BaseAudioContext

The BaseAudioContext interface acts as a supervisor of audio-processing graphs. It provides key processing parameters such as current time, output destination or sample rate. Additionally, it is responsible for nodes creation and audio-processing graph's lifecycle management. However, BaseAudioContext itself cannot be directly utilized, instead its functionalities must be accessed through one of its derived interfaces: AudioContext, OfflineAudioContext.

Audio graph

An audio graph is a structured representation of audio processing elements and their connections within an audio context. The graph consists of various types of nodes, each performing specific audio operations, connected in a network that defines the audio signal flow. In general we can distinguish four types of nodes:

• Source nodes (e.g AudioBufferSourceNode, OscillatorNode)
• Effect nodes (e.g GainNode, BiquadFilterNode)
• Analysis nodes (e.g AnalyserNode)
• Destination nodes (e.g AudioDestinationNode)

Rendering audio graph

Audio graph rendering is done in blocks of sample-frames. The number of sample-frames in a block is called render quantum size, and the block itself is called a render quantum. By default render quantum size value is 128 and it is constant.

The AudioContext rendering thread is driven by a system-level audio callback. Each call has a system-level audio callback buffer size, which is a varying number of sample-frames that needs to be computed on time before the next system-level audio callback arrives, but render quantum size does not have to be a divisor of the system-level audio callback buffer size.

info

Concept of system-level audio callback does not apply to OfflineAudioContext.

Properties

Name	Type	Description
currentTime	number	Double value representing an ever-increasing hardware time in seconds, starting from 0.	Read only
destination	AudioDestinationNode	Final output destination associated with the context.	Read only
sampleRate	number	Float value representing the sample rate (in samples per seconds) used by all nodes in this context.	Read only
state	ContextState	Enumerated value represents the current state of the context.	Read only

Methods

createAnalyser

The above method lets you create AnalyserNode.

Returns AnalyserNode.

createRecorderAdapter

The above method lets you create RecorderAdapterNode.

Returns RecorderAdapterNode

createBuffer

The above method lets you create AudioBuffer.

Parameters	Type	Description
numOfChannels	number	An integer representing the number of channels of the buffer.
length	number	An integer representing the length of the buffer in sampleFrames. Two seconds buffer has length equals to 2 * sampleRate.
sampleRate	number	A float representing the sample rate of the buffer.

Errors

Error type	Description
NotSupportedError	numOfChannels is outside the nominal range [1, 32].
NotSupportedError	sampleRate is outside the nominal range [8000, 96000].
NotSupportedError	length is less then 1.

Returns AudioBuffer.

createBufferSource

The above method lets you create AudioBufferSourceNode.

Parameters	Type	Description
pitchCorrection Optional	AudioBufferSourceNodeOptions	Dictionary object that specifies if pitch correction has to be available.

Returns AudioBufferSourceNode.

createBufferQueueSource

Mobile only

The above method lets you create AudioBufferQueueSourceNode.

Returns AudioBufferQueueSourceNode.

createGain

The above method lets you create GainNode.

Returns GainNode.

createOscillator

The above method lets you create OscillatorNode.

Returns OscillatorNode.

createStreamer

Mobile only

The above method lets you create StreamerNode.

Returns StreamerNode.

createPeriodicWave

The above method lets you create PeriodicWave. This waveform specifies a repeating pattern that an OscillatorNode can use to generate its output sound.

Parameters	Type	Description
real	Float32Array	An array of cosine terms.
imag	Float32Array	An array of sine terms.
constraints Optional	PeriodicWaveConstraints	An object that specifies if normalization is disabled.

Errors

Error type	Description
InvalidAccessError	real and imag arrays do not have same length.

Returns PeriodicWave.

createStereoPanner

The above method lets you create StereoPannerNode.

Returns StereoPannerNode.

createBiquadFilter

The above method lets you create BiquadFilterNode.

Returns BiquadFilterNode.

caution

Supported file formats:

• mp3
• wav
• flac
• opus
• ogg
• m4a
• aac
• mp4

decodeAudioData

Example

const url = ... // url to an audio

const buffer = await fetch(url)
  .then((response) => response.arrayBuffer())
  .then((arrayBuffer) => this.audioContext.decodeAudioData(arrayBuffer))
  .catch((error) => {
    console.error('Error decoding audio data source:', error);
    return null;
  });

The above method lets you decode audio data. It decodes with in memory audio data block.

Parameters	Type	Description
arrayBuffer	ArrayBuffer	ArrayBuffer with audio data.

Returns Promise<AudioBuffer>.

decodeAudioDataSource

Example using expo-asset library

import { Asset } from 'expo-asset';

const buffer = await Asset.fromModule(require('@/assets/music/example.mp3'))
    .downloadAsync()
    .then((asset) =>  {
      if (!asset.localUri) {
        throw new Error('Failed to load audio asset');
      }
      return this.audioContext.decodeAudioDataSource(asset.localUri);
    })

The above method lets you decode audio data file.

Parameters	Type	Description
sourcePath	string	Path to audio file located on the device.

Returns Promise<AudioBuffer>.

decodePCMInBase64Data

Mobile only

Example

const data = ... // data encoded in base64 string
const buffer = await this.audioContext.decodePCMInBase64Data(data);

The above method lets you decode audio data. It decodes with PCM data in Base64.

Parameters	Type	Description
base64	string	Base64 string with audio data.
playbackRate Optional	number	Number that represents audio speed, which will be applied during decoding.

Returns Promise<AudioBuffer>.

Remarks

currentTime

• Timer starts when context is created, stops when context is suspended.

ContextState

Details

Acceptable values:

• suspended

The audio context has been suspended (with one of suspend or OfflineAudioContext.suspend).

• running

The audio context is running normally.

• closed

The audio context has been closed (with close method).

AudioBufferSourceNodeOptions

Details

AudioBufferSourceNodeOptions is a dictionary object specifies if pitch correction algorithm has to be available.

interface AudioBufferSourceNodeOptions {
  pitchCorrection: boolean
}

PeriodicWaveConstraints

Details

PeriodicWaveConstraints is a dictionary object specifies whether normalization should be disabled during creating periodic wave. If not specified normalization is enabled. If normalized, periodic wave will have maximum peak value of 1 and minimum peak value of -1.

interface PeriodicWaveConstraints {
  disableNormalization: boolean;
}