@wasm-audio-decoders/aac

@wasm-audio-decoders/aac is a Web Assembly AAC audio decoder.

• 226.4 KiB minified bundle size
• Supports ADTS and ADIF streams, and raw AAC frames using an AudioSpecificConfig
• Browser and NodeJS support
• Built in Web Worker support
• Multichannel decoding (up to 8 channels)
• Based on faad2 and codec-parser

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 @wasm-audio-decoders/aac

  import { AACDecoder } from '@wasm-audio-decoders/aac';
  
  const decoder = new AACDecoder();

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

  <script src="aac-decoder.min.js" charset="UTF-8"></script>
  <script>
    const decoder = new window["aac-decoder"].AACDecoder();
  </script>

• Note that the script must be read using UTF-8 character encoding. If running in a browser, the <script> tag must include the charset="UTF-8" attribute; or, the loading HTML must include <meta charset="utf-8" />.

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 web worker asynchronously.

   Main thread synchronous decoding

   import { AACDecoder } from '@wasm-audio-decoders/aac';
   
   const decoder = new AACDecoder();
   
   // wait for the WASM to be compiled
   await decoder.ready;

   Web Worker asynchronous decoding

   import { AACDecoderWebWorker } from '@wasm-audio-decoders/aac';
   
   const decoder = new AACDecoderWebWorker();
   
   // wait for the WASM to be compiled
   await decoder.ready;

2. Begin decoding AAC data. ADTS and ADIF streams are detected automatically.

   const {channelData, samplesDecoded, sampleRate} = await decoder.decode(aacData);

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 AAC data.
   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: 44100, // sample rate of the decoded PCM
    errors: [ // array containing descriptions for any decode errors
      {
        message: "Unable to find ADTS syncword",
        frameLength: 400, // length of the frame or data in bytes that encountered an error
        frameNumber: 5, // position of error relative to total frames decoded 
        inputBytes: 11606, // position of error relative to total input bytes
        outputSamples: 20480, // position of error relative to total output samples
      }
    ]
}

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

Decoding will proceed through any errors. Any errors encountered may result in gaps in the decoded audio.

Multichannel Output

Each channel is assigned to a speaker location in a conventional surround arrangement, following the same speaker ordering convention as the other decoders in this repository (fronts ordered left, center, right; LFE last). Specific locations depend on the channel configuration of the AAC stream, and are given below in order of the corresponding channel indices.

• 1 channel: monophonic (mono).
• 2 channels: stereo (left, right).
• 3 channels: linear surround (front left, front center, front right).
• 4 channels: 4.0 surround (front left, front center, front right, back center).
• 5 channels: 5.0 surround (front left, front center, front right, back left, back right).
• 6 channels: 5.1 surround (front left, front center, front right, back left, back right, LFE).
• 7 channels: 6.1 surround (front left, front center, front right, side left, side right, back center, LFE).
• 8 channels: 7.1 surround (front left, front center, front right, side left, side right, back left, back right, LFE).

AACDecoder

Class that decodes AAC synchronously on the main thread.

Options

const decoder = new AACDecoder({
  audioSpecificConfig: undefined, // optional Uint8Array
});

• audioSpecificConfig optional
  • A Uint8Array containing an MPEG-4 AudioSpecificConfig, i.e. from the esds box of an MP4 file.
  • When set, the decoder is configured for raw AAC frames without any transport headers. Decode using decodeFrames(); decode() and decodeFile() are not supported in this mode.

Getters

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

Methods

• decoder.decode(aacData) async
  • aacData Uint8Array containing ADTS or ADIF AAC data.
  • Returns a promise that resolves with the decoded audio.
  • Use this when streaming audio into the decoder.
• decoder.flush() async
  • Returns a promise that resolves with any remaining data in the buffer.
  • Use this when you are finished piping audio in through the decode method to retrieve any remaining data in the buffer.
• decoder.decodeFile(aacData) async
  • aacData Uint8Array containing ADTS or ADIF AAC data.
  • Returns a promise that resolves with the decoded audio.
  • Use this when decoding an entire file.
• decoder.decodeFrames(aacFrames) async
  • aacFrames Array of Uint8Array containing complete ADTS frames, or raw AAC frames when the audioSpecificConfig option is set.
  • Returns a promise that resolves with the decoded audio.
  • Use this when you already have the AAC frames parsed and split into individual buffers.
• decoder.reset() async
  • Resets the decoder so that a new stream of AAC data 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 AAC data.

AACDecoderWebWorker

Class that decodes AAC 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 AACDecoderWebWorker({
  audioSpecificConfig: undefined, // optional Uint8Array
});

• audioSpecificConfig optional
  • A Uint8Array containing an MPEG-4 AudioSpecificConfig, i.e. from the esds box of an MP4 file.
  • When set, the decoder is configured for raw AAC frames without any transport headers. Decode using decodeFrames(); decode() and decodeFile() are not supported in this mode.

Getters

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

Methods

• decoder.decode(aacData) async
  • aacData Uint8Array containing ADTS or ADIF AAC data.
  • Returns a promise that resolves with the decoded audio.
  • Use this when streaming audio into the decoder.
• decoder.flush() async
  • Returns a promise that resolves with any remaining data in the buffer.
  • Use this when you are finished piping audio in through the decode method to retrieve any remaining data in the buffer.
• decoder.decodeFile(aacData) async
  • aacData Uint8Array containing ADTS or ADIF AAC data.
  • Returns a promise that resolves with the decoded audio.
  • Use this when decoding an entire file.
• decoder.decodeFrames(aacFrames) async
  • aacFrames Array of Uint8Array containing complete ADTS frames, or raw AAC frames when the audioSpecificConfig option is set.
  • Returns a promise that resolves with the decoded audio.
  • Use this when you already have the AAC frames parsed and split into individual buffers.
• decoder.reset() async
  • Resets the decoder so that a new stream of AAC data can be decoded.
• decoder.free() async
  • 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 AAC data.

Properly using the Web Worker interface

AACDecoderWebWorker 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 AACDecoderWebWorker 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.decode(data1).then(playAudio);
  decoder.decode(data2).then(playAudio);
  decoder.decode(data3).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.decode(data1); // blocks the main thread
  playAudio(decoded1);
  
  const decoded2 = await decoder.decode(data2); // blocks the main thread
  playAudio(decoded2);
  
  const decoded3 = await decoder.decode(data3); // blocks the main thread
  playAudio(decoded3);

Examples

Decoding multiple files using a single instance of AACDecoderWebWorker

This example shows how to decode multiple files using a single AACDecoderWebWorker instance. This code iterates over an array of input files (Array of Uint8Arrays) and queues up each file to be decoded one at a time.

First, wait for the decoder to become ready by calling decoder.ready.

For each iteration, decode() is called, it's result is pushed to the decodedFiles array, and decoder.reset() is called to prepare the decoder for a new file. These operations are queued up to the decoder instance and will complete one after another.

Finally, a call to decoder.free() is queued to clean up the memory stored by the decoder. This resolves when it and all of the other operations before it complete.

It's important to note that there is only one await operations in this example. Decoding can happen asynchronously and you only need to await when you need to use the results of the decode operation.

  const inputFiles = [file1, file2, file3] // Array of Uint8Array file data

  const decoder = new AACDecoderWebWorker();

  const decodedFiles = [];

  const decodePromise = decoder.ready // wait for the decoder to be ready
    .then(() => {
      for (const file of inputFiles) {
        decoder.decode(file) // queue the decode operation
          .then((result) => decodedFiles.push(result)); // save the decode result after decode completes
        decoder.reset(); // queue the reset operation
      }
    })
    .then(() => decoder.free()); // queue the free operation that will execute after the above operations

  // do sync operations here

  // await when you need to have the all of the audio data decoded
  await decodePromise;

Decoding multiple files using multiple instances of AACDecoderWebWorker

This example shows how to decode multiple files using multiple instances of AACDecoderWebWorker. This code iterates over an array of input files (Array of Uint8Arrays) and spawns a new AACDecoderWebWorker instance for each file and decodes the file. If you want to take full advantage of multi-core devices, this is the approach you will want to take since it will parallelize the decoding

For each input file, a new decoder is created, and the file is decoded using the decode() after decoder.ready is resolved. The result of the decode() operation is returned, and a finally() function on the promise calls decoder.free() to free up the instance after the decode operations are completed.

Finally, Promise.all() wraps this array of promises and resolves when all decode operations are complete.

It's important to note that there is only one await operation in this example. Decoding can happen asynchronously and you only need to await when you need to use the results of the decode operation.

  const inputFiles = [file1, file2, file3] // Array of Uint8Array file data

  // loops through each Uint8Array in `inputFiles` and decodes the files in separate threads
  const decodePromise = Promise.all(
    inputFiles.map((file) => {
      const decoder = new AACDecoderWebWorker();

      return decoder.ready
        .then(() => decoder.decode(file)) // decode the input file
        .finally(() => decoder.free()); // free the decoder after resolving the decode result
    })
  );

  // do sync operations here

  // await when you need to have the all of the audio data decoded
  const decodedFiles = await decodePromise;