148,7 @@ class Mp3AudioTrackBacking implements InputAudioTrackBacking {
 
 		return {
 			codec: 'mp3',
-			numberOfChannels: this.demuxer.firstFrameHeader.channelCount,
+			numberOfChannels: this.demuxer.firstFrameHeader.channel === 3 ? 1 : 2,
 			sampleRate: this.demuxer.firstFrameHeader.sampleRate,
 		};
 	}

src/mp3/mp3-misc.ts

@@ -0,0 +1,130 @@
+export const FRAME_HEADER_SIZE = 4;
+
+// These are in kbps:
+export const MPEG_V1_BITRATES: Record<number, number[]> = {
+	// Layer 3
+	1: [-1, 32, 40, 48, 56, 64, 80, 96, 112, 128, 160, 192, 224, 256, 320, -1],
+	// Layer 2
+	2: [-1, 32, 48, 56, 64, 80, 96, 112, 128, 160, 192, 224, 256, 320, 384, -1],
+	// Layer 1
+	3: [-1, 32, 64, 96, 128, 160, 192, 224, 256, 288, 320, 352, 384, 416, 448, -1],
+};
+export const MPEG_V2_BITRATES: Record<number, number[]> = {
+	// Layer 3
+	1: [-1, 32, 48, 56, 64, 80, 96, 112, 128, 144, 160, 176, 192, 224, 256, -1],
+	// Layer 2
+	2: [-1, 8, 16, 24, 32, 40, 48, 56, 64, 80, 96, 112, 128, 144, 160, -1],
+	// Layer 1
+	3: [-1, 8, 16, 24, 32, 40, 48, 56, 64, 80, 96, 112, 128, 144, 160, -1],
+};
+export const SAMPLING_RATES: Record<number, number[]> = {
+	// MPEG Version 2.5
+	0: [11025, 12000, 8000, -1],
+	// MPEG Version 2 (ISO/IEC 13818-3)
+	2: [22050, 24000, 16000, -1],
+	// MPEG Version 1 (ISO/IEC 11172-3)
+	3: [44100, 48000, 32000, -1],
+};
+
+/** 'Xing' */
+export const XING = 0x58696e67;
+/** 'Info' */
+export const INFO = 0x496e666f;
+
+export type FrameHeader = {
+	startPos: number;
+	mpegVersionId: number;
+	layer: number;
+	bitrate: number;
+	frequencyIndex: number;
+	sampleRate: number;
+	channel: number;
+	modeExtension: number;
+	copyright: number;
+	original: number;
+	emphasis: number;
+	totalSize: number;
+	dataStart: number;
+	dataSize: number;
+};
+
+export const computeMp3FrameSize = (bitrate: number, sampleRate: number, padding: number) => {
+	return Math.floor((144 * bitrate / sampleRate) + padding);
+};
+
+export const getXingOffset = (mpegVersionId: number, channel: number) => {
+	return mpegVersionId === 3
+		? (channel === 3 ? 21 : 36)
+		: (channel === 3 ? 13 : 21);
+};
+
+export const readFrameHeader = (word: number, reader: { pos: number; fileSize: number | null }): FrameHeader | null => {
+	const startPos = reader.pos;
+
+	const firstByte = word >>> 24;
+	const secondByte = (word >>> 16) & 0xff;
+	const thirdByte = (word >>> 8) & 0xff;
+	const fourthByte = word & 0xff;
+
+	if (firstByte !== 0xff && secondByte !== 0xff && thirdByte !== 0xff && fourthByte !== 0xff) {
+		reader.pos += 4;
+		return null;
+	}
+
+	reader.pos += 1;
+
+	if ((secondByte & 0xe0) !== 0xe0) {
+		return null;
+	}
+
+	const mpegVersionId = (secondByte >> 3) & 0x3;
+	const layer = (secondByte >> 1) & 0x3;
+
+	const bitrateIndex = (thirdByte >> 4) & 0xf;
+	const frequencyIndex = (thirdByte >> 2) & 0x3;
+	const padding = (thirdByte >> 1) & 0x1;
+
+	const channel = (fourthByte >> 6) & 0x3;
+	const modeExtension = (fourthByte >> 4) & 0x3;
+	const copyright = (fourthByte >> 3) & 0x1;
+	const original = (fourthByte >> 2) & 0x1;
+	const emphasis = fourthByte & 0x3;
+
+	const kilobitRate = mpegVersionId === 3
+		? MPEG_V1_BITRATES[layer]?.[bitrateIndex]
+		: MPEG_V2_BITRATES[layer]?.[bitrateIndex];
+	if (!kilobitRate || kilobitRate === -1) {
+		return null;
+	}
+
+	const bitrate = kilobitRate * 1000;
+
+	const sampleRate = SAMPLING_RATES[mpegVersionId]?.[frequencyIndex];
+	if (!sampleRate || sampleRate === -1) {
+		return null;
+	}
+
+	const frameLength = computeMp3FrameSize(bitrate, sampleRate, padding);
+
+	if (reader.fileSize !== null && reader.fileSize - startPos < frameLength) {
+		// The frame doesn't fit into the rest of the file
+		return null;
+	}
+
+	return {
+		startPos: startPos,
+		mpegVersionId,
+		layer,
+		bitrate,
+		frequencyIndex,
+		sampleRate,
+		channel,
+		modeExtension,
+		copyright,
+		original,
+		emphasis,
+		totalSize: frameLength,
+		dataStart: startPos + 4,
+		dataSize: frameLength - 4,
+	};
+};

src/mp3/mp3-muxer.ts

@@ -0,0 +1,125 @@
+import { assert } from '../misc';
+import { Muxer } from '../muxer';
+import { Output, OutputAudioTrack } from '../output';
+import { EncodedAudioSample } from '../sample';
+import { Writer } from '../writer';
+import { getXingOffset, INFO, readFrameHeader, XING } from './mp3-misc';
+import { Mp3Writer, XingFrameData } from './mp3-writer';
+
+export class Mp3Muxer extends Muxer {
+	private writer: Writer;
+	private mp3Writer: Mp3Writer;
+	private xingFrameData: XingFrameData | null = null;
+	private frameCount = 0;
+	private framePositions: number[] = [];
+
+	constructor(output: Output) {
+		super(output);
+
+		this.writer = output._writer;
+		this.mp3Writer = new Mp3Writer(output._writer);
+	}
+
+	async start() {
+		// Nothing needed here
+	}
+
+	async addEncodedVideoSample() {
+		throw new Error('MP3 does not support video.');
+	}
+
+	async addEncodedAudioSample(
+		track: OutputAudioTrack,
+		sample: EncodedAudioSample,
+	) {
+		const release = await this.mutex.acquire();
+
+		try {
+			if (!this.xingFrameData) {
+				const view = new DataView(sample.data.buffer, sample.data.byteOffset, sample.data.byteLength);
+				if (view.byteLength < 4) {
+					throw new Error('Invalid MP3 header in sample.');
+				}
+
+				const word = view.getUint32(0, false);
+				const header = readFrameHeader(word, { pos: 0, fileSize: null });
+				if (!header) {
+					throw new Error('Invalid MP3 header in sample.');
+				}
+
+				const xingOffset = getXingOffset(header.mpegVersionId, header.channel);
+				if (view.byteLength >= xingOffset + 4) {
+					const word = view.getUint32(xingOffset, false);
+					const isXing = word === XING || word === INFO;
+
+					if (isXing) {
+						// This is not a data frame, so let's completely ignore this sample
+						return;
+					}
+				}
+
+				this.xingFrameData = {
+					mpegVersionId: header.mpegVersionId,
+					layer: header.layer,
+					frequencyIndex: header.frequencyIndex,
+					channel: header.channel,
+					modeExtension: header.modeExtension,
+					copyright: header.copyright,
+					original: header.original,
+					emphasis: header.emphasis,
+
+					frameCount: null,
+					fileSize: null,
+					toc: null,
+				};
+
+				// Write a Xing frame because this muxer doesn't make any bitrate constraints, meaning we don't know if
+				// this will be a constant or variable bitrate file. Therefore, always write the Xing frame.
+				this.mp3Writer.writeXingFrame(this.xingFrameData);
+
+				this.frameCount++;
+			}
+
+			this.validateAndNormalizeTimestamp(track, sample.timestamp, sample.type === 'key');
+
+			this.framePositions.push(this.writer.getPos());
+			this.writer.write(sample.data);
+			this.frameCount++;
+
+			await this.writer.flush();
+		} finally {
+			release();
+		}
+	}
+
+	async addSubtitleCue() {
+		throw new Error('MP3 does not support subtitles.');
+	}
+
+	async finalize() {
+		if (!this.xingFrameData) {
+			return;
+		}
+
+		const endPos = this.writer.getPos();
+
+		this.writer.seek(0);
+
+		const toc = new Uint8Array(100);
+		for (let i = 0; i < 100; i++) {
+			const index = Math.floor(this.framePositions.length * (i / 100));
+			assert(index !== -1 && index < this.framePositions.length);
+
+			const byteOffset = this.framePositions[index]!;
+			toc[i] = 256 * (byteOffset / endPos);
+		}
+
+		this.xingFrameData.frameCount = this.frameCount;
+		this.xingFrameData.fileSize = endPos;
+		this.xingFrameData.toc = toc;
+
+		this.mp3Writer.writeXingFrame(this.xingFrameData);
+
+		this.writer.seek(endPos);
+	}
+}

src/mp3/mp3-reader.ts

@@ -1,44 +1,6 @@
 import { assert } from '../misc';
 import { Reader } from '../reader';
-
-const FRAME_HEADER_SIZE = 4;
-
-// These are in kbps:
-const MPEG_V1_BITRATES: Record<number, number[]> = {
-	// Layer 3
-	1: [-1, 32, 40, 48, 56, 64, 80, 96, 112, 128, 160, 192, 224, 256, 320, -1],
-	// Layer 2
-	2: [-1, 32, 48, 56, 64, 80, 96, 112, 128, 160, 192, 224, 256, 320, 384, -1],
-	// Layer 1
-	3: [-1, 32, 64, 96, 128, 160, 192, 224, 256, 288, 320, 352, 384, 416, 448, -1],
-};
-const MPEG_V2_BITRATES: Record<number, number[]> = {
-	// Layer 3
-	1: [-1, 32, 48, 56, 64, 80, 96, 112, 128, 144, 160, 176, 192, 224, 256, -1],
-	// Layer 2
-	2: [-1, 8, 16, 24, 32, 40, 48, 56, 64, 80, 96, 112, 128, 144, 160, -1],
-	// Layer 1
-	3: [-1, 8, 16, 24, 32, 40, 48, 56, 64, 80, 96, 112, 128, 144, 160, -1],
-};
-const SAMPLING_RATES: Record<number, number[]> = {
-	// MPEG Version 2.5
-	0: [11025, 12000, 8000, -1],
-	// MPEG Version 2 (ISO/IEC 13818-3)
-	2: [22050, 24000, 16000, -1],
-	// MPEG Version 1 (ISO/IEC 11172-3)
-	3: [44100, 48000, 32000, -1],
-};
-
-export type FrameHeader = {
-	startPos: number;
-	mpegVersionId: number;
-	bitrate: number;
-	sampleRate: number;
-	channelCount: number;
-	totalSize: number;
-	dataStart: number;
-	dataSize: number;
-};
+import { FRAME_HEADER_SIZE, FrameHeader, readFrameHeader } from './mp3-misc';
 
 export class Mp3Reader {
 	pos = 0;
@@ -96,66 +58,13 @@ export class Mp3Reader {
 		until ??= this.fileSize;
 
 		while (this.pos < until - FRAME_HEADER_SIZE) {
-			const startPos = this.pos;
 			const word = this.readU32();
+			this.pos -= 4;
 
-			const firstByte = word >>> 24;
-			const secondByte = (word >>> 16) & 0xff;
-			const thirdByte = (word >>> 8) & 0xff;
-			const fourthByte = word & 0xff;
-
-			if (firstByte !== 0xff && secondByte !== 0xff && thirdByte !== 0xff && fourthByte !== 0xff) {
-				continue;
+			const header = readFrameHeader(word, this);
+			if (header) {
+				return header;
 			}
-
-			this.pos -= 3; // For when we continue
-
-			if ((secondByte & 0xe0) !== 0xe0) {
-				continue;
-			}
-
-			const mpegVersionId = (secondByte >> 3) & 0x3;
-			const layer = (secondByte >> 1) & 0x3;
-
-			const bitrateIndex = (thirdByte >> 4) & 0xf;
-			const frequencyIndex = (thirdByte >> 2) & 0x3;
-			const padding = (thirdByte >> 1) & 0x1;
-
-			const channel = (fourthByte >> 6) & 0x3;
-
-			const kilobitRate = mpegVersionId === 3
-				? MPEG_V1_BITRATES[layer]?.[bitrateIndex]
-				: MPEG_V2_BITRATES[layer]?.[bitrateIndex];
-			if (!kilobitRate || kilobitRate === -1) {
-				continue;
-			}
-
-			const bitrate = kilobitRate * 1000;
-
-			const sampleRate = SAMPLING_RATES[mpegVersionId]?.[frequencyIndex];
-			if (!sampleRate || sampleRate === -1) {
-				continue;
-			}
-
-			const channelCount = channel === 3 ? 1 : 2;
-
-			const frameLength = Math.floor((144 * bitrate / sampleRate) + padding);
-
-			if (this.fileSize - startPos < frameLength) {
-				// The frame doesn't fit into the rest of the file
-				return null;
-			}
-
-			return {
-				startPos,
-				mpegVersionId,
-				bitrate,
-				sampleRate,
-				channelCount,
-				totalSize: frameLength,
-				dataStart: startPos + 4,
-				dataSize: frameLength - 4,
-			};
 		}
 
 		return null;

src/mp3/mp3-writer.ts

@@ -0,0 +1,104 @@
+import { Writer } from '../writer';
+import {
+	computeMp3FrameSize,
+	getXingOffset,
+	MPEG_V1_BITRATES,
+	MPEG_V2_BITRATES,
+	SAMPLING_RATES,
+	XING,
+} from './mp3-misc';
+
+export type XingFrameData = {
+	mpegVersionId: number;
+	layer: number;
+	frequencyIndex: number;
+	channel: number;
+	modeExtension: number;
+	copyright: number;
+	original: number;
+	emphasis: number;
+
+	frameCount: number | null;
+	fileSize: number | null;
+	toc: Uint8Array | null;
+};
+
+export class Mp3Writer {
+	private helper = new Uint8Array(8);
+	private helperView = new DataView(this.helper.buffer);
+
+	constructor(private writer: Writer) {}
+
+	writeU32(value: number) {
+		this.helperView.setUint32(0, value, false);
+		this.writer.write(this.helper.subarray(0, 4));
+	}
+
+	writeXingFrame(data: XingFrameData) {
+		const startPos = this.writer.getPos();
+
+		const firstByte = 0xff;
+		const secondByte = 0xe0 | (data.mpegVersionId << 3) | (data.layer << 1);
+
+		const bitrateGroup = data.mpegVersionId === 3 ? MPEG_V1_BITRATES : MPEG_V2_BITRATES;
+		const bitrates = bitrateGroup?.[data.layer];
+		if (!bitrates) {
+			throw new Error('Invalid MPEG version and layer combination.');
+		}
+
+		const sampleRate = SAMPLING_RATES[data.mpegVersionId]?.[data.frequencyIndex];
+		if (!sampleRate || sampleRate === -1) {
+			throw new Error('Invalid MPEG version and frequency index combination.');
+		}
+
+		const padding = 0;
+		const neededBytes = 155;
+
+		// Let's find the lowest bitrate for which the frame size is sufficiently large to fit all the data
+		const bitrateIndex = bitrates.findIndex((kbr) => {
+			return computeMp3FrameSize(1000 * kbr, sampleRate, padding) >= neededBytes;
+		});
+		if (bitrateIndex === -1) {
+			throw new Error('No suitable bitrate found.');
+		}
+
+		const thirdByte = (bitrateIndex << 4) | (data.frequencyIndex << 2) | padding << 1;
+		const fourthByte = (data.channel << 6)
+			| (data.modeExtension << 4)
+			| (data.copyright << 3)
+			| (data.original << 2)
+			| data.emphasis;
+
+		this.helper[0] = firstByte;
+		this.helper[1] = secondByte;
+		this.helper[2] = thirdByte;
+		this.helper[3] = fourthByte;
+
+		this.writer.write(this.helper.subarray(0, 4));
+
+		const xingOffset = getXingOffset(data.mpegVersionId, data.channel);
+
+		this.writer.seek(startPos + xingOffset);
+		this.writeU32(XING);
+
+		let flags = 0;
+		if (data.frameCount !== null) {
+			flags |= 1;
+		}
+		if (data.fileSize !== null) {
+			flags |= 2;
+		}
+		if (data.toc !== null) {
+			flags |= 4;
+		}
+
+		this.writeU32(flags);
+
+		this.writeU32(data.frameCount ?? 0);
+		this.writeU32(data.fileSize ?? 0);
+		this.writer.write(data.toc ?? new Uint8Array(100));
+
+		const frameSize = computeMp3FrameSize(1000 * bitrates[bitrateIndex]!, sampleRate, padding);
+		this.writer.seek(startPos + frameSize);
+	}
+}

src/muxer.ts

@@ -14,12 +14,12 @@ export abstract class Muxer {
 	abstract start(): Promise<void>;
 	abstract addEncodedVideoSample(
 		track: OutputVideoTrack,
-		chunk: EncodedVideoSample,
+		sample: EncodedVideoSample,
 		meta?: EncodedVideoChunkMetadata
 	): Promise<void>;
 	abstract addEncodedAudioSample(
 		track: OutputAudioTrack,
-		chunk: EncodedAudioSample,
+		sample: EncodedAudioSample,
 		meta?: EncodedAudioChunkMetadata
 	): Promise<void>;
 	abstract addSubtitleCue(track: OutputSubtitleTrack, cue: SubtitleCue, meta?: SubtitleMetadata): Promise<void>;

src/output-format.ts

@@ -11,6 +11,7 @@ import {
 } from './codec';
 import { IsobmffMuxer } from './isobmff/isobmff-muxer';
 import { MatroskaMuxer } from './matroska/matroska-muxer';
+import { Mp3Muxer } from './mp3/mp3-muxer';
 import { Muxer } from './muxer';
 import { Output, TrackType } from './output';
 import { WaveMuxer } from './wave/wave-muxer';
@@ -256,6 +257,40 @@ export class WebMOutputFormat extends MkvOutputFormat {
 	}
 }
 
+/** @public */
+export class Mp3OutputFormat extends OutputFormat {
+	/** @internal */
+	_createMuxer(output: Output) {
+		return new Mp3Muxer(output);
+	}
+
+	/** @internal */
+	_getName() {
+		return 'MP3';
+	}
+
+	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 '.mp3';
+	}
+
+	getSupportedCodecs() {
+		return Mp3OutputFormat.getSupportedCodecs();
+	}
+
+	static getSupportedCodecs(): MediaCodec[] {
+		return ['mp3'];
+	}
+}
+
 /** @public */
 export class WaveOutputFormat extends OutputFormat {
 	/** @internal */

src/wave/wave-muxer.ts

@@ -24,7 +24,7 @@ export class WaveMuxer extends Muxer {
 	}
 
 	async addEncodedVideoSample() {
-		throw new Error('WAVE format does not support video.');
+		throw new Error('WAVE does not support video.');
 	}
 
 	async addEncodedAudioSample(
@@ -46,19 +46,17 @@ export class WaveMuxer extends Muxer {
 
 			this.validateAndNormalizeTimestamp(track, sample.timestamp, sample.type === 'key');
 
-			if (sample.data) {
-				this.output._writer.write(sample.data);
-				this.dataSize += sample.data.byteLength;
-			}
+			this.writer.write(sample.data);
+			this.dataSize += sample.data.byteLength;
 
-			await this.output._writer.flush();
+			await this.writer.flush();
 		} finally {
 			release();
 		}
 	}
 
 	async addSubtitleCue() {
-		throw new Error('WAVE format does not support subtitles.');
+		throw new Error('WAVE does not support subtitles.');
 	}
 
 	private writeHeader(track: OutputAudioTrack, config: AudioDecoderConfig) {
@@ -102,7 +100,7 @@ export class WaveMuxer extends Muxer {
 	}
 
 	async finalize() {
-		const currentPos = this.writer.getPos();
+		const endPos = this.writer.getPos();
 
 		// Write file size
 		this.writer.seek(4);
@@ -112,6 +110,6 @@ export class WaveMuxer extends Muxer {
 		this.writer.seek(40);
 		this.riffWriter.writeU32(this.dataSize);
 
-		this.writer.seek(currentPos);
+		this.writer.seek(endPos);
 	}
 }