opus-decoder

opus-decoder

opus-decoder is a Web Assembly Opus audio decoder.

• 82.4 KiB minified bundle size
• Browser and NodeJS support
• Built in Web Worker support
• Based on libopus

See the homepage of this repository for more Web Assembly audio decoders like this one.

Checkout the demo here

Installing

• Install from NPM.

  Run npm i opus-decoder

  import { OpusDecoder } from 'opus-decoder';
  
  const decoder = new OpusDecoder();
  

• Or download the build and include it as a script.

  <script src="opus-decoder.min.js"></script>
  <script>
    const decoder = new window["opus-decoder"].OpusDecoder();
  </script>
  

Usage

1. Create a new instance and wait for the WASM to finish compiling. Decoding can be done on the main thread synchronously, or in a webworker asynchronously.

   Main thread synchronous decoding

   import { OpusDecoder } from 'opus-decoder';
   
   const decoder = new OpusDecoder();
   
   // wait for the WASM to be compiled
   await decoder.ready;
   

   Web Worker asynchronous decoding

   import { OpusDecoderWebWorker } from 'opus-decoder';
   
   const decoder = new OpusDecoderWebWorker();
   
   // wait for the WASM to be compiled
   await decoder.ready;
   

2. Begin decoding Opus frames.

   // Decode an individual Opus frame
   const {channelData, samplesDecoded, sampleRate} = decoder.decodeFrame(opusFrame);
   
   // Decode an array of individual Opus frames
   const {channelData, samplesDecoded, sampleRate} = decoder.decodeFrames(opusFrameArray);
   

3. When done decoding, reset the decoder to decode a new stream, or free up the memory being used by the WASM module if you have no more audio to decode.

   // `reset()` clears the decoder state and allows you do decode a new stream of Opus frames.
   await decoder.reset();
   
   // `free()` de-allocates the memory used by the decoder. You will need to create a new instance after calling `free()` to start decoding again.
   decoder.free();
   

API

Decoded audio is always returned in the below structure.

{
    channelData: [
      leftAudio, // Float32Array of PCM samples for the left channel
      rightAudio, // Float32Array of PCM samples for the right channel
      ... // additional channels
    ],
    samplesDecoded: 1234, // number of PCM samples that were decoded per channel
    sampleRate: 48000 // sample rate of the decoded PCM
}

Each Float32Array within channelData can be used directly in the WebAudio API for playback.

Multichannel Output

Each channel is assigned to a speaker location in a conventional surround arrangement. Specific locations depend on the number of channels, and are given below in order of the corresponding channel indices. This set of surround options and speaker location orderings is the same as those used by the Vorbis codec.

• 1 channel: monophonic (mono).
• 2 channels: stereo (left, right).
• 3 channels: linear surround (left, center, right).
• 4 channels: quadraphonic (front left, front right, rear left, rear right).
• 5 channels: 5.0 surround (front left, front center, front right, rear left, rear right).
• 6 channels: 5.1 surround (front left, front center, front right, rear left, rear right, LFE).
• 7 channels: 6.1 surround (front left, front center, front right, side left, side right, rear center, LFE).
• 8 channels: 7.1 surround (front left, front center, front right, side left, side right, rear left, rear right, LFE).

See: https://datatracker.ietf.org/doc/html/rfc7845.html#section-5.1.1.2

Each Float32Array within channelData can be used directly in the WebAudio API for playback.

OpusDecoder

Class that decodes Opus frames synchronously on the main thread.

Options

const decoder = new OpusDecoder({ 
  preSkip: 0,
  channels: 2,
  streamCount: 1,
  coupledStreamCount: 1,
  channelMappingTable: [0, 1]
});

The below options should be obtained from the Opus Header.

See this documentation on the Opus header for more information. If you don't have access to the Opus header, the default values will successfully decode most stereo Opus streams.

• preSkip optional, defaults to 0
  • Number of samples to skip at the beginning reported by the Opus header.

Required for Multichannel Decoding. (Channel Mapping Family >= 1)

• channels optional, defaults to 2
  • Number of channels reported by the Opus header.
• streamCount optional, defaults to 1
  • Number of streams reported by the Opus header.
• coupledStreamCount optional, defaults to: 1 when 2 channels, 0 when 1 channel
  • Number of coupled streams reported by the Opus header.
• channelMappingTable optional, defaults to [0, 1] when 2 channels, [0] when 1 channel
  • Channel mapping reported by the Opus header.

Getters

• decoder.ready async
  • Returns a promise that is resolved when the WASM is compiled and ready to use.

Methods

• decoder.decodeFrame(opusFrame)
  • opusFrame Uint8Array containing a single Opus frame.
  • Returns decoded audio.
• decoder.decodeFrames(opusFrames)
  • opusFrames Array of Uint8Arrays containing Opus frames.
  • Returns decoded audio.
• decoder.reset() async
  • Resets the decoder so that a new stream of Opus frames can be decoded.
• decoder.free()
  • De-allocates the memory used by the decoder.
  • After calling free(), the current instance is made unusable, and a new instance will need to be created to decode additional Opus frames.

OpusDecoderWebWorker

Class that decodes Opus frames asynchronously within a web worker. Decoding is performed in a separate, non-blocking thread. Each new instance spawns a new worker allowing you to run multiple workers for concurrent decoding of multiple streams.

Options

const decoder = new OpusDecoderWebWorker({ 
  preSkip: 0,
  channels: 2,
  streamCount: 1,
  coupledStreamCount: 1,
  channelMappingTable: [0, 1]
});

The below options should be obtained from the Opus Header.

See this documentation on the Opus header for more information. If you don't have access to the Opus header, the default values will successfully decode most stereo Opus streams.

• preSkip optional, defaults to 0
  • Number of samples to skip at the beginning reported by the Opus header.

Required for Multichannel Decoding. (Channel Mapping Family >= 1)

• channels optional, defaults to 2
  • Number of channels reported by the Opus header.
• streamCount optional, defaults to 1
  • Number of streams reported by the Opus header.
• coupledStreamCount optional, defaults to: 1 when 2 channels, 0 when 1 channel
  • Number of coupled streams reported by the Opus header.
• channelMappingTable optional, defaults to [0, 1] when 2 channels, [0] when 1 channel
  • Channel mapping reported by the Opus header.

Getters

• decoder.ready async
  • Returns a promise that is resolved when the WASM is compiled and ready to use.

Methods

• decoder.decodeFrame(opusFrame) async
  • opusFrame Uint8Array containing a single Opus frame.
  • Returns a promise that resolves with the decoded audio.
• decoder.decodeFrames(opusFrames) async
  • opusFrames Array of Uint8Arrays containing Opus frames.
  • Returns a promise that resolves with the decoded audio.
• decoder.reset() async
  • Resets the decoder so that a new stream of Opus frames can be decoded.
• decoder.free() async
  • De-allocates the memory used by the decoder and terminates the web worker.
  • After calling free(), the current instance is made unusable, and a new instance will need to be created to decode additional Opus frames.

Properly using the Web Worker interface

OpusDecoderWebWorker uses async functions to send operations to the web worker without blocking the main thread. To fully take advantage of the concurrency provided by web workers, your code should avoid using await on decode operations where it will block the main thread.

Each method call on a OpusDecoderWebWorker instance will queue up an operation to the web worker. Operations will complete within the web worker thread one at a time and in the same order in which the methods were called.

• Good Main thread is not blocked during each decode operation. The example playAudio function is called when each decode operation completes. Also, the next decode operation can begin while playAudio is doing work on the main thread.

  const playAudio = ({ channelData, samplesDecoded, sampleRate }) => {
    // does something to play the audio data.
  }
  
  decoder.decodeFrame(frameData1).then(playAudio);
  decoder.decodeFrame(frameData2).then(playAudio);
  decoder.decodeFrame(frameData3).then(playAudio);
  
  // do some other operations while the audio is decoded
  

• Bad Main thread is being blocked by await during each decode operation. Synchronous code is halted while decoding completes, negating the benefits of using a webworker.

  const decoded1 = await decoder.decodeFrame(frameData1); // blocks the main thread
  playAudio(decoded1);
  
  const decoded2 = await decoder.decodeFrame(frameData2); // blocks the main thread
  playAudio(decoded2);
  
  const decoded3 = await decoder.decodeFrame(frameData3); // blocks the main thread
  playAudio(decoded3);