Web Audio API 1.1

Introduction

Audio on the web has been fairly primitive up to this point and until very recently has had to be delivered through plugins such as Flash and QuickTime. The introduction of the audio element in HTML5 is very important, allowing for basic streaming audio playback. But, it is not powerful enough to handle more complex audio applications. For sophisticated web-based games or interactive applications, another solution is required. It is a goal of this specification to include the capabilities found in modern game audio engines as well as some of the mixing, processing, and filtering tasks that are found in modern desktop audio production applications.

The APIs have been designed with a wide variety of use cases [webaudio-usecases] in mind. Ideally, it should be able to support any use case which could reasonably be implemented with an optimized C++ engine controlled via script and run in a browser. That said, modern desktop audio software can have very advanced capabilities, some of which would be difficult or impossible to build with this system. Apple’s Logic Audio is one such application which has support for external MIDI controllers, arbitrary plugin audio effects and synthesizers, highly optimized direct-to-disk audio file reading/writing, tightly integrated time-stretching, and so on. Nevertheless, the proposed system will be quite capable of supporting a large range of reasonably complex games and interactive applications, including musical ones. And it can be a very good complement to the more advanced graphics features offered by WebGL. The API has been designed so that more advanced capabilities can be added at a later time.

Features

The API supports these primary features:

• Modular routing for simple or complex mixing/effect architectures.

• High dynamic range, using 32-bit floats for internal processing.

• Sample-accurate scheduled sound playback with low latency for musical applications requiring a very high degree of rhythmic precision such as drum machines and sequencers. This also includes the possibility of dynamic creation of effects.

• Automation of audio parameters for envelopes, fade-ins / fade-outs, granular effects, filter sweeps, LFOs etc.

• Flexible handling of channels in an audio stream, allowing them to be split and merged.

• Processing of audio sources from an audio or video media element.

• Processing live audio input using a MediaStream from getUserMedia().

• Integration with WebRTC

  • Processing audio received from a remote peer using a MediaStreamTrackAudioSourceNode and [webrtc].

  • Sending a generated or processed audio stream to a remote peer using a MediaStreamAudioDestinationNode and [webrtc].

• Audio stream synthesis and processing directly using scripts.

• Spatialized audio supporting a wide range of 3D games and immersive environments:

  • Panning models: equalpower, HRTF, pass-through

  • Distance Attenuation

  • Sound Cones

  • Obstruction / Occlusion

  • Source / Listener based

• A convolution engine for a wide range of linear effects, especially very high-quality room effects. Here are some examples of possible effects:

  • Small / large room

  • Cathedral

  • Concert hall

  • Cave

  • Tunnel

  • Hallway

  • Forest

  • Amphitheater

  • Sound of a distant room through a doorway

  • Extreme filters

  • Strange backwards effects

  • Extreme comb filter effects

• Dynamics compression for overall control and sweetening of the mix

• Efficient real-time time-domain and frequency-domain analysis / music visualizer support.

• Efficient biquad filters for lowpass, highpass, and other common filters.

• A Waveshaping effect for distortion and other non-linear effects

• Oscillators

Modular Routing

Modular routing allows arbitrary connections between different AudioNode objects. Each node can have inputs and/or outputs. A source node has no inputs and a single output. A destination node has one input and no outputs. Other nodes such as filters can be placed between the source and destination nodes. The developer doesn’t have to worry about low-level stream format details when two objects are connected together; the right thing just happens. For example, if a mono audio stream is connected to a stereo input it should just mix to left and right channels appropriately.

In the simplest case, a single source can be routed directly to the output. All routing occurs within an AudioContext containing a single AudioDestinationNode:

Illustrating this simple routing, here’s a simple example playing a single sound:

const context = new AudioContext();

function playSound() {
    const source = context.createBufferSource();
    source.buffer = dogBarkingBuffer;
    source.connect(context.destination);
    source.start(0);
}

Here’s a more complex example with three sources and a convolution reverb send with a dynamics compressor at the final output stage:

let context;let compressor;let reverb;let source1, source2, source3;let lowpassFilter;let waveShaper;let panner;let dry1, dry2, dry3;let wet1, wet2, wet3;let mainDry;let mainWet;function setupRoutingGraph () {    context = new AudioContext();    // Create the effects nodes.    lowpassFilter = context.createBiquadFilter();    waveShaper = context.createWaveShaper();    panner = context.createPanner();    compressor = context.createDynamicsCompressor();    reverb = context.createConvolver();    // Create main wet and dry.    mainDry = context.createGain();    mainWet = context.createGain();    // Connect final compressor to final destination.    compressor.connect(context.destination);    // Connect main dry and wet to compressor.    mainDry.connect(compressor);    mainWet.connect(compressor);    // Connect reverb to main wet.    reverb.connect(mainWet);    // Create a few sources.    source1 = context.createBufferSource();    source2 = context.createBufferSource();    source3 = context.createOscillator();    source1.buffer = manTalkingBuffer;    source2.buffer = footstepsBuffer;    source3.frequency.value = 440;    // Connect source1    dry1 = context.createGain();    wet1 = context.createGain();    source1.connect(lowpassFilter);    lowpassFilter.connect(dry1);    lowpassFilter.connect(wet1);    dry1.connect(mainDry);    wet1.connect(reverb);    // Connect source2    dry2 = context.createGain();    wet2 = context.createGain();    source2.connect(waveShaper);    waveShaper.connect(dry2);    waveShaper.connect(wet2);    dry2.connect(mainDry);    wet2.connect(reverb);    // Connect source3    dry3 = context.createGain();    wet3 = context.createGain();    source3.connect(panner);    panner.connect(dry3);    panner.connect(wet3);    dry3.connect(mainDry);    wet3.connect(reverb);    // Start the sources now.    source1.start(0);    source2.start(0);    source3.start(0);}

Modular routing also permits the output of AudioNodes to be routed to an AudioParam parameter that controls the behavior of a different AudioNode. In this scenario, the output of a node can act as a modulation signal rather than an input signal.

function setupRoutingGraph() {    const context = new AudioContext();    // Create the low frequency oscillator that supplies the modulation signal    const lfo = context.createOscillator();    lfo.frequency.value = 1.0;    // Create the high frequency oscillator to be modulated    const hfo = context.createOscillator();    hfo.frequency.value = 440.0;    // Create a gain node whose gain determines the amplitude of the modulation signal    const modulationGain = context.createGain();    modulationGain.gain.value = 50;    // Configure the graph and start the oscillators    lfo.connect(modulationGain);    modulationGain.connect(hfo.detune);    hfo.connect(context.destination);    hfo.start(0);    lfo.start(0);}

API Overview

The interfaces defined are:

• An AudioContext interface, which contains an audio signal graph representing connections between AudioNodes.

• An AudioNode interface, which represents audio sources, audio outputs, and intermediate processing modules. AudioNodes can be dynamically connected together in a modular fashion. AudioNodes exist in the context of an AudioContext.

• An AnalyserNode interface, an AudioNode for use with music visualizers, or other visualization applications.

• An AudioBuffer interface, for working with memory-resident audio assets. These can represent one-shot sounds, or longer audio clips.

• An AudioBufferSourceNode interface, an AudioNode which generates audio from an AudioBuffer.

• An AudioDestinationNode interface, an AudioNode subclass representing the final destination for all rendered audio.

• An AudioParam interface, for controlling an individual aspect of an AudioNode's functioning, such as volume.

• An AudioListener interface, which works with a PannerNode for spatialization.

• An AudioWorklet interface representing a factory for creating custom nodes that can process audio directly using scripts.

• An AudioWorkletGlobalScope interface, the context in which AudioWorkletProcessor processing scripts run.

• An AudioWorkletNode interface, an AudioNode representing a node processed in an AudioWorkletProcessor.

• An AudioWorkletProcessor interface, representing a single node instance inside an audio worker.

• A BiquadFilterNode interface, an AudioNode for common low-order filters such as:

  • Low Pass

  • High Pass

  • Band Pass

  • Low Shelf

  • High Shelf

  • Peaking

  • Notch

  • Allpass

• A ChannelMergerNode interface, an AudioNode for combining channels from multiple audio streams into a single audio stream.

• A ChannelSplitterNode interface, an AudioNode for accessing the individual channels of an audio stream in the routing graph.

• A ConstantSourceNode interface, an AudioNode for generating a nominally constant output value with an AudioParam to allow automation of the value.

• A ConvolverNode interface, an AudioNode for applying a real-time linear effect (such as the sound of a concert hall).

• A DelayNode interface, an AudioNode which applies a dynamically adjustable variable delay.

• A DynamicsCompressorNode interface, an AudioNode for dynamics compression.

• A GainNode interface, an AudioNode for explicit gain control.

• An IIRFilterNode interface, an AudioNode for a general IIR filter.

• A MediaElementAudioSourceNode interface, an AudioNode which is the audio source from an audio, video, or other media element.

• A MediaStreamAudioSourceNode interface, an AudioNode which is the audio source from a MediaStream such as live audio input, or from a remote peer.

• A MediaStreamTrackAudioSourceNode interface, an AudioNode which is the audio source from a MediaStreamTrack.

• A MediaStreamAudioDestinationNode interface, an AudioNode which is the audio destination to a MediaStream sent to a remote peer.

• A PannerNode interface, an AudioNode for spatializing / positioning audio in 3D space.

• A PeriodicWave interface for specifying custom periodic waveforms for use by the OscillatorNode.

• An OscillatorNode interface, an AudioNode for generating a periodic waveform.

• A StereoPannerNode interface, an AudioNode for equal-power positioning of audio input in a stereo stream.

• A WaveShaperNode interface, an AudioNode which applies a non-linear waveshaping effect for distortion and other more subtle warming effects.

There are also several features that have been deprecated from the Web Audio API but not yet removed, pending implementation experience of their replacements:

• A ScriptProcessorNode interface, an AudioNode for generating or processing audio directly using scripts.

• An AudioProcessingEvent interface, which is an event type used with ScriptProcessorNode objects.

1. The Audio API

1.1. The BaseAudioContext Interface

This interface represents a set of AudioNode objects and their connections. It allows for arbitrary routing of signals to an AudioDestinationNode. Nodes are created from the context and are then connected together.

BaseAudioContext is not instantiated directly, but is instead extended by the concrete interfaces AudioContext (for real-time rendering) and OfflineAudioContext (for offline rendering).

BaseAudioContext are created with an internal slot [[pending promises]] that is an initially empty ordered list of promises.

Each BaseAudioContext has a unique media element event task source. Additionally, a BaseAudioContext has several private slots [[rendering thread state]] and [[control thread state]] that take values from AudioContextState, and that are both initially set to "suspended" , and a private slot [[render quantum size]] that is an unsigned integer.

enum AudioContextState {
    "suspended",
    "running",
    "closed"
};

enum AudioContextRenderSizeCategory {
    "default",
    "hardware"
};

callback DecodeErrorCallback = undefined (DOMException error);

callback DecodeSuccessCallback = undefined (AudioBuffer decodedData);

[Exposed=Window]
interface BaseAudioContext : EventTarget {
    readonly attribute AudioDestinationNode destination;
    readonly attribute float sampleRate;
    readonly attribute double currentTime;
    readonly attribute AudioListener listener;
    readonly attribute AudioContextState state;
    readonly attribute unsigned long renderQuantumSize;
    [SameObject, SecureContext]
    readonly attribute AudioWorklet audioWorklet;
    attribute EventHandler onstatechange;

    AnalyserNode createAnalyser ();
    BiquadFilterNode createBiquadFilter ();
    AudioBuffer createBuffer (unsigned long numberOfChannels,
                                unsigned long length,
                                float sampleRate);
    AudioBufferSourceNode createBufferSource ();
    ChannelMergerNode createChannelMerger (optional unsigned long numberOfInputs = 6);
    ChannelSplitterNode createChannelSplitter (
        optional unsigned long numberOfOutputs = 6);
    ConstantSourceNode createConstantSource ();
    ConvolverNode createConvolver ();
    DelayNode createDelay (optional double maxDelayTime = 1.0);
    DynamicsCompressorNode createDynamicsCompressor ();
    GainNode createGain ();
    IIRFilterNode createIIRFilter (sequence<double> feedforward,
                                    sequence<double> feedback);
    OscillatorNode createOscillator ();
    PannerNode createPanner ();
    PeriodicWave createPeriodicWave (sequence<float> real,
                                        sequence<float> imag,
                                        optional PeriodicWaveConstraints constraints = {});
    ScriptProcessorNode createScriptProcessor(
        optional unsigned long bufferSize = 0,
        optional unsigned long numberOfInputChannels = 2,
        optional unsigned long numberOfOutputChannels = 2);
    StereoPannerNode createStereoPanner ();
    WaveShaperNode createWaveShaper ();

    Promise<AudioBuffer> decodeAudioData (
        ArrayBuffer audioData,
        optional DecodeSuccessCallback? successCallback,
        optional DecodeErrorCallback? errorCallback);
};

1.1.1. Attributes

audioWorklet, of type AudioWorklet, readonly

Allows access to the Worklet object that can import a script containing AudioWorkletProcessor class definitions via the algorithms defined by [HTML] and AudioWorklet.

currentTime, of type double, readonly

This is the time in seconds of the sample frame immediately following the last sample-frame in the block of audio most recently processed by the context’s rendering graph. If the context’s rendering graph has not yet processed a block of audio, then currentTime has a value of zero.

In the time coordinate system of currentTime, the value of zero corresponds to the first sample-frame in the first block processed by the graph. Elapsed time in this system corresponds to elapsed time in the audio stream generated by the BaseAudioContext, which may not be synchronized with other clocks in the system. (For an OfflineAudioContext, since the stream is not being actively played by any device, there is not even an approximation to real time.)

All scheduled times in the Web Audio API are relative to the value of currentTime.

When the BaseAudioContext is in the "running" state, the value of this attribute is monotonically increasing and is updated by the rendering thread in uniform increments, corresponding to one render quantum. Thus, for a running context, currentTime increases steadily as the system processes audio blocks, and always represents the time of the start of the next audio block to be processed. It is also the earliest possible time when any change scheduled in the current state might take effect.

currentTime MUST be read atomically on the control thread before being returned.

destination, of type AudioDestinationNode, readonly

An AudioDestinationNode with a single input representing the final destination for all audio. Usually this will represent the actual audio hardware. All AudioNodes actively rendering audio will directly or indirectly connect to destination.

listener, of type AudioListener, readonly

An AudioListener which is used for 3D spatialization.

onstatechange, of type EventHandler

A property used to set an event handler for an event that is dispatched to BaseAudioContext when the state of the AudioContext has changed (i.e. when the corresponding promise would have resolved). The event type of this event handler is statechange. An event that uses the Event interface will be dispatched to the event handler, which can query the AudioContext’s state directly. A newly-created AudioContext will always begin in the suspended state, and a state change event will be fired whenever the state changes to a different state. This event is fired before the complete event is fired.

sampleRate, of type float, readonly

The sample rate (in sample-frames per second) at which the BaseAudioContext handles audio. It is assumed that all AudioNodes in the context run at this rate. In making this assumption, sample-rate converters or "varispeed" processors are not supported in real-time processing. The Nyquist frequency is half this sample-rate value.

state, of type AudioContextState, readonly

Describes the current state of the BaseAudioContext. Getting this attribute returns the contents of the [[control thread state]] slot.

renderQuantumSize, of type unsigned long, readonly

Getting this attribute returns the value of [[render quantum size]] slot.

1.1.2. Methods

createAnalyser()

Factory method for an AnalyserNode.

No parameters.

Return type: AnalyserNode

createBiquadFilter()

Factory method for a BiquadFilterNode representing a second order filter which can be configured as one of several common filter types.

No parameters.

Return type: BiquadFilterNode

createBuffer(numberOfChannels, length, sampleRate)

Creates an AudioBuffer of the given size. The audio data in the buffer will be zero-initialized (silent). A NotSupportedError exception MUST be thrown if any of the arguments is negative, zero, or outside its nominal range.

Arguments for the BaseAudioContext.createBuffer() method.

Parameter	Type	Nullable	Optional	Description
numberOfChannels	unsigned long	✘	✘	Determines how many channels the buffer will have. An implementation MUST support at least 32 channels.
length	unsigned long	✘	✘	Determines the size of the buffer in sample-frames. This MUST be at least 1.
sampleRate	float	✘	✘	Describes the sample-rate of the linear PCM audio data in the buffer in sample-frames per second. An implementation MUST support sample rates in at least the range 8000 to 96000.

Return type: AudioBuffer

createBufferSource()

Factory method for a AudioBufferSourceNode.

No parameters.

Return type: AudioBufferSourceNode

createChannelMerger(numberOfInputs)

Factory method for a ChannelMergerNode representing a channel merger. An IndexSizeError exception MUST be thrown if numberOfInputs is less than 1 or is greater than the number of supported channels.

Arguments for the BaseAudioContext.createChannelMerger(numberOfInputs) method.

Parameter	Type	Nullable	Optional	Description
numberOfInputs	unsigned long	✘	✔	Determines the number of inputs. Values of up to 32 MUST be supported. If not specified, then 6 will be used.

Return type: ChannelMergerNode

createChannelSplitter(numberOfOutputs)

Factory method for a ChannelSplitterNode representing a channel splitter. An IndexSizeError exception MUST be thrown if numberOfOutputs is less than 1 or is greater than the number of supported channels.

Arguments for the BaseAudioContext.createChannelSplitter(numberOfOutputs) method.

Parameter	Type	Nullable	Optional	Description
numberOfOutputs	unsigned long	✘	✔	The number of outputs. Values of up to 32 MUST be supported. If not specified, then 6 will be used.

Return type: ChannelSplitterNode

createConstantSource()

Factory method for a ConstantSourceNode.

No parameters.

Return type: ConstantSourceNode

createConvolver()

Factory method for a ConvolverNode.

No parameters.

Return type: ConvolverNode

createDelay(maxDelayTime)

Factory method for a DelayNode. The initial default delay time will be 0 seconds.

Arguments for the BaseAudioContext.createDelay(maxDelayTime) method.

Parameter	Type	Nullable	Optional	Description
maxDelayTime	double	✘	✔	Specifies the maximum delay time in seconds allowed for the delay line. If specified, this value MUST be greater than zero and less than three minutes or a NotSupportedError exception MUST be thrown. If not specified, then 1 will be used.

Return type: DelayNode

createDynamicsCompressor()

Factory method for a DynamicsCompressorNode.

No parameters.

Return type: DynamicsCompressorNode

createGain()

Factory method for GainNode.

No parameters.

Return type: GainNode

createIIRFilter(feedforward, feedback)

Arguments for the BaseAudioContext.createIIRFilter() method.

Parameter	Type	Nullable	Optional	Description
feedforward	sequence<double>	✘	✘	An array of the feedforward (numerator) coefficients for the transfer function of the IIR filter. The maximum length of this array is 20. If all of the values are zero, an InvalidStateError MUST be thrown. A NotSupportedError MUST be thrown if the array length is 0 or greater than 20.
feedback	sequence<double>	✘	✘	An array of the feedback (denominator) coefficients for the transfer function of the IIR filter. The maximum length of this array is 20. If the first element of the array is 0, an InvalidStateError MUST be thrown. A NotSupportedError MUST be thrown if the array length is 0 or greater than 20.

Return type: IIRFilterNode

createOscillator()

Factory method for an OscillatorNode.

No parameters.

Return type: OscillatorNode

createPanner()

Factory method for a PannerNode.

No parameters.

Return type: PannerNode

createPeriodicWave(real, imag, constraints)

Factory method to create a PeriodicWave.

When calling this method, execute these steps:

1. If real and imag are not of the same length, an IndexSizeError MUST be thrown.

2. Let o be a new object of type PeriodicWaveOptions.

3. Respectively set the real and imag parameters passed to this factory method to the attributes of the same name on o.

4. Set the disableNormalization attribute on o to the value of the disableNormalization attribute of the constraints attribute passed to the factory method.

5. Construct a new PeriodicWave p, passing the BaseAudioContext this factory method has been called on as a first argument, and o.

6. Return p.

Arguments for the BaseAudioContext.createPeriodicWave() method.

Parameter	Type	Nullable	Optional	Description
real	sequence<float>	✘	✘	A sequence of cosine parameters. See its real constructor argument for a more detailed description.
imag	sequence<float>	✘	✘	A sequence of sine parameters. See its imag constructor argument for a more detailed description.
constraints	PeriodicWaveConstraints	✘	✔	If not given, the waveform is normalized. Otherwise, the waveform is normalized according the value given by constraints.

Return type: PeriodicWave

createScriptProcessor(bufferSize, numberOfInputChannels, numberOfOutputChannels)

Factory method for a ScriptProcessorNode. This method is DEPRECATED, as it is intended to be replaced by AudioWorkletNode. Creates a ScriptProcessorNode for direct audio processing using scripts. An IndexSizeError exception MUST be thrown if bufferSize or numberOfInputChannels or numberOfOutputChannels are outside the valid range.

It is invalid for both numberOfInputChannels and numberOfOutputChannels to be zero. In this case an IndexSizeError MUST be thrown.

Arguments for the BaseAudioContext.createScriptProcessor(bufferSize, numberOfInputChannels, numberOfOutputChannels) method.

Parameter	Type	Nullable	Optional	Description
bufferSize	unsigned long	✘	✔	The bufferSize parameter determines the buffer size in units of sample-frames. If it’s not passed in, or if the value is 0, then the implementation will choose the best buffer size for the given environment, which will be constant power of 2 throughout the lifetime of the node. Otherwise if the author explicitly specifies the bufferSize, it MUST be one of the following values: 256, 512, 1024, 2048, 4096, 8192, 16384. This value controls how frequently the audioprocess event is dispatched and how many sample-frames need to be processed each call. Lower values for bufferSize will result in a lower (better) latency. Higher values will be necessary to avoid audio breakup and glitches. It is recommended for authors to not specify this buffer size and allow the implementation to pick a good buffer size to balance between latency and audio quality. If the value of this parameter is not one of the allowed power-of-2 values listed above, an IndexSizeError MUST be thrown.
numberOfInputChannels	unsigned long	✘	✔	This parameter determines the number of channels for this node’s input. The default value is 2. Values of up to 32 must be supported. A NotSupportedError must be thrown if the number of channels is not supported.
numberOfOutputChannels	unsigned long	✘	✔	This parameter determines the number of channels for this node’s output. The default value is 2. Values of up to 32 must be supported. A NotSupportedError must be thrown if the number of channels is not supported.

Return type: ScriptProcessorNode

createStereoPanner()

Factory method for a StereoPannerNode.

No parameters.

Return type: StereoPannerNode

createWaveShaper()

Factory method for a WaveShaperNode representing a non-linear distortion.

No parameters.

Return type: WaveShaperNode

decodeAudioData(audioData, successCallback, errorCallback)

Asynchronously decodes the audio file data contained in the ArrayBuffer. The ArrayBuffer can, for example, be loaded from an XMLHttpRequest’s response attribute after setting the responseType to "arraybuffer". Audio file data can be in any of the formats supported by the audio element. The buffer passed to decodeAudioData() has its content-type determined by sniffing, as described in [mimesniff].

Although the primary method of interfacing with this function is via its promise return value, the callback parameters are provided for legacy reasons.

Encourage implementation to warn authors in case of a corrupted file. It isn’t possible to throw because this would be a breaking change.

Note: If the compressed audio data byte-stream is corrupted but the decoding can otherwise proceed, implementations are encouraged to warn authors for example via the developer tools.

When decodeAudioData is called, the following steps MUST be performed on the control thread:

1. If this's relevant global object's associated Document is not fully active then return a promise rejected with "InvalidStateError" DOMException.

2. Let promise be a new Promise.

3. If audioData is detached, execute the following steps:

   1. Append promise to [[pending promises]].

   2. Detach the audioData ArrayBuffer. If this operations throws, jump to the step 3.

   3. Queue a decoding operation to be performed on another thread.

4. Else, execute the following error steps:

   1. Let error be a DataCloneError.

   2. Reject promise with error, and remove it from [[pending promises]].

   3. Queue a media element task to invoke errorCallback with error.

5. Return promise.

When queuing a decoding operation to be performed on another thread, the following steps MUST happen on a thread that is not the control thread nor the rendering thread, called the decoding thread.

Note: Multiple decoding threads can run in parallel to service multiple calls to decodeAudioData.

1. Let can decode be a boolean flag, initially set to true.

2. Attempt to determine the MIME type of audioData, using MIME Sniffing § 6.2 Matching an audio or video type pattern. If the audio or video type pattern matching algorithm returns undefined, set can decode to false.

3. If can decode is true, attempt to decode the encoded audioData into linear PCM. In case of failure, set can decode to false.

   If the media byte-stream contains multiple audio tracks, only decode the first track to linear pcm.

   Note: Authors who need more control over the decoding process can use [WEBCODECS].

4. If can decode is false, queue a media element task to execute the following steps:

   1. Let error be a DOMException whose name is EncodingError.

      2. Reject promise with error, and remove it from [[pending promises]].

   2. If errorCallback is not missing, invoke errorCallback with error.

5. Otherwise:

   1. Take the result, representing the decoded linear PCM audio data, and resample it to the sample-rate of the BaseAudioContext if it is different from the sample-rate of audioData.

   2. queue a media element task to execute the following steps:

      1. Let buffer be an AudioBuffer containing the final result (after possibly performing sample-rate conversion).

      2. Resolve promise with buffer.

      3. If successCallback is not missing, invoke successCallback with buffer.

Arguments for the BaseAudioContext.decodeAudioData() method.

Parameter	Type	Nullable	Optional	Description
audioData	ArrayBuffer	✘	✘	An ArrayBuffer containing compressed audio data.
successCallback	DecodeSuccessCallback?	✔	✔	A callback function which will be invoked when the decoding is finished. The single argument to this callback is an AudioBuffer representing the decoded PCM audio data.
errorCallback	DecodeErrorCallback?	✔	✔	A callback function which will be invoked if there is an error decoding the audio file.

Return type: Promise<AudioBuffer>

1.1.3. Callback DecodeSuccessCallback() Parameters

decodedData, of type AudioBuffer

The AudioBuffer containing the decoded audio data.

1.1.4. Callback DecodeErrorCallback() Parameters

error, of type DOMException

The error that occurred while decoding.

1.1.5. Lifetime

Once created, an AudioContext will continue to play sound until it has no more sound to play, or the page goes away.

1.1.6. Lack of Introspection or Serialization Primitives

The Web Audio API takes a fire-and-forget approach to audio source scheduling. That is, source nodes are created for each note during the lifetime of the AudioContext, and never explicitly removed from the graph. This is incompatible with a serialization API, since there is no stable set of nodes that could be serialized.

Moreover, having an introspection API would allow content script to be able to observe garbage collections.

1.1.7. System Resources Associated with BaseAudioContext Subclasses

The subclasses AudioContext and OfflineAudioContext should be considered expensive objects. Creating these objects may involve creating a high-priority thread, or using a low-latency system audio stream, both having an impact on energy consumption. It is usually not necessary to create more than one AudioContext in a document.

Constructing or resuming a BaseAudioContext subclass involves acquiring system resources for that context. For AudioContext, this also requires creation of a system audio stream. These operations return when the context begins generating output from its associated audio graph.

Additionally, a user-agent can have an implementation-defined maximum number of AudioContexts, after which any attempt to create a new AudioContext will fail, throwing NotSupportedError.

suspend and close allow authors to release system resources, including threads, processes and audio streams. Suspending a BaseAudioContext permits implementations to release some of its resources, and allows it to continue to operate later by invoking resume. Closing an AudioContext permits implementations to release all of its resources, after which it cannot be used or resumed again.

Note: For example, this can involve waiting for the audio callbacks to fire regularly, or to wait for the hardware to be ready for processing.

1.2. The AudioContext Interface

This interface represents an audio graph whose AudioDestinationNode is routed to a real-time output device that produces a signal directed at the user. In most use cases, only a single AudioContext is used per document.

enum AudioContextLatencyCategory {
        "balanced",
        "interactive",
        "playback"
};

enum AudioSinkType {
    "none"
};

[Exposed=Window]
interface AudioContext : BaseAudioContext {
    constructor (optional AudioContextOptions contextOptions = {});
    readonly attribute double baseLatency;
    readonly attribute double outputLatency;
    [SecureContext] readonly attribute (DOMString or AudioSinkInfo) sinkId;
    [SecureContext] readonly attribute AudioRenderCapacity renderCapacity;
    attribute EventHandler onsinkchange;
    attribute EventHandler onerror;
    AudioTimestamp getOutputTimestamp ();
    Promise<undefined> resume ();
    Promise<undefined> suspend ();
    Promise<undefined> close ();
    [SecureContext] Promise<undefined> setSinkId ((DOMString or AudioSinkOptions) sinkId);
    MediaElementAudioSourceNode createMediaElementSource (HTMLMediaElement mediaElement);
    MediaStreamAudioSourceNode createMediaStreamSource (MediaStream mediaStream);
    MediaStreamTrackAudioSourceNode createMediaStreamTrackSource (
        MediaStreamTrack mediaStreamTrack);
    MediaStreamAudioDestinationNode createMediaStreamDestination ();
};

An AudioContext is said to be allowed to start if the user agent allows the context state to transition from "suspended" to "running". A user agent may disallow this initial transition, and to allow it only when the AudioContext's relevant global object has sticky activation.

AudioContext has following internal slots:

[[suspended by user]]

A boolean flag representing whether the context is suspended by user code. The initial value is false.

[[sink ID]]

A DOMString or an AudioSinkInfo representing the identifier or the information of the current audio output device respectively. The initial value is "", which means the default audio output device.

[[pending resume promises]]

An ordered list to store pending Promises created by resume(). It is initially empty.

1.2.1. Constructors

AudioContext(contextOptions)

If the current settings object's relevant global object's associated Document is NOT fully active, throw an "InvalidStateError" and abort these steps.

When creating an AudioContext, execute these steps:

1. Let context be a new AudioContext object.

2. Set a [[control thread state]] to suspended on context.

3. Set a [[rendering thread state]] to suspended on context.

4. Let messageChannel be a new MessageChannel.

5. Let controlSidePort be the value of messageChannel’s port1 attribute.

6. Let renderingSidePort be the value of messageChannel’s port2 attribute.

7. Let serializedRenderingSidePort be the result of StructuredSerializeWithTransfer(renderingSidePort, « renderingSidePort »).

8. Set this audioWorklet's port to controlSidePort.

9. Queue a control message to set the MessagePort on the AudioContextGlobalScope, with serializedRenderingSidePort.

10. If contextOptions is given, perform the following substeps:

    1. If sinkId is specified, let sinkId be the value of contextOptions.sinkId and run the following substeps:

       1. If both sinkId and [[sink ID]] are a type of DOMString, and they are equal to each other, abort these substeps.

       2. If sinkId is a type of AudioSinkOptions and [[sink ID]] is a type of AudioSinkInfo, and type in sinkId and type in [[sink ID]] are equal, abort these substeps.

       3. Let validationResult be the return value of sink identifier validation of sinkId.

       4. If validationResult is a type of DOMException, throw an exception with validationResult and abort these substeps.

       5. If sinkId is a type of DOMString, set [[sink ID]] to sinkId and abort these substeps.

       6. If sinkId is a type of AudioSinkOptions, set [[sink ID]] to a new instance of AudioSinkInfo created with the value of type of sinkId.

    2. Set the internal latency of context according to contextOptions.latencyHint, as described in latencyHint.

    3. If contextOptions.sampleRate is specified, set the sampleRate of context to this value. Otherwise, follow these substeps:

       1. If sinkId is the empty string or a type of AudioSinkOptions, use the sample rate of the default output device. Abort these substeps.

       2. If sinkId is a DOMString, use the sample rate of the output device identified by sinkId. Abort these substeps.

       If contextOptions.sampleRate differs from the sample rate of the output device, the user agent MUST resample the audio output to match the sample rate of the output device.

       Note: If resampling is required, the latency of context may be affected, possibly by a large amount.

11. If context is allowed to start, send a control message to start processing.

12. Return context.

Sending a control message to start processing means executing the following steps:

1. Let document be the current settings object's relevant global object's associated Document.

2. Attempt to acquire system resources to use a following audio output device based on [[sink ID]] for rendering:

   • The default audio output device for the empty string.

   • A audio output device identified by [[sink ID]].

   1. If resource acquisition fails, execute the following steps:

      1. If document is not allowed to use the feature identified by "speaker-selection", abort these substeps.

      2. Queue a media element task to fire an event named error at the AudioContext, and abort the following steps.

3. Set this [[rendering thread state]] to running on the AudioContext.

4. Queue a media element task to execute the following steps:

   1. Set the state attribute of the AudioContext to "running".

   2. fire an event named statechange at the AudioContext.

NOTE: In cases where an AudioContext is constructed with no arguments and resource acquisition fails, the User-Agent will attempt to silently render the audio graph using a mechanism that emulates an audio output device.

Sending a control message to set the MessagePort on the AudioWorkletGlobalScope means executing the following steps, on the rendering thread, with serializedRenderingSidePort, that has been transfered to the AudioWorkletGlobalScope:

1. Let deserializedPort be the result of StructuredDeserialize(serializedRenderingSidePort, the current Realm).

2. Set port to deserializedPort.

Arguments for the AudioContext.constructor(contextOptions) method.

Parameter	Type	Nullable	Optional	Description
contextOptions	AudioContextOptions	✘	✔	User-specified options controlling how the AudioContext should be constructed.

1.2.2. Attributes

baseLatency, of type double, readonly

This represents the number of seconds of processing latency incurred by the AudioContext passing the audio from the AudioDestinationNode to the audio subsystem. It does not include any additional latency that might be caused by any other processing between the output of the AudioDestinationNode and the audio hardware and specifically does not include any latency incurred the audio graph itself.

For example, if the audio context is running at 44.1 kHz with default render quantum size, and the AudioDestinationNode implements double buffering internally and can process and output audio each render quantum, then the processing latency is \((2\cdot128)/44100 = 5.805 \mathrm{ ms}\), approximately.

outputLatency, of type double, readonly

The estimation in seconds of audio output latency, i.e., the interval between the time the UA requests the host system to play a buffer and the time at which the first sample in the buffer is actually processed by the audio output device. For devices such as speakers or headphones that produce an acoustic signal, this latter time refers to the time when a sample’s sound is produced.

An outputLatency attribute value depends on the platform and the connected audio output device hardware. The outputLatency attribute value may change while the context is running or the associated audio output device changes. It is useful to query this value frequently when accurate synchronization is required.

renderCapacity, of type AudioRenderCapacity, readonly

Returns an AudioRenderCapacity instance associated with an AudioContext.

sinkId, of type (DOMString or AudioSinkInfo), readonly

Returns the value of [[sink ID]] internal slot. This attribute is cached upon update, and it returns the same object after caching.

onsinkchange, of type EventHandler

An event handler for setSinkId(). The event type of this event handler is sinkchange. This event will be dispatched when changing the output device is completed.

NOTE: This is not dispatched for the initial device selection in the construction of AudioContext. The statechange event is available to check the readiness of the initial output device.

onerror, of type EventHandler

An event handler for the Event dispatched from an AudioContext. The event type of this handler is error and the user agent can dispatch this event in the following cases:

• When initializing and activating a selected audio device encounters failures.

• When the audio output device associated with an AudioContext is disconnected while the context is running.

• When the operating system reports an audio device malfunction.

1.2.3. Methods

close()

Closes the AudioContext, releasing the system resources being used. This will not automatically release all AudioContext-created objects, but will suspend the progression of the AudioContext's currentTime, and stop processing audio data.

When close is called, execute these steps:

1. If this's relevant global object's associated Document is not fully active then return a promise rejected with "InvalidStateError" DOMException.

2. Let promise be a new Promise.

3. If the [[control thread state]] flag on the AudioContext is closed reject the promise with InvalidStateError, abort these steps, returning promise.

4. Set the [[control thread state]] flag on the AudioContext to closed.

5. Queue a control message to close the AudioContext.

6. Return promise.

Running a control message to close an AudioContext means running these steps on the rendering thread:

1. Attempt to release system resources.

2. Set the [[rendering thread state]] to suspended.

   This will stop rendering.

3. If this control message is being run in a reaction to the document being unloaded, abort this algorithm.

   There is no need to notify the control thread in this case.

4. queue a media element task to execute the following steps:

   1. Resolve promise.

   2. If the state attribute of the AudioContext is not already "closed":

      1. Set the state attribute of the AudioContext to "closed".

      2. queue a media element task to fire an event named statechange at the AudioContext.

When an AudioContext is closed, any MediaStreams and HTMLMediaElements that were connected to an AudioContext will have their output ignored. That is, these will no longer cause any output to speakers or other output devices. For more flexibility in behavior, consider using HTMLMediaElement.captureStream().

Note: When an AudioContext has been closed, implementation can choose to aggressively release more resources than when suspending.

No parameters.

Return type: Promise<undefined>

createMediaElementSource(mediaElement)

Creates a MediaElementAudioSourceNode given an HTMLMediaElement. As a consequence of calling this method, audio playback from the HTMLMediaElement will be re-routed into the processing graph of the AudioContext.

Arguments for the AudioContext.createMediaElementSource() method.

Parameter	Type	Nullable	Optional	Description
mediaElement	HTMLMediaElement	✘	✘	The media element that will be re-routed.

Return type: MediaElementAudioSourceNode

createMediaStreamDestination()

Creates a MediaStreamAudioDestinationNode

No parameters.

Return type: MediaStreamAudioDestinationNode

createMediaStreamSource(mediaStream)

Creates a MediaStreamAudioSourceNode.

Arguments for the AudioContext.createMediaStreamSource() method.

Parameter	Type	Nullable	Optional	Description
mediaStream	MediaStream	✘	✘	The media stream that will act as source.

Return type: MediaStreamAudioSourceNode

createMediaStreamTrackSource(mediaStreamTrack)

Creates a MediaStreamTrackAudioSourceNode.

Arguments for the AudioContext.createMediaStreamTrackSource() method.

Parameter	Type	Nullable	Optional	Description
mediaStreamTrack	MediaStreamTrack	✘	✘	The MediaStreamTrack that will act as source. The value of its kind attribute must be equal to "audio", or an InvalidStateError exception MUST be thrown.

Return type: MediaStreamTrackAudioSourceNode

getOutputTimestamp()

Returns a new AudioTimestamp instance containing two related audio stream position values for the context: the contextTime member contains the time of the sample frame which is currently being rendered by the audio output device (i.e., output audio stream position), in the same units and origin as context’s currentTime; the performanceTime member contains the time estimating the moment when the sample frame corresponding to the stored contextTime value was rendered by the audio output device, in the same units and origin as performance.now() (described in [hr-time-3]).

If the context’s rendering graph has not yet processed a block of audio, then getOutputTimestamp call returns an AudioTimestamp instance with both members containing zero.

After the context’s rendering graph has started processing of blocks of audio, its currentTime attribute value always exceeds the contextTime value obtained from getOutputTimestamp method call.

The value returned from getOutputTimestamp method can be used to get performance time estimation for the slightly later context’s time value:

function outputPerformanceTime(contextTime) {
    const timestamp = context.getOutputTimestamp();
    const elapsedTime = contextTime - timestamp.contextTime;
    return timestamp.performanceTime + elapsedTime * 1000;
}

In the above example the accuracy of the estimation depends on how close the argument value is to the current output audio stream position: the closer the given contextTime is to timestamp.contextTime, the better the accuracy of the obtained estimation.

Note: The difference between the values of the context’s currentTime and the contextTime obtained from getOutputTimestamp method call cannot be considered as a reliable output latency estimation because currentTime may be incremented at non-uniform time intervals, so outputLatency attribute should be used instead.

No parameters.

Return type: AudioTimestamp

resume()

Resumes the progression of the AudioContext's currentTime when it has been suspended.

When resume is called, execute these steps:

1. If this's relevant global object's associated Document is not fully active then return a promise rejected with "InvalidStateError" DOMException.

2. Let promise be a new Promise.

3. If the [[control thread state]] on the AudioContext is closed reject the promise with InvalidStateError, abort these steps, returning promise.

4. Set [[suspended by user]] to false.

5. If the context is not allowed to start, append promise to [[pending promises]] and [[pending resume promises]] and abort these steps, returning promise.

6. Set the [[control thread state]] on the AudioContext to running.

7. Queue a control message to resume the AudioContext.

8. Return promise.

Running a control message to resume an AudioContext means running these steps on the rendering thread:

1. Attempt to acquire system resources.

2. Set the [[rendering thread state]] on the AudioContext to running.

3. Start rendering the audio graph.

4. In case of failure, queue a media element task to execute the following steps:

   1. Reject all promises from [[pending resume promises]] in order, then clear [[pending resume promises]].

   2. Additionally, remove those promises from [[pending promises]].

5. queue a media element task to execute the following steps:

   1. Resolve all promises from [[pending resume promises]] in order.

   2. Clear [[pending resume promises]]. Additionally, remove those promises from [[pending promises]].

   3. Resolve promise.

   4. If the state attribute of the AudioContext is not already "running":

      1. Set the state attribute of the AudioContext to "running".

      2. Queue a media element task to fire an event named statechange at the AudioContext.

No parameters.

Return type: Promise<undefined>

suspend()

Suspends the progression of AudioContext's currentTime, allows any current context processing blocks that are already processed to be played to the destination, and then allows the system to release its claim on audio hardware. This is generally useful when the application knows it will not need the AudioContext for some time, and wishes to temporarily release system resource associated with the AudioContext. The promise resolves when the frame buffer is empty (has been handed off to the hardware), or immediately (with no other effect) if the context is already suspended. The promise is rejected if the context has been closed.

When suspend is called, execute these steps:

1. If this's relevant global object's associated Document is not fully active then return a promise rejected with "InvalidStateError" DOMException.

2. Let promise be a new Promise.

3. If the [[control thread state]] on the AudioContext is closed reject the promise with InvalidStateError, abort these steps, returning promise.

4. Append promise to [[pending promises]].

5. Set [[suspended by user]] to true.

6. Set the [[control thread state]] on the AudioContext to suspended.

7. Queue a control message to suspend the AudioContext.

8. Return promise.

Running a control message to suspend an AudioContext means running these steps on the rendering thread:

1. Attempt to release system resources.

2. Set the [[rendering thread state]] on the AudioContext to suspended.

3. queue a media element task to execute the following steps:

   1. Resolve promise.

   2. If the state attribute of the AudioContext is not already "suspended":

      1. Set the state attribute of the AudioContext to "suspended".

      2. Queue a media element task to fire an event named statechange at the AudioContext.

While an AudioContext is suspended, MediaStreams will have their output ignored; that is, data will be lost by the real time nature of media streams. HTMLMediaElements will similarly have their output ignored until the system is resumed. AudioWorkletNodes and ScriptProcessorNodes will cease to have their processing handlers invoked while suspended, but will resume when the context is resumed. For the purpose of AnalyserNode window functions, the data is considered as a continuous stream - i.e. the resume()/suspend() does not cause silence to appear in the AnalyserNode's stream of data. In particular, calling AnalyserNode functions repeatedly when a AudioContext is suspended MUST return the same data.

No parameters.

Return type: Promise<undefined>

setSinkId((DOMString or AudioSinkOptions) sinkId)

Sets the identifier of an output device. When this method is invoked, the user agent MUST run the following steps:

1. Let sinkId be the method’s first argument.

2. If sinkId is equal to [[sink ID]], return a promise, resolve it immediately and abort these steps.

3. Let validationResult be the return value of sink identifier validation of sinkId.

4. If validationResult is not null, return a promise rejected with validationResult. Abort these steps.

5. Let p be a new promise.

6. Send a control message with p and sinkId to start processing.

7. Return p.

Sending a control message to start processing during setSinkId() means executing the following steps:

1. Let p be the promise passed into this algorithm.

2. Let sinkId be the sink identifier passed into this algorithm.

3. If both sinkId and [[sink ID]] are a type of DOMString, and they are equal to each other, queue a media element task to resolve p and abort these steps.

4. If sinkId is a type of AudioSinkOptions and [[sink ID]] is a type of AudioSinkInfo, and type in sinkId and type in [[sink ID]] are equal, queue a media element task to resolve p and abort these steps.

5. Let wasRunning be true.

6. Set wasRunning to false if the [[rendering thread state]] on the AudioContext is "suspended".

7. Pause the renderer after processing the current render quantum.

8. Attempt to release system resources.

9. If wasRunning is true:

   1. Set the [[rendering thread state]] on the AudioContext to "suspended".

   2. Queue a media element task to execute the following steps:

      1. If the state attribute of the AudioContext is not already "suspended":

         1. Set the state attribute of the AudioContext to "suspended".

         2. Fire an event named statechange at the associated AudioContext.

10. Attempt to acquire system resources to use a following audio output device based on [[sink ID]] for rendering:

    • The default audio output device for the empty string.

    • A audio output device identified by [[sink ID]].

    In case of failure, reject p with "InvalidAccessError" abort the following steps.

11. Queue a media element task to execute the following steps:

    1. If sinkId is a type of DOMString, set [[sink ID]] to sinkId. Abort these steps.

    2. If sinkId is a type of AudioSinkOptions and [[sink ID]] is a type of DOMString, set [[sink ID]] to a new instance of AudioSinkInfo created with the value of type of sinkId.

    3. If sinkId is a type of AudioSinkOptions and [[sink ID]] is a type of AudioSinkInfo, set type of [[sink ID]] to the type value of sinkId.

    4. Resolve p.

    5. Fire an event named sinkchange at the associated AudioContext.

12. If wasRunning is true:

    1. Set the [[rendering thread state]] on the AudioContext to "running".

    2. Queue a media element task to execute the following steps:

       1. If the state attribute of the AudioContext is not already "running":

          1. Set the state attribute of the AudioContext to "running".

          2. Fire an event named statechange at the associated AudioContext.

1.2.4. Validating sinkId

This algorithm is used to validate the information provided to modify sinkId:

1.2.5. AudioContextOptions

The AudioContextOptions dictionary is used to specify user-specified options for an AudioContext.

dictionary AudioContextOptions {
    (AudioContextLatencyCategory or double) latencyHint = "interactive";
    float sampleRate;
    (DOMString or AudioSinkOptions) sinkId;
    (AudioContextRenderSizeCategory or unsigned long) renderSizeHint = "default";
};

1.2.5.1. Dictionary AudioContextOptions Members

latencyHint, of type (AudioContextLatencyCategory or double), defaulting to "interactive"

Identify the type of playback, which affects tradeoffs between audio output latency and power consumption.

The preferred value of the latencyHint is a value from AudioContextLatencyCategory. However, a double can also be specified for the number of seconds of latency for finer control to balance latency and power consumption. It is at the browser’s discretion to interpret the number appropriately. The actual latency used is given by AudioContext’s baseLatency attribute.

sampleRate, of type float

Set the sampleRate to this value for the AudioContext that will be created. The supported values are the same as the sample rates for an AudioBuffer. A NotSupportedError exception MUST be thrown if the specified sample rate is not supported.

If sampleRate is not specified, the preferred sample rate of the output device for this AudioContext is used.

sinkId, of type (DOMString or AudioSinkOptions)

The identifier or associated information of the audio output device. See sinkId for more details.

renderSizeHint, of type (AudioContextRenderSizeCategory or unsigned long), defaulting to "default"

This allows users to ask for a particular render quantum size when an integer is passed, to use the default of 128 frames if nothing or "default" is passed, or to ask the User-Agent to pick a good render quantum size if "hardware" is specified.

It is a hint that might not be honored.

1.2.6. AudioSinkOptions

The AudioSinkOptions dictionary is used to specify options for sinkId.

dictionary AudioSinkOptions {
    required AudioSinkType type;
};

1.2.6.1. Dictionary AudioSinkOptions Members

type, of type AudioSinkType

A value of AudioSinkType to specify the type of the device.

1.2.7. AudioSinkInfo

The AudioSinkInfo interface is used to get information on the current audio output device via sinkId.

[Exposed=Window]
interface AudioSinkInfo {
    readonly attribute AudioSinkType type;
};

1.2.7.1. Attributes

type, of type AudioSinkType, readonly

A value of AudioSinkType that represents the type of the device.

1.2.8. AudioTimestamp

dictionary AudioTimestamp {
    double contextTime;
    DOMHighResTimeStamp performanceTime;
};

1.2.8.1. Dictionary AudioTimestamp Members

contextTime, of type double

Represents a point in the time coordinate system of BaseAudioContext’s currentTime.

performanceTime, of type DOMHighResTimeStamp

Represents a point in the time coordinate system of a Performance interface implementation (described in [hr-time-3]).

1.2.9. AudioRenderCapacity

[Exposed=Window]
interface AudioRenderCapacity : EventTarget {
    undefined start(optional AudioRenderCapacityOptions options = {});
        undefined stop();
        attribute EventHandler onupdate;
};

This interface provides rendering performance metrics of an AudioContext. In order to calculate them, the renderer collects a load value per system-level audio callback.

1.2.9.1. Attributes

onupdate, of type EventHandler

The event type of this event handler is update. Events dispatched to the event handler will use the AudioRenderCapacityEvent interface.

1.2.9.2. Methods

start(options)

Starts metric collection and analysis. This will repeatedly fire an event named update at the AudioRenderCapacity, using AudioRenderCapacityEvent, with the given update interval in AudioRenderCapacityOptions.

stop()

Stops metric collection and analysis. It also stops dispatching update events.

1.2.10. AudioRenderCapacityOptions

The AudioRenderCapacityOptions dictionary can be used to provide user options for an AudioRenderCapacity.

dictionary AudioRenderCapacityOptions {
        double updateInterval = 1;
};

1.2.10.1. Dictionary AudioRenderCapacityOptions Members

updateInterval, of type double, defaulting to 1

An update interval (in second) for dispaching AudioRenderCapacityEvents. A load value is calculated per system-level audio callback, and multiple load values will be collected over the specified interval period. For example, if the renderer runs at a 48Khz sample rate and the system-level audio callback’s buffer size is 192 frames, 250 load values will be collected over 1 second interval.

If the given value is smaller than the duration of the system-level audio callback, NotSupportedError is thrown.

1.2.11. AudioRenderCapacityEvent

[Exposed=Window]
interface AudioRenderCapacityEvent : Event {
    constructor (DOMString type, optional AudioRenderCapacityEventInit eventInitDict = {});
        readonly attribute double timestamp;
        readonly attribute double averageLoad;
        readonly attribute double peakLoad;
        readonly attribute double underrunRatio;
};

dictionary AudioRenderCapacityEventInit : EventInit {
    double timestamp = 0;
    double averageLoad = 0;
    double peakLoad = 0;
    double underrunRatio = 0;
};

1.2.11.1. Attributes

timestamp, of type double, readonly

The start time of the data collection period in terms of the associated AudioContext's currentTime.

averageLoad, of type double, readonly

An average of collected load values over the given update interval. The precision is limited to 1/100th.

peakLoad, of type double, readonly

A maximum value from collected load values over the given update interval. The precision is also limited to 1/100th.

underrunRatio, of type double, readonly

A ratio between the number of buffer underruns (when a load value is greater than 1.0) and the total number of system-level audio callbacks over the given update interval.

Where \(u\) is the number of buffer underruns and \(N\) is the number of system-level audio callbacks over the given update interval, the buffer underrun ratio is:

• 0.0 if \(u\) = 0.

• Otherwise, compute \(u/N\) and take a ceiling value of the nearest 100th.

1.3. The OfflineAudioContext Interface

OfflineAudioContext is a particular type of BaseAudioContext for rendering/mixing-down (potentially) faster than real-time. It does not render to the audio hardware, but instead renders as quickly as possible, fulfilling the returned promise with the rendered result as an AudioBuffer.

[Exposed=Window]
interface OfflineAudioContext : BaseAudioContext {
    constructor(OfflineAudioContextOptions contextOptions);
    constructor(unsigned long numberOfChannels, unsigned long length, float sampleRate);
    Promise<AudioBuffer> startRendering();
    Promise<undefined> resume();
    Promise<undefined> suspend(double suspendTime);
    readonly attribute unsigned long length;
    attribute EventHandler oncomplete;
};

1.3.1. Constructors

OfflineAudioContext(contextOptions)

If the current settings object's relevant global object's associated Document is NOT fully active, throw an InvalidStateError and abort these steps.

Let c be a new OfflineAudioContext object. Initialize c as follows:

1. Set the [[control thread state]] for c to "suspended".

2. Set the [[rendering thread state]] for c to "suspended".

3. Determine the [[render quantum size]] for this OfflineAudioContext, based on the value of the renderSizeHint:

   1. If it has the default value of "default" or "hardware", set the [[render quantum size]] private slot to 128.

   2. Else, if an integer has been passed, the User-Agent can decide to honour this value by setting it to the [[render quantum size]] private slot.

4. Construct an AudioDestinationNode with its channelCount set to contextOptions.numberOfChannels.

5. Let messageChannel be a new MessageChannel.

6. Let controlSidePort be the value of messageChannel’s port1 attribute.

7. Let renderingSidePort be the value of messageChannel’s port2 attribute.

8. Let serializedRenderingSidePort be the result of StructuredSerializeWithTransfer(renderingSidePort, « renderingSidePort »).

9. Set this audioWorklet's port to controlSidePort.

10. Queue a control message to set the MessagePort on the AudioContextGlobalScope, with serializedRenderingSidePort.

Arguments for the OfflineAudioContext.constructor(contextOptions) method.

Parameter	Type	Nullable	Optional	Description
contextOptions				The initial parameters needed to construct this context.

OfflineAudioContext(numberOfChannels, length, sampleRate)

The OfflineAudioContext can be constructed with the same arguments as AudioContext.createBuffer. A NotSupportedError exception MUST be thrown if any of the arguments is negative, zero, or outside its nominal range.

The OfflineAudioContext is constructed as if

new OfflineAudioContext({
        numberOfChannels: numberOfChannels,
        length: length,
        sampleRate: sampleRate
})

were called instead.

Arguments for the OfflineAudioContext.constructor(numberOfChannels, length, sampleRate) method.

Parameter	Type	Nullable	Optional	Description
numberOfChannels	unsigned long	✘	✘	Determines how many channels the buffer will have. See createBuffer() for the supported number of channels.
length	unsigned long	✘	✘	Determines the size of the buffer in sample-frames.
sampleRate	float	✘	✘	Describes the sample-rate of the linear PCM audio data in the buffer in sample-frames per second. See createBuffer() for valid sample rates.

1.3.2. Attributes

length, of type unsigned long, readonly

The size of the buffer in sample-frames. This is the same as the value of the length parameter for the constructor.

oncomplete, of type EventHandler

The event type of this event handler is complete. The event dispatched to the event handler will use the OfflineAudioCompletionEvent interface. It is the last event fired on an OfflineAudioContext.

1.3.3. Methods

startRendering()

Given the current connections and scheduled changes, starts rendering audio.

Although the primary method of getting the rendered audio data is via its promise return value, the instance will also fire an event named complete for legacy reasons.

Let [[rendering started]] be an internal slot of this OfflineAudioContext. Initialize this slot to false.

When startRendering is called, the following steps MUST be performed on the control thread:

1. If this's relevant global object's associated Document is not fully active then return a promise rejected with "InvalidStateError" DOMException.
2. If the [[rendering started]] slot on the OfflineAudioContext is true, return a rejected promise with InvalidStateError, and abort these steps.
3. Set the [[rendering started]] slot of the OfflineAudioContext to true.
4. Let promise be a new promise.
5. Create a new AudioBuffer, with a number of channels, length and sample rate equal respectively to the numberOfChannels, length and sampleRate values passed to this instance’s constructor in the contextOptions parameter. Assign this buffer to an internal slot [[rendered buffer]] in the OfflineAudioContext.
6. If an exception was thrown during the preceding AudioBuffer constructor call, reject promise with this exception.
7. Otherwise, in the case that the buffer was successfully constructed, begin offline rendering.
8. Append promise to [[pending promises]].
9. Return promise.

To begin offline rendering, the following steps MUST happen on a rendering thread that is created for the occasion.

1. Given the current connections and scheduled changes, start rendering length sample-frames of audio into [[rendered buffer]]
2. For every render quantum, check and suspend rendering if necessary.
3. If a suspended context is resumed, continue to render the buffer.
4. Once the rendering is complete, queue a media element task to execute the following steps:
   1. Resolve the promise created by startRendering() with [[rendered buffer]].
   2. Queue a media element task to fire an event named complete at the OfflineAudioContext using OfflineAudioCompletionEvent whose renderedBuffer property is set to [[rendered buffer]].

No parameters.

Return type: Promise<AudioBuffer>

resume()

Resumes the progression of the OfflineAudioContext's currentTime when it has been suspended.

When resume is called, execute these steps:

1. If this's relevant global object's associated Document is not fully active then return a promise rejected with "InvalidStateError" DOMException.

2. Let promise be a new Promise.

3. Abort these steps and reject promise with InvalidStateError when any of following conditions is true:

   • The [[control thread state]] on the OfflineAudioContext is closed.

   • The [[rendering started]] slot on the OfflineAudioContext is false.

4. Set the [[control thread state]] flag on the OfflineAudioContext to running.

5. Queue a control message to resume the OfflineAudioContext.

6. Return promise.

Running a control message to resume an OfflineAudioContext means running these steps on the rendering thread:

1. Set the [[rendering thread state]] on the OfflineAudioContext to running.

2. Start rendering the audio graph.

3. In case of failure, queue a media element task to reject promise and abort the remaining steps.

4. queue a media element task to execute the following steps:

   1. Resolve promise.

   2. If the state attribute of the OfflineAudioContext is not already "running":

      1. Set the state attribute of the OfflineAudioContext to "running".

      2. Queue a media element task to fire an event named statechange at the OfflineAudioContext.

No parameters.

Return type: Promise<undefined>

suspend(suspendTime)

Schedules a suspension of the time progression in the audio context at the specified time and returns a promise. This is generally useful when manipulating the audio graph synchronously on OfflineAudioContext.

Note that the maximum precision of suspension is the size of the render quantum and the specified suspension time will be rounded up to the nearest render quantum boundary. For this reason, it is not allowed to schedule multiple suspends at the same quantized frame. Also, scheduling should be done while the context is not running to ensure precise suspension.

Arguments for the OfflineAudioContext.suspend() method.

Parameter	Type	Nullable	Optional	Description
suspendTime	double	✘	✘	Schedules a suspension of the rendering at the specified time, which is quantized and rounded up to the render quantum size. If the quantized frame number is negative or is less than or equal to the current time or is greater than or equal to the total render duration or is scheduled by another suspend for the same time, then the promise is rejected with InvalidStateError.

Return type: Promise<undefined>

1.3.4. OfflineAudioContextOptions

This specifies the options to use in constructing an OfflineAudioContext.

dictionary OfflineAudioContextOptions {
    unsigned long numberOfChannels = 1;
    required unsigned long length;
    required float sampleRate;
    (AudioContextRenderSizeCategory or unsigned long) renderSizeHint = "default";
};

1.3.4.1. Dictionary OfflineAudioContextOptions Members

length, of type unsigned long

The length of the rendered AudioBuffer in sample-frames.

numberOfChannels, of type unsigned long, defaulting to 1

The number of channels for this OfflineAudioContext.

sampleRate, of type float

The sample rate for this OfflineAudioContext.

renderSizeHint, of type (AudioContextRenderSizeCategory or unsigned long), defaulting to "default"

A hint for the render quantum size of this OfflineAudioContext.

1.3.5. The OfflineAudioCompletionEvent Interface

This is an Event object which is dispatched to OfflineAudioContext for legacy reasons.

[Exposed=Window]
interface OfflineAudioCompletionEvent : Event {
    constructor (DOMString type, OfflineAudioCompletionEventInit eventInitDict);
    readonly attribute AudioBuffer renderedBuffer;
};

1.3.5.1. Attributes

renderedBuffer, of type AudioBuffer, readonly

An AudioBuffer containing the rendered audio data.

1.3.5.2. OfflineAudioCompletionEventInit

dictionary OfflineAudioCompletionEventInit : EventInit {
    required AudioBuffer renderedBuffer;
};

1.3.5.2.1. Dictionary OfflineAudioCompletionEventInit Members

renderedBuffer, of type AudioBuffer

Value to be assigned to the renderedBuffer attribute of the event.

1.4. The AudioBuffer Interface

This interface represents a memory-resident audio asset. It can contain one or more channels with each channel appearing to be 32-bit floating-point linear PCM values with a nominal range of \([-1,1]\) but the values are not limited to this range. Typically, it would be expected that the length of the PCM data would be fairly short (usually somewhat less than a minute). For longer sounds, such as music soundtracks, streaming should be used with the audio element and MediaElementAudioSourceNode.

An AudioBuffer may be used by one or more AudioContexts, and can be shared between an OfflineAudioContext and an AudioContext.

AudioBuffer has four internal slots:

[[number of channels]]

The number of audio channels for this AudioBuffer, which is an unsigned long.

[[length]]

The length of each channel of this AudioBuffer, which is an unsigned long.

[[sample rate]]

The sample-rate, in Hz, of this AudioBuffer, a float.

[[internal data]]

A data block holding the audio sample data.

[Exposed=Window]
interface AudioBuffer {
    constructor (AudioBufferOptions options);
    readonly attribute float sampleRate;
    readonly attribute unsigned long length;
    readonly attribute double duration;
    readonly attribute unsigned long numberOfChannels;
    Float32Array getChannelData (unsigned long channel);
    undefined copyFromChannel (Float32Array destination,
                               unsigned long channelNumber,
                               optional unsigned long bufferOffset = 0);
    undefined copyToChannel (Float32Array source,
                             unsigned long channelNumber,
                             optional unsigned long bufferOffset = 0);
};

1.4.1. Constructors

AudioBuffer(options)

1. If any of the values in options lie outside its nominal range, throw a NotSupportedError exception and abort the following steps.

2. Let b be a new AudioBuffer object.

3. Respectively assign the values of the attributes numberOfChannels, length, sampleRate of the AudioBufferOptions passed in the constructor to the internal slots [[number of channels]], [[length]], [[sample rate]].

4. Set the internal slot [[internal data]] of this AudioBuffer to the result of calling CreateByteDataBlock([[length]] * [[number of channels]]).

   Note: This initializes the underlying storage to zero.

5. Return b.

Arguments for the AudioBuffer.constructor() method.

Parameter	Type	Nullable	Optional	Description
options	AudioBufferOptions	✘	✘	An AudioBufferOptions that determine the properties for this AudioBuffer.

1.4.2. Attributes

duration, of type double, readonly

Duration of the PCM audio data in seconds.

This is computed from the [[sample rate]] and the [[length]] of the AudioBuffer by performing a division between the [[length]] and the [[sample rate]].

length, of type unsigned long, readonly

Length of the PCM audio data in sample-frames. This MUST return the value of [[length]].

numberOfChannels, of type unsigned long, readonly

The number of discrete audio channels. This MUST return the value of [[number of channels]].

sampleRate, of type float, readonly

The sample-rate for the PCM audio data in samples per second. This MUST return the value of [[sample rate]].

1.4.3. Methods

copyFromChannel(destination, channelNumber, bufferOffset)

The copyFromChannel() method copies the samples from the specified channel of the AudioBuffer to the destination array.

Let buffer be the AudioBuffer with \(N_b\) frames, let \(N_f\) be the number of elements in the destination array, and \(k\) be the value of bufferOffset. Then the number of frames copied from buffer to destination is \(\max(0, \min(N_b - k, N_f))\). If this is less than \(N_f\), then the remaining elements of destination are not modified.

Arguments for the AudioBuffer.copyFromChannel() method.

Parameter	Type	Nullable	Optional	Description
destination	Float32Array	✘	✘	The array the channel data will be copied to.
channelNumber	unsigned long	✘	✘	The index of the channel to copy the data from. If channelNumber is greater or equal than the number of channels of the AudioBuffer, an IndexSizeError MUST be thrown.
bufferOffset	unsigned long	✘	✔	An optional offset, defaulting to 0. Data from the AudioBuffer starting at this offset is copied to the destination.

Return type: undefined

copyToChannel(source, channelNumber, bufferOffset)

The copyToChannel() method copies the samples to the specified channel of the AudioBuffer from the source array.

A UnknownError may be thrown if source cannot be copied to the buffer.

Let buffer be the AudioBuffer with \(N_b\) frames, let \(N_f\) be the number of elements in the source array, and \(k\) be the value of bufferOffset. Then the number of frames copied from source to the buffer is \(\max(0, \min(N_b - k, N_f))\). If this is less than \(N_f\), then the remaining elements of buffer are not modified.

Arguments for the AudioBuffer.copyToChannel() method.

Parameter	Type	Nullable	Optional	Description
source	Float32Array	✘	✘	The array the channel data will be copied from.
channelNumber	unsigned long	✘	✘	The index of the channel to copy the data to. If channelNumber is greater or equal than the number of channels of the AudioBuffer, an IndexSizeError MUST be thrown.
bufferOffset	unsigned long	✘	✔	An optional offset, defaulting to 0. Data from the source is copied to the AudioBuffer starting at this offset.

Return type: undefined

getChannelData(channel)

According to the rules described in acquire the content either allow writing into or getting a copy of the bytes stored in [[internal data]] in a new Float32Array

A UnknownError may be thrown if the [[internal data]] or the new Float32Array cannot be created.

Arguments for the AudioBuffer.getChannelData() method.

Parameter	Type	Nullable	Optional	Description
channel	unsigned long	✘	✘	This parameter is an index representing the particular channel to get data for. An index value of 0 represents the first channel. This index value MUST be less than [[number of channels]] or an IndexSizeError exception MUST be thrown.

Return type: Float32Array

Note: The methods copyToChannel() and copyFromChannel() can be used to fill part of an array by passing in a Float32Array that’s a view onto the larger array. When reading data from an AudioBuffer's channels, and the data can be processed in chunks, copyFromChannel() should be preferred to calling getChannelData() and accessing the resulting array, because it may avoid unnecessary memory allocation and copying.

An internal operation acquire the contents of an AudioBuffer is invoked when the contents of an AudioBuffer are needed by some API implementation. This operation returns immutable channel data to the invoker.

The acquire the contents of an AudioBuffer operation is invoked in the following cases:

• When AudioBufferSourceNode.start is called, it acquires the contents of the node’s buffer. If the operation fails, nothing is played.

• When the buffer of an AudioBufferSourceNode is set and AudioBufferSourceNode.start has been previously called, the setter acquires the content of the AudioBuffer. If the operation fails, nothing is played.

• When a ConvolverNode's buffer is set to an AudioBuffer it acquires the content of the AudioBuffer.

• When the dispatch of an AudioProcessingEvent completes, it acquires the contents of its outputBuffer.

Note: This means that copyToChannel() cannot be used to change the content of an AudioBuffer currently in use by an AudioNode that has acquired the content of an AudioBuffer since the AudioNode will continue to use the data previously acquired.

1.4.4. AudioBufferOptions

This specifies the options to use in constructing an AudioBuffer. The length and sampleRate members are required.

dictionary AudioBufferOptions {
    unsigned long numberOfChannels = 1;
    required unsigned long length;
    required float sampleRate;
};

1.4.4.1. Dictionary AudioBufferOptions Members

The allowed values for the members of this dictionary are constrained. See createBuffer().

length, of type unsigned long

The length in sample frames of the buffer. See length for constraints.

numberOfChannels, of type unsigned long, defaulting to 1

The number of channels for the buffer. See numberOfChannels for constraints.

sampleRate, of type float

The sample rate in Hz for the buffer. See sampleRate for constraints.

1.5. The AudioNode Interface

AudioNodes are the building blocks of an AudioContext. This interface represents audio sources, the audio destination, and intermediate processing modules. These modules can be connected together to form processing graphs for rendering audio to the audio hardware. Each node can have inputs and/or outputs. A source node has no inputs and a single output. Most processing nodes such as filters will have one input and one output. Each type of AudioNode differs in the details of how it processes or synthesizes audio. But, in general, an AudioNode will process its inputs (if it has any), and generate audio for its outputs (if it has any).

Each output has one or more channels. The exact number of channels depends on the details of the specific AudioNode.

An output may connect to one or more AudioNode inputs, thus fan-out is supported. An input initially has no connections, but may be connected from one or more AudioNode outputs, thus fan-in is supported. When the connect() method is called to connect an output of an AudioNode to an input of an AudioNode, we call that a connection to the input.

Each AudioNode input has a specific number of channels at any given time. This number can change depending on the connection(s) made to the input. If the input has no connections then it has one channel which is silent.

For each input, an AudioNode performs a mixing of all connections to that input. Please see § 4 Channel Up-Mixing and Down-Mixing for normative requirements and details.

The processing of inputs and the internal operations of an AudioNode take place continuously with respect to AudioContext time, regardless of whether the node has connected outputs, and regardless of whether these outputs ultimately reach an AudioContext's AudioDestinationNode.

[Exposed=Window]
interface AudioNode : EventTarget {
    AudioNode connect (AudioNode destinationNode,
                       optional unsigned long output = 0,
                       optional unsigned long input = 0);
    undefined connect (AudioParam destinationParam, optional unsigned long output = 0);
    undefined disconnect ();
    undefined disconnect (unsigned long output);
    undefined disconnect (AudioNode destinationNode);
    undefined disconnect (AudioNode destinationNode, unsigned long output);
    undefined disconnect (AudioNode destinationNode,
                          unsigned long output,
                          unsigned long input);
    undefined disconnect (AudioParam destinationParam);
    undefined disconnect (AudioParam destinationParam, unsigned long output);
    readonly attribute BaseAudioContext context;
    readonly attribute unsigned long numberOfInputs;
    readonly attribute unsigned long numberOfOutputs;
    attribute unsigned long channelCount;
    attribute ChannelCountMode channelCountMode;
    attribute ChannelInterpretation channelInterpretation;
};

1.5.1. AudioNode Creation

AudioNodes can be created in two ways: by using the constructor for this particular interface, or by using the factory method on the BaseAudioContext or AudioContext.

The BaseAudioContext passed as first argument of the constructor of an AudioNodes is called the associated BaseAudioContext of the AudioNode to be created. Similarly, when using the factory method, the associated BaseAudioContext of the AudioNode is the BaseAudioContext this factory method is called on.

The associated interface for a factory method is the interface of the objects that are returned from this method. The associated option object for an interface is the option object that can be passed to the constructor for this interface.

AudioNodes are EventTargets, as described in [DOM]. This means that it is possible to dispatch events to AudioNodes the same way that other EventTargets accept events.

enum ChannelCountMode {
    "max",
    "clamped-max",
    "explicit"
};

The ChannelCountMode, in conjuction with the node’s channelCount and channelInterpretation values, is used to determine the computedNumberOfChannels that controls how inputs to a node are to be mixed. The computedNumberOfChannels is determined as shown below. See § 4 Channel Up-Mixing and Down-Mixing for more information on how mixing is to be done.

enum ChannelInterpretation {
    "speakers",
    "discrete"
};

1.5.2. AudioNode Tail-Time

An AudioNode can have a tail-time. This means that even when the AudioNode is fed silence, the output can be non-silent.

AudioNodes have a non-zero tail-time if they have internal processing state such that input in the past affects the future output. AudioNodes may continue to produce non-silent output for the calculated tail-time even after the input transitions from non-silent to silent.

1.5.3. AudioNode Lifetime

AudioNode can be actively processing during a render quantum, if any of the following conditions hold.

• An AudioScheduledSourceNode is actively processing if and only if it is playing for at least part of the current rendering quantum.

• A MediaElementAudioSourceNode is actively processing if and only if its mediaElement is playing for at least part of the current rendering quantum.

• A MediaStreamAudioSourceNode or a MediaStreamTrackAudioSourceNode are actively processing when the associated MediaStreamTrack object has a readyState attribute equal to "live", a muted attribute equal to false and an enabled attribute equal to true.

• A DelayNode in a cycle is actively processing only when the absolute value of any output sample for the current render quantum is greater than or equal to \( 2^{-126} \).

• A ScriptProcessorNode is actively processing when its input or output is connected.

• An AudioWorkletNode is actively processing when its AudioWorkletProcessor's [[callable process]] returns true and either its active source flag is true or any AudioNode connected to one of its inputs is actively processing.

• All other AudioNodes start actively processing when any AudioNode connected to one of its inputs is actively processing, and stops actively processing when the input that was received from other actively processing AudioNode no longer affects the output.

Note: This takes into account AudioNodes that have a tail-time.

AudioNodes that are not actively processing output a single channel of silence.

1.5.4. Attributes

channelCount, of type unsigned long

channelCount is the number of channels used when up-mixing and down-mixing connections to any inputs to the node. The default value is 2 except for specific nodes where its value is specially determined. This attribute has no effect for nodes with no inputs. If this value is set to zero or to a value greater than the implementation’s maximum number of channels the implementation MUST throw a NotSupportedError exception.

In addition, some nodes have additional channelCount constraints on the possible values for the channel count:

AudioDestinationNode

The behavior depends on whether the destination node is the destination of an AudioContext or OfflineAudioContext:

AudioContext

The channel count MUST be between 1 and maxChannelCount. An IndexSizeError exception MUST be thrown for any attempt to set the count outside this range.

OfflineAudioContext

The channel count cannot be changed. An InvalidStateError exception MUST be thrown for any attempt to change the value.

AudioWorkletNode

See § 1.32.4.3.2 Configuring Channels with AudioWorkletNodeOptions Configuring Channels with AudioWorkletNodeOptions.

ChannelMergerNode

The channel count cannot be changed, and an InvalidStateError exception MUST be thrown for any attempt to change the value.

ChannelSplitterNode

The channel count cannot be changed, and an InvalidStateError exception MUST be thrown for any attempt to change the value.

ConvolverNode

The channel count cannot be greater than two, and a NotSupportedError exception MUST be thrown for any attempt to change it to a value greater than two.

DynamicsCompressorNode

The channel count cannot be greater than two, and a NotSupportedError exception MUST be thrown for any attempt to change it to a value greater than two.

PannerNode

The channel count cannot be greater than two, and a NotSupportedError exception MUST be thrown for any attempt to change it to a value greater than two.

ScriptProcessorNode

The channel count cannot be changed, and an NotSupportedError exception MUST be thrown for any attempt to change the value.

StereoPannerNode

The channel count cannot be greater than two, and a NotSupportedError exception MUST be thrown for any attempt to change it to a value greater than two.

See § 4 Channel Up-Mixing and Down-Mixing for more information on this attribute.

channelCountMode, of type ChannelCountMode

channelCountMode determines how channels will be counted when up-mixing and down-mixing connections to any inputs to the node. The default value is "max". This attribute has no effect for nodes with no inputs.

In addition, some nodes have additional channelCountMode constraints on the possible values for the channel count mode:

AudioDestinationNode

If the AudioDestinationNode is the destination node of an OfflineAudioContext, then the channel count mode cannot be changed. An InvalidStateError exception MUST be thrown for any attempt to change the value.

ChannelMergerNode

The channel count mode cannot be changed from "explicit" and an InvalidStateError exception MUST be thrown for any attempt to change the value.

ChannelSplitterNode

The channel count mode cannot be changed from "explicit" and an InvalidStateError exception MUST be thrown for any attempt to change the value.

ConvolverNode

The channel count mode cannot be set to "max", and a NotSupportedError exception MUST be thrown for any attempt to set it to "max".

DynamicsCompressorNode

The channel count mode cannot be set to "max", and a NotSupportedError exception MUST be thrown for any attempt to set it to "max".

PannerNode

The channel count mode cannot be set to "max", and a NotSupportedError exception MUST be thrown for any attempt to set it to "max".

ScriptProcessorNode

The channel count mode cannot be changed from "explicit" and an NotSupportedError exception MUST be thrown for any attempt to change the value.

StereoPannerNode

The channel count mode cannot be set to "max", and a NotSupportedError exception MUST be thrown for any attempt to set it to "max".

See the § 4 Channel Up-Mixing and Down-Mixing section for more information on this attribute.

channelInterpretation, of type ChannelInterpretation

channelInterpretation determines how individual channels will be treated when up-mixing and down-mixing connections to any inputs to the node. The default value is "speakers". This attribute has no effect for nodes with no inputs.

In addition, some nodes have additional channelInterpretation constraints on the possible values for the channel interpretation:

ChannelSplitterNode

The channel intepretation can not be changed from "discrete" and a InvalidStateError exception MUST be thrown for any attempt to change the value.

See § 4 Channel Up-Mixing and Down-Mixing for more information on this attribute.

context, of type BaseAudioContext, readonly

The BaseAudioContext which owns this AudioNode.

numberOfInputs, of type unsigned long, readonly

The number of inputs feeding into the AudioNode. For source nodes, this will be 0. This attribute is predetermined for many AudioNode types, but some AudioNodes, like the ChannelMergerNode and the AudioWorkletNode, have variable number of inputs.

numberOfOutputs, of type unsigned long, readonly

The number of outputs coming out of the AudioNode. This attribute is predetermined for some AudioNode types, but can be variable, like for the ChannelSplitterNode and the AudioWorkletNode.

1.5.5. Methods

connect(destinationNode, output, input)

There can only be one connection between a given output of one specific node and a given input of another specific node. Multiple connections with the same termini are ignored.

For example:

nodeA.connect(nodeB);
nodeA.connect(nodeB);

will have the same effect as

nodeA.connect(nodeB);

This method returns destination AudioNode object.

Arguments for the AudioNode.connect(destinationNode, output, input) method.

Parameter	Type	Nullable	Optional	Description
destinationNode				The destination parameter is the AudioNode to connect to. If the destination parameter is an AudioNode that has been created using another AudioContext, an InvalidAccessError MUST be thrown. That is, AudioNodes cannot be shared between AudioContexts. Multiple AudioNodes can be connected to the same AudioNode, this is described in Channel Upmixing and down mixing section.
output	unsigned long	✘	✔	The output parameter is an index describing which output of the AudioNode from which to connect. If this parameter is out-of-bounds, an IndexSizeError exception MUST be thrown. It is possible to connect an AudioNode output to more than one input with multiple calls to connect(). Thus, "fan-out" is supported.
input				The input parameter is an index describing which input of the destination AudioNode to connect to. If this parameter is out-of-bounds, an IndexSizeError exception MUST be thrown. It is possible to connect an AudioNode to another AudioNode which creates a cycle: an AudioNode may connect to another AudioNode, which in turn connects back to the input or AudioParam of the first AudioNode.

Return type: AudioNode

connect(destinationParam, output)

Connects the AudioNode to an AudioParam, controlling the parameter value with an a-rate signal.

It is possible to connect an AudioNode output to more than one AudioParam with multiple calls to connect(). Thus, "fan-out" is supported.

It is possible to connect more than one AudioNode output to a single AudioParam with multiple calls to connect(). Thus, "fan-in" is supported.

An AudioParam will take the rendered audio data from any AudioNode output connected to it and convert it to mono by down-mixing if it is not already mono, then mix it together with other such outputs and finally will mix with the intrinsic parameter value (the value the AudioParam would normally have without any audio connections), including any timeline changes scheduled for the parameter.

The down-mixing to mono is equivalent to the down-mixing for an AudioNode with channelCount = 1, channelCountMode = "explicit", and channelInterpretation = "speakers".

There can only be one connection between a given output of one specific node and a specific AudioParam. Multiple connections with the same termini are ignored.

For example:

nodeA.connect(param);
nodeA.connect(param);

will have the same effect as

nodeA.connect(param);

Arguments for the AudioNode.connect(destinationParam, output) method.

Parameter	Type	Nullable	Optional	Description
destinationParam	AudioParam	✘	✘	The destination parameter is the AudioParam to connect to. This method does not return the destination AudioParam object. If destinationParam belongs to an AudioNode that belongs to a BaseAudioContext that is different from the BaseAudioContext that has created the AudioNode on which this method was called, an InvalidAccessError MUST be thrown.
output	unsigned long	✘	✔	The output parameter is an index describing which output of the AudioNode from which to connect. If the parameter is out-of-bounds, an IndexSizeError exception MUST be thrown.

Return type: undefined

disconnect()

Disconnects all outgoing connections from the AudioNode.

No parameters.

Return type: undefined

disconnect(output)

Disconnects a single output of the AudioNode from any other AudioNode or AudioParam objects to which it is connected.

Arguments for the AudioNode.disconnect(output) method.

Parameter	Type	Nullable	Optional	Description
output	unsigned long	✘	✘	This parameter is an index describing which output of the AudioNode to disconnect. It disconnects all outgoing connections from the given output. If this parameter is out-of-bounds, an IndexSizeError exception MUST be thrown.

Return type: undefined

disconnect(destinationNode)

Disconnects all outputs of the AudioNode that go to a specific destination AudioNode.

Arguments for the AudioNode.disconnect(destinationNode) method.

Parameter	Type	Nullable	Optional	Description
destinationNode				The destinationNode parameter is the AudioNode to disconnect. It disconnects all outgoing connections to the given destinationNode. If there is no connection to the destinationNode, an InvalidAccessError exception MUST be thrown.

Return type: undefined

disconnect(destinationNode, output)

Disconnects a specific output of the AudioNode from any and all inputs of some destination AudioNode.

Arguments for the AudioNode.disconnect(destinationNode, output) method.

Parameter	Type	Nullable	Optional	Description
destinationNode				The destinationNode parameter is the AudioNode to disconnect. If there is no connection to the destinationNode from the given output, an InvalidAccessError exception MUST be thrown.
output	unsigned long	✘	✘	The output parameter is an index describing which output of the AudioNode from which to disconnect. If this parameter is out-of-bounds, an IndexSizeError exception MUST be thrown.

Return type: undefined

disconnect(destinationNode, output, input)

Disconnects a specific output of the AudioNode from a specific input of some destination AudioNode.

Arguments for the AudioNode.disconnect(destinationNode, output, input) method.

Parameter	Type	Nullable	Optional	Description
destinationNode				The destinationNode parameter is the AudioNode to disconnect. If there is no connection to the destinationNode from the given output to the given input, an InvalidAccessError exception MUST be thrown.
output	unsigned long	✘	✘	The output parameter is an index describing which output of the AudioNode from which to disconnect. If this parameter is out-of-bounds, an IndexSizeError exception MUST be thrown.
input				The input parameter is an index describing which input of the destination AudioNode to disconnect. If this parameter is out-of-bounds, an IndexSizeError exception MUST be thrown.

Return type: undefined

disconnect(destinationParam)

Disconnects all outputs of the AudioNode that go to a specific destination AudioParam. The contribution of this AudioNode to the computed parameter value goes to 0 when this operation takes effect. The intrinsic parameter value is not affected by this operation.

Arguments for the AudioNode.disconnect(destinationParam) method.

Parameter	Type	Nullable	Optional	Description
destinationParam	AudioParam	✘	✘	The destinationParam parameter is the AudioParam to disconnect. If there is no connection to the destinationParam, an InvalidAccessError exception MUST be thrown.

Return type: undefined

disconnect(destinationParam, output)

Disconnects a specific output of the AudioNode from a specific destination AudioParam. The contribution of this AudioNode to the computed parameter value goes to 0 when this operation takes effect. The intrinsic parameter value is not affected by this operation.

Arguments for the AudioNode.disconnect(destinationParam, output) method.

Parameter	Type	Nullable	Optional	Description
destinationParam	AudioParam	✘	✘	The destinationParam parameter is the AudioParam to disconnect. If there is no connection to the destinationParam, an InvalidAccessError exception MUST be thrown.
output	unsigned long	✘	✘	The output parameter is an index describing which output of the AudioNode from which to disconnect. If the parameter is out-of-bounds, an IndexSizeError exception MUST be thrown.

Return type: undefined

1.5.6. AudioNodeOptions

This specifies the options that can be used in constructing all AudioNodes. All members are optional. However, the specific values used for each node depends on the actual node.

dictionary AudioNodeOptions {
    unsigned long channelCount;
    ChannelCountMode channelCountMode;
    ChannelInterpretation channelInterpretation;
};

1.5.6.1. Dictionary AudioNodeOptions Members

channelCount, of type unsigned long

Desired number of channels for the channelCount attribute.

channelCountMode, of type ChannelCountMode

Desired mode for the channelCountMode attribute.

channelInterpretation, of type ChannelInterpretation

Desired mode for the channelInterpretation attribute.

1.6. The AudioParam Interface

AudioParam controls an individual aspect of an AudioNode's functionality, such as volume. The parameter can be set immediately to a particular value using the value attribute. Or, value changes can be scheduled to happen at very precise times (in the coordinate system of AudioContext's currentTime attribute), for envelopes, volume fades, LFOs, filter sweeps, grain windows, etc. In this way, arbitrary timeline-based automation curves can be set on any AudioParam. Additionally, audio signals from the outputs of AudioNodes can be connected to an AudioParam, summing with the intrinsic parameter value.

Some synthesis and processing AudioNodes have AudioParams as attributes whose values MUST be taken into account on a per-audio-sample basis. For other AudioParams, sample-accuracy is not important and the value changes can be sampled more coarsely. Each individual AudioParam will specify that it is either an a-rate parameter which means that its values MUST be taken into account on a per-audio-sample basis, or it is a k-rate parameter.

Implementations MUST use block processing, with each AudioNode processing one render quantum.

For each render quantum, the value of a k-rate parameter MUST be sampled at the time of the very first sample-frame, and that value MUST be used for the entire block. a-rate parameters MUST be sampled for each sample-frame of the block. Depending on the AudioParam, its rate can be controlled by setting the automationRate attribute to either "a-rate" or "k-rate". See the description of the individual AudioParams for further details.

Each AudioParam includes minValue and maxValue attributes that together form the simple nominal range for the parameter. In effect, value of the parameter is clamped to the range \([\mathrm{minValue}, \mathrm{maxValue}]\). See § 1.6.3 Computation of Value for full details.

For many AudioParams the minValue and maxValue is intended to be set to the maximum possible range. In this case, maxValue should be set to the most-positive-single-float value, which is 3.4028235e38. (However, in JavaScript which only supports IEEE-754 double precision float values, this must be written as 3.4028234663852886e38.) Similarly, minValue should be set to the most-negative-single-float value, which is the negative of the most-positive-single-float: -3.4028235e38. (Similarly, this must be written in JavaScript as -3.4028234663852886e38.)

An AudioParam maintains a list of zero or more automation events. Each automation event specifies changes to the parameter’s value over a specific time range, in relation to its automation event time in the time coordinate system of the AudioContext's currentTime attribute. The list of automation events is maintained in ascending order of automation event time.

The behavior of a given automation event is a function of the AudioContext's current time, as well as the automation event times of this event and of adjacent events in the list. The following automation methods change the event list by adding a new event to the event list, of a type specific to the method:

• setValueAtTime() - SetValue

• linearRampToValueAtTime() - LinearRampToValue

• exponentialRampToValueAtTime() - ExponentialRampToValue

• setTargetAtTime() - SetTarget

• setValueCurveAtTime() - SetValueCurve

The following rules will apply when calling these methods:

• Automation event times are not quantized with respect to the prevailing sample rate. Formulas for determining curves and ramps are applied to the exact numerical times given when scheduling events.

• If one of these events is added at a time where there is already one or more events, then it will be placed in the list after them, but before events whose times are after the event.

• If setValueCurveAtTime() is called for time \(T\) and duration \(D\) and there are any events having a time strictly greater than \(T\), but strictly less than \(T + D\), then a NotSupportedError exception MUST be thrown. In other words, it’s not ok to schedule a value curve during a time period containing other events, but it’s ok to schedule a value curve exactly at the time of another event.

• Similarly a NotSupportedError exception MUST be thrown if any automation method is called at a time which is contained in \([T, T+D)\), \(T\) being the time of the curve and \(D\) its duration.

Note: AudioParam attributes are read only, with the exception of the value attribute.

The automation rate of an AudioParam can be selected setting the automationRate attribute with one of the following values. However, some AudioParams have constraints on whether the automation rate can be changed.

enum AutomationRate {
    "a-rate",
    "k-rate"
};

Each AudioParam has an internal slot [[current value]], initially set to the AudioParam's defaultValue.

[Exposed=Window]
interface AudioParam {
    attribute float value;
    attribute AutomationRate automationRate;
    readonly attribute float defaultValue;
    readonly attribute float minValue;
    readonly attribute float maxValue;
    AudioParam setValueAtTime (float value, double startTime);
    AudioParam linearRampToValueAtTime (float value, double endTime);
    AudioParam exponentialRampToValueAtTime (float value, double endTime);
    AudioParam setTargetAtTime (float target, double startTime, float timeConstant);
    AudioParam setValueCurveAtTime (sequence<float> values,
                                    double startTime,
                                    double duration);
    AudioParam cancelScheduledValues (double cancelTime);
    AudioParam cancelAndHoldAtTime (double cancelTime);
};

1.6.1. Attributes

automationRate, of type AutomationRate

The automation rate for the AudioParam. The default value depends on the actual AudioParam; see the description of each individual AudioParam for the default value.

Some nodes have additional automation rate constraints as follows:

AudioBufferSourceNode

The AudioParams playbackRate and detune MUST be "k-rate". An InvalidStateError must be thrown if the rate is changed to "a-rate".

DynamicsCompressorNode

The AudioParams threshold, knee, ratio, attack, and release MUST be "k-rate". An InvalidStateError must be thrown if the rate is changed to "a-rate".

PannerNode

If the panningModel is "HRTF", the setting of the automationRate for any AudioParam of the PannerNode is ignored. Likewise, the setting of the automationRate for any AudioParam of the AudioListener is ignored. In this case, the AudioParam behaves as if the automationRate were set to "k-rate".

defaultValue, of type float, readonly

Initial value for the value attribute.

maxValue, of type float, readonly

The nominal maximum value that the parameter can take. Together with minValue, this forms the nominal range for this parameter.

minValue, of type float, readonly

The nominal minimum value that the parameter can take. Together with maxValue, this forms the nominal range for this parameter.

value, of type float

The parameter’s floating-point value. This attribute is initialized to the defaultValue.

Getting this attribute returns the contents of the [[current value]] slot. See § 1.6.3 Computation of Value for the algorithm for the value that is returned.

Setting this attribute has the effect of assigning the requested value to the [[current value]] slot, and calling the setValueAtTime() method with the current AudioContext's currentTime and [[current value]]. Any exceptions that would be thrown by setValueAtTime() will also be thrown by setting this attribute.

1.6.2. Methods

cancelAndHoldAtTime(cancelTime)

This is similar to cancelScheduledValues() in that it cancels all scheduled parameter changes with times greater than or equal to cancelTime. However, in addition, the automation value that would have happened at cancelTime is then proprogated for all future time until other automation events are introduced.

The behavior of the timeline in the face of cancelAndHoldAtTime() when automations are running and can be introduced at any time after calling cancelAndHoldAtTime() and before cancelTime is reached is quite complicated. The behavior of cancelAndHoldAtTime() is therefore specified in the following algorithm.

Let \(t_c\) be the value of cancelTime. Then

1. Let \(E_1\) be the event (if any) at time \(t_1\) where \(t_1\) is the largest number satisfying \(t_1 \le t_c\).

2. Let \(E_2\) be the event (if any) at time \(t_2\) where \(t_2\) is the smallest number satisfying \(t_c \lt t_2\).

3. If \(E_2\) exists:

   1. If \(E_2\) is a linear or exponential ramp,

      1. Effectively rewrite \(E_2\) to be the same kind of ramp ending at time \(t_c\) with an end value that would be the value of the original ramp at time \(t_c\).

      2. Go to step 5.

   2. Otherwise, go to step 4.

4. If \(E_1\) exists:

   1. If \(E_1\) is a setTarget event,

      1. Implicitly insert a setValueAtTime event at time \(t_c\) with the value that the setTarget would have at time \(t_c\).

      2. Go to step 5.

   2. If \(E_1\) is a setValueCurve with a start time of \(t_3\) and a duration of \(d\)

      1. If \(t_c \gt t_3 + d\), go to step 5.

      2. Otherwise,

         1. Effectively replace this event with a setValueCurve event with a start time of \(t_3\) and a new duration of \(t_c-t_3\). However, this is not a true replacement; this automation MUST take care to produce the same output as the original, and not one computed using a different duration. (That would cause sampling of the value curve in a slightly different way, producing different results.)

         2. Go to step 5.

5. Remove all events with time greater than \(t_c\).

If no events are added, then the automation value after cancelAndHoldAtTime() is the constant value that the original timeline would have had at time \(t_c\).

Arguments for the AudioParam.cancelAndHoldAtTime() method.

Parameter	Type	Nullable	Optional	Description
cancelTime	double	✘	✘	The time after which any previously scheduled parameter changes will be cancelled. It is a time in the same time coordinate system as the AudioContext's currentTime attribute. A RangeError exception MUST be thrown if cancelTime is negative. If cancelTime is less than currentTime, it is clamped to currentTime.

Return type: AudioParam

cancelScheduledValues(cancelTime)

Cancels all scheduled parameter changes with times greater than or equal to cancelTime. Cancelling a scheduled parameter change means removing the scheduled event from the event list. Any active automations whose automation event time is less than cancelTime are also cancelled, and such cancellations may cause discontinuities because the original value (from before such automation) is restored immediately. Any hold values scheduled by cancelAndHoldAtTime() are also removed if the hold time occurs after cancelTime.

For a setValueCurveAtTime(), let \(T_0\) and \(T_D\) be the corresponding startTime and duration, respectively of this event. Then if cancelTime is in the range \([T_0, T_0 + T_D]\), the event is removed from the timeline.

Arguments for the AudioParam.cancelScheduledValues() method.

Parameter	Type	Nullable	Optional	Description
cancelTime	double	✘	✘	The time after which any previously scheduled parameter changes will be cancelled. It is a time in the same time coordinate system as the AudioContext's currentTime attribute. A RangeError exception MUST be thrown if cancelTime is negative. If cancelTime is less than currentTime, it is clamped to currentTime.

Return type: AudioParam

exponentialRampToValueAtTime(value, endTime)

Schedules an exponential continuous change in parameter value from the previous scheduled parameter value to the given value. Parameters representing filter frequencies and playback rate are best changed exponentially because of the way humans perceive sound.

The value during the time interval \(T_0 \leq t < T_1\) (where \(T_0\) is the time of the previous event and \(T_1\) is the endTime parameter passed into this method) will be calculated as:

$$
    v(t) = V_0 \left(\frac{V_1}{V_0}\right)^\frac{t - T_0}{T_1 - T_0}
$$

where \(V_0\) is the value at the time \(T_0\) and \(V_1\) is the value parameter passed into this method. If \(V_0\) and \(V_1\) have opposite signs or if \(V_0\) is zero, then \(v(t) = V_0\) for \(T_0 \le t \lt T_1\).

This also implies an exponential ramp to 0 is not possible. A good approximation can be achieved using setTargetAtTime() with an appropriately chosen time constant.

If there are no more events after this ExponentialRampToValue event then for \(t \geq T_1\), \(v(t) = V_1\).

If there is no event preceding this event, the exponential ramp behaves as if setValueAtTime(value, currentTime) were called where value is the current value of the attribute and currentTime is the context currentTime at the time exponentialRampToValueAtTime() is called.

If the preceding event is a SetTarget event, \(T_0\) and \(V_0\) are chosen from the current time and value of SetTarget automation. That is, if the SetTarget event has not started, \(T_0\) is the start time of the event, and \(V_0\) is the value just before the SetTarget event starts. In this case, the ExponentialRampToValue event effectively replaces the SetTarget event. If the SetTarget event has already started, \(T_0\) is the current context time, and \(V_0\) is the current SetTarget automation value at time \(T_0\). In both cases, the automation curve is continuous.

Arguments for the AudioParam.exponentialRampToValueAtTime() method.

Parameter	Type	Nullable	Optional	Description
value	float	✘	✘	The value the parameter will exponentially ramp to at the given time. A RangeError exception MUST be thrown if this value is equal to 0.
endTime	double	✘	✘	The time in the same time coordinate system as the AudioContext's currentTime attribute where the exponential ramp ends. A RangeError exception MUST be thrown if endTime is negative or is not a finite number. If endTime is less than currentTime, it is clamped to currentTime.

Return type: AudioParam

linearRampToValueAtTime(value, endTime)

Schedules a linear continuous change in parameter value from the previous scheduled parameter value to the given value.

The value during the time interval \(T_0 \leq t < T_1\) (where \(T_0\) is the time of the previous event and \(T_1\) is the endTime parameter passed into this method) will be calculated as:

$$
    v(t) = V_0 + (V_1 - V_0) \frac{t - T_0}{T_1 - T_0}
$$

where \(V_0\) is the value at the time \(T_0\) and \(V_1\) is the value parameter passed into this method.

If there are no more events after this LinearRampToValue event then for \(t \geq T_1\), \(v(t) = V_1\).

If there is no event preceding this event, the linear ramp behaves as if setValueAtTime(value, currentTime) were called where value is the current value of the attribute and currentTime is the context currentTime at the time linearRampToValueAtTime() is called.

If the preceding event is a SetTarget event, \(T_0\) and \(V_0\) are chosen from the current time and value of SetTarget automation. That is, if the SetTarget event has not started, \(T_0\) is the start time of the event, and \(V_0\) is the value just before the SetTarget event starts. In this case, the LinearRampToValue event effectively replaces the SetTarget event. If the SetTarget event has already started, \(T_0\) is the current context time, and \(V_0\) is the current SetTarget automation value at time \(T_0\). In both cases, the automation curve is continuous.

Arguments for the AudioParam.linearRampToValueAtTime() method.

Parameter	Type	Nullable	Optional	Description
value	float	✘	✘	The value the parameter will linearly ramp to at the given time.
endTime	double	✘	✘	The time in the same time coordinate system as the AudioContext's currentTime attribute at which the automation ends. A RangeError exception MUST be thrown if endTime is negative or is not a finite number. If endTime is less than currentTime, it is clamped to currentTime.

Return type: AudioParam

setTargetAtTime(target, startTime, timeConstant)

Start exponentially approaching the target value at the given time with a rate having the given time constant. Among other uses, this is useful for implementing the "decay" and "release" portions of an ADSR envelope. Please note that the parameter value does not immediately change to the target value at the given time, but instead gradually changes to the target value.

During the time interval: \(T_0 \leq t\), where \(T_0\) is the startTime parameter:

$$
    v(t) = V_1 + (V_0 - V_1)\, e^{-\left(\frac{t - T_0}{\tau}\right)}
$$

where \(V_0\) is the initial value (the [[current value]] attribute) at \(T_0\) (the startTime parameter), \(V_1\) is equal to the target parameter, and \(\tau\) is the timeConstant parameter.

If a LinearRampToValue or ExponentialRampToValue event follows this event, the behavior is described in linearRampToValueAtTime() or exponentialRampToValueAtTime(), respectively. For all other events, the SetTarget event ends at the time of the next event.

Arguments for the AudioParam.setTargetAtTime() method.

Parameter	Type	Nullable	Optional	Description
target	float	✘	✘	The value the parameter will start changing to at the given time.
startTime	double	✘	✘	The time at which the exponential approach will begin, in the same time coordinate system as the AudioContext's currentTime attribute. A RangeError exception MUST be thrown if start is negative or is not a finite number. If startTime is less than currentTime, it is clamped to currentTime.
timeConstant	float	✘	✘	The time-constant value of first-order filter (exponential) approach to the target value. The larger this value is, the slower the transition will be. The value MUST be non-negative or a RangeError exception MUST be thrown. If timeConstant is zero, the output value jumps immediately to the final value. More precisely, timeConstant is the time it takes a first-order linear continuous time-invariant system to reach the value \(1 - 1/e\) (around 63.2%) given a step input response (transition from 0 to 1 value).

Return type: AudioParam

setValueAtTime(value, startTime)

Schedules a parameter value change at the given time.

If there are no more events after this SetValue event, then for \(t \geq T_0\), \(v(t) = V\), where \(T_0\) is the startTime parameter and \(V\) is the value parameter. In other words, the value will remain constant.

If the next event (having time \(T_1\)) after this SetValue event is not of type LinearRampToValue or ExponentialRampToValue, then, for \(T_0 \leq t < T_1\):

$$
    v(t) = V
$$

In other words, the value will remain constant during this time interval, allowing the creation of "step" functions.

If the next event after this SetValue event is of type LinearRampToValue or ExponentialRampToValue then please see linearRampToValueAtTime() or exponentialRampToValueAtTime(), respectively.

Arguments for the AudioParam.setValueAtTime() method.

Parameter	Type	Nullable	Optional	Description
value	float	✘	✘	The value the parameter will change to at the given time.
startTime	double	✘	✘	The time in the same time coordinate system as the BaseAudioContext's currentTime attribute at which the parameter changes to the given value. A RangeError exception MUST be thrown if startTime is negative or is not a finite number. If startTime is less than currentTime, it is clamped to currentTime.

Return type: AudioParam

setValueCurveAtTime(values, startTime, duration)

Sets an array of arbitrary parameter values starting at the given time for the given duration. The number of values will be scaled to fit into the desired duration.

Let \(T_0\) be startTime, \(T_D\) be duration, \(V\) be the values array, and \(N\) be the length of the values array. Then, during the time interval: \(T_0 \le t < T_0 + T_D\), let

$$
    \begin{align*} k &= \left\lfloor \frac{N - 1}{T_D}(t-T_0) \right\rfloor \\
    \end{align*}
$$

Then \(v(t)\) is computed by linearly interpolating between \(V[k]\) and \(V[k+1]\),

After the end of the curve time interval (\(t \ge T_0 + T_D\)), the value will remain constant at the final curve value, until there is another automation event (if any).

An implicit call to setValueAtTime() is made at time \(T_0 + T_D\) with value \(V[N-1]\) so that following automations will start from the end of the setValueCurveAtTime() event.

Arguments for the AudioParam.setValueCurveAtTime() method.

Parameter	Type	Nullable	Optional	Description
values	sequence<float>	✘	✘	A sequence of float values representing a parameter value curve. These values will apply starting at the given time and lasting for the given duration. When this method is called, an internal copy of the curve is created for automation purposes. Subsequent modifications of the contents of the passed-in array therefore have no effect on the AudioParam. An InvalidStateError MUST be thrown if this attribute is a sequence<float> object that has a length less than 2.
startTime	double	✘	✘	The start time in the same time coordinate system as the AudioContext's currentTime attribute at which the value curve will be applied. A RangeError exception MUST be thrown if startTime is negative or is not a finite number. If startTime is less than currentTime, it is clamped to currentTime.
duration	double	✘	✘	The amount of time in seconds (after the startTime parameter) where values will be calculated according to the values parameter. A RangeError exception MUST be thrown if duration is not strictly positive or is not a finite number.

Return type: AudioParam