wasm-media-encoders

The LAME MP3 and Ogg Vorbis audio encoders, compiled to minimal WebAssembly and ready for use in the browser or Node.

Why?

While some browsers have native encoding using the MediaRecorder API, it only supports a small number of codecs.

This package aims to fill the gap by compiling the reference LAME and Ogg Vorbis encoders to WebAssembly. The resulting package is tiny and fast:

Encoder	JS*	WASM	Combined + Gzipped
MP3	3.3 KiB	130 KiB	66 KiB
Ogg Vorbis	3.3 KiB	440 KiB	158 KiB

_{*When using ESNext syntax and no polyfills}

Installation

yarn add wasm-media-encoders

or

npm install wasm-media-encoders

Getting started

With webpack or .mjs file in Node 12+:

import { createMp3Enoder, createOggEncoder } from "wasm-media-encoders";

createOggEncoder().then((encoder) => {
  /* Configure and use the encoder */
});
createMp3Encoder().then((encoder) => {
  /* Configure and use the encoder */
});

With a <script> tag:

<script src="https://unpkg.com/wasm-media-encoders/dist/umd/WasmMediaEncoder.min.js"></script>
<script>
  // The UMD package will fetch() the WASM binaries from
  // unpkg.com by default to reduce size

  WasmMediaEncoder.createMp3Encoder().then((encoder) => {
    /* Configure and use the encoder */
  });
</script>

Using CommonJS in Node:

const { createMp3Encoder } = require("wasm-media-encoders");
createMp3Encoder().then((encoder) => {
  /* Configure and use the encoder */
});

If you'd like to use ESNext syntax and transpile yourself, use the wasm-media-encoders/esnext path.

Example usage

createMp3Encoder().then((encoder) => {
  encoder.configure({
    sampleRate: 48000,
    channels: 2,
    vbrQuality: 2,
  });

  let outBuffer = new Uint8Array(1024 * 1024);
  let offset = 0;
  let moreData = true;

  while (true) {
    const mp3Data = moreData
      ? encoder.encode([
          pcm_l /* Float32Array of left channel PCM data */,
          pcm_r /* Float32Array of right channel PCM data */,
        ])
      : /* finalize() returns the last few frames */
        encoder.finalize();

    /* mp3Data is a Uint8Array that is still owned by the encoder and MUST be copied */

    if (mp3Data.length + offset > outBuffer.length) {
      const newBuffer = new Uint8Array(mp3Data.length + offset);
      newBuffer.set(outBuffer);
      outBuffer = newBuffer;
    }

    outBuffer.set(mp3Data, offset);
    offset += mp3Data.length;

    if (!moreData) {
      break;
    }

    moreData = false;
  }

  return new Uint8Array(outBuffer.buffer, 0, offset);

  /* Or encoder can be reused without calling createEncoder() again:

  encoder.configure({...new params})
  encoder.encode()
  encoder.finalize() */
});

Reducing bundle size

By default, this package inlines the WASM binary as a base64-encoded data URL. This make importing the encoder easy, but also increases the bundle size by about 30%. With webpack, you can load the WASM directly (found in wasm-media-encoders/wasm/(mp3|ogg).wasm), passing the URL as the second parameter to createEncoder().

Webpack v4

Besides using file-loader, you also need to disable WASM parsing in the webpack config:

import { createEncoder } from "wasm-media-encoders";
import wasm from "file-loader!wasm-media-encoders/wasm/mp3.wasm";

createEncoder("audio/mpeg", wasm).then((encoder) => {
  /* Now mp3.wasm will be copied to output dir by webpack and fetch()ed at runtime */
});

// webpack.config.js

module.exports = {
  // ...
  module: {
    rules: [
      {
        test: /\.wasm$/,
        type: "javascript/auto",
      },
    ],
  },
};

Webpack v5

Using asset modules and URL assets is easy in webpack v5:

import { createEncoder } from "wasm-media-encoders";

createEncoder(
  "audio/mpeg",
  new URL("wasm-media-encoder/wasm/mp3", import.meta.url)
).then((encoder) => {
  /* Now mp3.wasm will be copied to output dir by webpack and fetch()ed at runtime */
});

API

Named exports:

createMp3Encoder(): Promise<WasmMediaEncoder>

createOggEncoder(): Promise<WasmMediaEncoder>

These two named exports use inline base-64 encoded WASM binaries (or fetch() from unpkg.com in the case of UMD). Tree-shaking in bundlers should prevent unused encoders from being included in the final bundle.

createEncoder(mimeType, wasm, moduleCallback?): Promise<WasmMediaEncoder>

Parameter	Type	Description
mimeType	string	The MIME type of the encoder to create. Supported values are 'audio/mpeg' (MP3) or 'audio/ogg' (Ogg Vorbis)
wasm	string | ArrayBuffer | Uint8Array | WebAssembly.Module	A URL, base64 data URL, buffer, or compiled WebAssembly.Module representing the WASM binary for the specific mimeType. The WASM binaries are included in the package under wasm-media-encoders/wasm/(mp3|ogg).wasm. Non-data URLs are not supported before Node 16; use node-fetch if this is something you need.
moduleCallback?	((module: WebAssembly.Module, wasmVersion: string, wasmMimeType: string) => void) | undefined	Optionally, a callback that will be called with the compiled WebAssembly module and the module's version and MIME type. You can cache this module client-side and pass it as the wasm parameter to avoid fetching and compiling it every time. If you cache the WASM module, be sure the wasmVersion parameter matches the return value of jsLibraryVersion().

jsLibraryVersion(): string

Returns an opaque version string for the Javascript portion of this library. This string must match the WASM version string provided in the moduleCallback wasmVersion parameter documented above.

WasmMediaEncoder

configure(options): void

Configures the encoder. This method can be called at any time to reset the state of the encoder, including after finalize().

The options object is a union of common properties and encoder-specific ones. All common options are required.

Common options:

Property	Type	Description
channels	Number	The number of channels to be encoded. Currently only 1 or 2 channels are supported.
sampleRate	Number	Sample rate of data to be encoded, in hertz.

Options for MIME type audio/mpeg (MP3):

vbrQuality and bitrate are mutually exclusive.

Property	Type	Default	Description
vbrQuality?	Number | undefined	4.0	Variable Bitrate (VBR) quality for the LAME encoder, from 0.0 (best quality) to 9.999 (worst quality). See here for details.
bitrate?	Number | undefined		Constant bitrate in kbit/s. Valid bitrates are 8, 16, 24, 32, 40, 48, 64, 80, 96, 112, 128, 160, 192, 224, 256, or 320.
outputSampleRate?	Number | undefined		Output sample rate in hertz. By default, LAME will pick the best sample rate based on the amount of compression. Valid sample rates for MPEG1 are 32000, 44100, and 48000. Valid sample rates for MPEG2 are 16000, 22050, and 24000. Valid sample rates for MPEG2.5 are 8000, 11025, and 12000.

Options for MIME type audio/ogg (Ogg Vorbis):

Property	Type	Default	Description
vbrQuality?	Number | undefined	3.0	Variable Bitrate (VBR) quality for the Vorbis encoder, from -1.0 (worst quality) to 10.0 (best quality). See here for approximate bitrates.

encode(samples): Uint8Array

Encodes PCM samples and returns a Uint8Array. You may call this method repeatedly to add more samples (e.g. if streaming in PCM data). May return a Uint8Array of length zero. The returned Uint8Array is owned by the encoder and MUST be copied before any other encoder methods are called.

Parameter	Type	Description
samples	Float32Array[]	A channels-length array of Float32Array representing the PCM data to encode. Each sample must be in the range of -1.0 to 1.0

finalize(): Uint8Array

Flushes the encoder and returns the last few encoded samples. May return a Uint8Array of length zero. The returned Uint8Array is owned by the encoder and MUST be copied before any other encoder methods are called.

Building

Dev Container

The easiest way to get started is by creating a development container using the included .devcontainer/devcontainer.json. Tools that support dev containers can be found here. Development containers require a Docker or Podman installation. Full instructions for Visual Studio Code can be found here.

Manually setting up environment

To manually set up a development environment, you must install the following prerequisites:

1. Emscripten SDK (v2.0.8)

2. Node 20

Note that the Emscripten SDK includes Node 12 and will include it in the PATH. Node 20 must appear before Node 12 in the PATH.

Enable yarn using corepack:

corepack enable
# If no network access, install the checked-in version:
# corepack install -g .devcontainer/corepack.tgz

Building

To create a release build:

make

Testing

make test

License

This project is licensed under the terms of the MIT license.