Web Audio API

Abstract

This specification describes a high-level JavaScript API for processing and synthesizing audio in web applications. The primary paradigm is of an audio routing graph, where a number of AudioNode objects are connected together to define the overall audio rendering. The actual processing will primarily take place in the underlying implementation (typically optimized Assembly / C / C++ code), but direct JavaScript processing and synthesis is also supported.

The introductory section covers the motivation behind this specification.

This API is designed to be used in conjunction with other APIs and elements on the web platform, notably: XMLHttpRequest (using the responseType and response attributes). For games and interactive applications, it is anticipated to be used with the canvas 2D and WebGL 3D graphics APIs.

Status of this Document

This section describes the status of this document at the time of its publication. Other documents may supersede this document. A list of current W3C publications and the latest revision of this technical report can be found in the W3C technical reports index at http://www.w3.org/TR/.

This is the third public Working Draft of the Web Audio API specification. It has been produced by the W3C Audio Working Group , which is part of the W3C WebApps Activity.

Please send comments about this document to <public-audio@w3.org> (public archives of the W3C audio mailing list). Web content and browser developers are encouraged to review this draft.

Publication as a Working Draft does not imply endorsement by the W3C Membership. This is a draft document and may be updated, replaced or obsoleted by other documents at any time. It is inappropriate to cite this document as other than work in progress.

This document was produced by a group operating under the 5 February 2004 W3C Patent Policy. W3C maintains a public list of any patent disclosures made in connection with the deliverables of the group; that page also includes instructions for disclosing a patent. An individual who has actual knowledge of a patent which the individual believes contains Essential Claim(s) must disclose the information in accordance with section 6 of the W3C Patent Policy.

2. Conformance

Everything in this specification is normative except for examples and sections marked as being informative.

The keywords “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “RECOMMENDED”, “MAY” and “OPTIONAL” in this document are to be interpreted as described in Key words for use in RFCs to Indicate Requirement Levels [RFC2119].

The following conformance classes are defined by this specification:

conforming implementation

A user agent is considered to be a conforming implementation if it satisfies all of the MUST-, REQUIRED- and SHALL-level criteria in this specification that apply to implementations.

3. Terminology and Algorithms

This specification includes algorithms (steps) as part of the definition of methods. Conforming implementations (referred to as "user agents" from here on) MAY use other algorithms in the implementation of these methods, provided the end result is the same.

4. The Audio API

4.1. The AudioContext Interface

This interface represents a set of AudioNode objects and their connections. It allows for arbitrary routing of signals to the AudioDestinationNode (what the user ultimately hears). Nodes are created from the context and are then connected together. In most use cases, only a single AudioContext is used per document. An AudioContext is constructed as follows:

        var context = new AudioContext();
        

 

        interface AudioContext {

            readonly attribute AudioDestinationNode destination;
            readonly attribute float sampleRate;
            readonly attribute float currentTime;
            readonly attribute AudioListener listener;
            readonly attribute unsigned long activeSourceCount;

            AudioBuffer createBuffer(in unsigned long numberOfChannels, in unsigned long length, in float sampleRate)
                raises(DOMException);

            AudioBuffer createBuffer(in ArrayBuffer buffer, in boolean mixToMono)
                raises(DOMException);
            
            void decodeAudioData(in ArrayBuffer audioData,
                                 in [Callback] AudioBufferCallback successCallback,
                                 in [Optional, Callback] AudioBufferCallback errorCallback)
                raises(DOMException);
            

            // AudioNode creation          
            AudioBufferSourceNode createBufferSource();

            MediaElementAudioSourceNode createMediaElementSource(in HTMLMediaElement mediaElement)
                raises(DOMException);

            MediaStreamAudioSourceNode createMediaStreamSource(in MediaStream mediaStream)
                raises(DOMException);

            JavaScriptAudioNode createJavaScriptNode(in unsigned long bufferSize,
                                                     in [Optional] unsigned long numberOfInputChannels = 2,
                                                     in [Optional] unsigned long numberOfOutputChannels = 2)
                raises(DOMException);

            RealtimeAnalyserNode createAnalyser();
            AudioGainNode createGainNode();
            DelayNode createDelayNode(in [Optional] double maxDelayTime);
            BiquadFilterNode createBiquadFilter();
            AudioPannerNode createPanner();
            ConvolverNode createConvolver();

            AudioChannelSplitter createChannelSplitter(in [Optional] unsigned long numberOfOutputs = 6)
                raises(DOMException);

            AudioChannelMerger createChannelMerger(in [Optional] unsigned long numberOfInputs = 6);
                raises(DOMException);

            DynamicsCompressorNode createDynamicsCompressor();

            Oscillator createOscillator();
            WaveTable createWaveTable(in Float32Array real, in Float32Array imag)
                raises(DOMException);

        }
        

4.2. 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. An AudioSourceNode has no inputs and a single output. An AudioDestinationNode has one input and no outputs and represents the final destination to the audio hardware. Most processing nodes such as filters will have one input and one output.

For performance reasons, practical implementations will need to use block processing, with each AudioNode processing a fixed number of sample-frames of size block-size. In order to get uniform behavior across implementations, we will define this value explicitly. block-size is defined to be 128 sample-frames which corresponds to roughly 3ms at a sample-rate of 44.1KHz.

 

    interface AudioNode {

        void connect(in AudioNode destination, in [Optional] unsigned long output = 0, in [Optional] unsigned long input = 0)
            raises(DOMException);

        void connect(in AudioParam destination, in [Optional] unsigned long output = 0)
            raises(DOMException);

        void disconnect(in [Optional] unsigned long output = 0)
            raises(DOMException);

        readonly attribute AudioContext context;
        readonly attribute unsigned long numberOfInputs;
        readonly attribute unsigned long numberOfOutputs;

    }
    

4.3. The AudioSourceNode Interface

This is an abstract interface representing an audio source, an AudioNode which has no inputs and a single output:

      numberOfInputs  : 0
      numberOfOutputs : 1
      

Subclasses of AudioSourceNode will implement specific types of audio sources.

 

    interface AudioSourceNode : AudioNode {

    }
    

4.4. The AudioDestinationNode Interface

This is an AudioNode representing the final audio destination and is what the user will ultimately hear. It can be considered as an audio output device which is connected to speakers. All rendered audio to be heard will be routed to this node, a "terminal" node in the AudioContext's routing graph. There is only a single AudioDestinationNode per AudioContext, provided through the destination attribute of AudioContext.

      numberOfInputs  : 1
      numberOfOutputs : 0
      

 

    interface AudioDestinationNode : AudioNode {

        readonly attribute unsigned long maxNumberOfChannels;
        attribute unsigned long numberOfChannels;

    }
    

4.5. The AudioParam Interface

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

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 whose value changes must be taken into account at least at a 3ms resolution, but can be more precise than this.

Because practical implementations will use block processing, and will process a fixed number of sample-frames at a time (block-size sample-frames). For each block, the value of a k-rate parameter will be sampled at the time of the very first sample-frame, and that value will be used for the entire block.

 

    interface AudioParam {

        attribute float value;
        readonly attribute float minValue;
        readonly attribute float maxValue;
        readonly attribute float defaultValue;

        // Parameter automation. 
        void setValueAtTime(in float value, in float time);
        void linearRampToValueAtTime(in float value, in float time);
        void exponentialRampToValueAtTime(in float value, in float time);

        // Exponentially approach the target value with a rate having the given time constant. 
        void setTargetValueAtTime(in float targetValue, in float time, in float timeConstant);

        // Sets an array of arbitrary parameter values starting at time for the given duration. 
        // The number of values will be scaled to fit into the desired duration. 
        void setValueCurveAtTime(in Float32Array values, in float time, in float duration);
        
        // Cancels all scheduled parameter changes with times greater than or equal to startTime. 
        void cancelScheduledValues(in float startTime);

    }
    

4.5.3. AudioParam Automation Example

Example

ECMAScript

 
var t0 = 0;
var t1 = 0.1;
var t2 = 0.2;
var t3 = 0.3;
var t4 = 0.4;
var t5 = 0.6;
var t6 = 0.7;
var t7 = 1.0;

var curveLength = 44100;
var curve = new Float32Array(curveLength);
for (var i = 0; i < curveLength; ++i)
    curve[i] = Math.sin(Math.PI * i / curveLength);

param.setValueAtTime(0.2, t0);
param.setValueAtTime(0.3, t1);
param.setValueAtTime(0.4, t2);
param.linearRampToValueAtTime(1, t3);
param.linearRampToValueAtTime(0.15, t4);
param.exponentialRampToValueAtTime(0.75, t5);
param.exponentialRampToValueAtTime(0.05, t6);
param.setValueCurveAtTime(curve, t6, t7 - t6);

4.6. AudioGain

This interface is a particular type of AudioParam which specifically controls the gain (volume) of some aspect of the audio processing. The unit type is "linear gain". The nominal minValue is 0, but may be set negative for phase inversion. The nominal maxValue is 1, but higher values are allowed (no exception thrown).

 

  interface AudioGain : AudioParam {

  };
  

4.7. The AudioGainNode Interface

Changing the gain of an audio signal is a fundamental operation in audio applications. The AudioGainNode is one of the building blocks for creating mixers. This interface is an AudioNode with a single input and single output:

    numberOfInputs  : 1
    numberOfOutputs : 1
    

which multiplies the input audio signal by the (possibly time-varying) gain attribute, copying the result to the output. By default, it will take the input and pass it through to the output unchanged, which represents a constant gain change of 1.

As with other AudioParams, the gain parameter represents a mapping from time (in the coordinate system of AudioContext.currentTime) to floating-point value. Every PCM audio sample in the input is multiplied by the gain parameter's value for the specific time corresponding to that audio sample. This multiplied value represents the PCM audio sample for the output.

The number of channels of the output will always equal the number of channels of the input, with each channel of the input being multiplied by the gain values and being copied into the corresponding channel of the output.

The implementation must make gain changes to the audio stream smoothly, without introducing noticeable clicks or glitches. This process is called "de-zippering".

 

    interface AudioGainNode : AudioNode {

        AudioGain gain;

    }
    

4.8. The DelayNode Interface

A delay-line is a fundamental building block in audio applications. This interface is an AudioNode with a single input and single output:

    numberOfInputs  : 1
    numberOfOutputs : 1
    

which delays the incoming audio signal by a certain amount. The default amount is 0 seconds (no delay). When the delay time is changed, the implementation must make the transition smoothly, without introducing noticeable clicks or glitches to the audio stream.

 

    interface DelayNode : AudioNode {

        AudioParam delayTime;

    }
    

4.9. The AudioBuffer Interface

This interface represents a memory-resident audio asset (for one-shot sounds and other short audio clips). Its format is non-interleaved IEEE 32-bit linear PCM with a nominal range of -1 -> +1. It can contain one or more channels. It is analogous to a WebGL texture. 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.

 

    interface AudioBuffer {

        readonly attribute float sampleRate;
        readonly attribute long length;

        // in seconds          
        readonly attribute float duration;  

        readonly attribute int numberOfChannels;

        Float32Array getChannelData(in unsigned long channel);

    }
    

4.10. The AudioBufferSourceNode Interface

This interface represents an audio source from an in-memory audio asset in an AudioBuffer. It generally will be used for short audio assets which require a high degree of scheduling flexibility (can playback in rhythmically perfect ways). The playback state of an AudioBufferSourceNode goes through distinct stages during its lifetime in this order: UNSCHEDULED_STATE, SCHEDULED_STATE, PLAYING_STATE, FINISHED_STATE. The noteOn() method causes a transition from the UNSCHEDULED_STATE to SCHEDULED_STATE. Depending on the time argument passed to noteOn(), a transition is made from the SCHEDULED_STATE to PLAYING_STATE, at which time sound is first generated. Following this, a transition from the PLAYING_STATE to FINISHED_STATE happens when either the buffer's audio data has been completely played (if the loop attribute is false), or when the noteOff() method has been called and the specified time has been reached. Please see more details in the noteOn() and noteOff() description. Once an AudioBufferSourceNode has reached the FINISHED state it will no longer emit any sound. Thus noteOn() and noteOff() may not be issued multiple times for a given AudioBufferSourceNode.

    numberOfInputs  : 0
    numberOfOutputs : 1
    

 

    interface AudioBufferSourceNode : AudioSourceNode {

        const unsigned short UNSCHEDULED_STATE = 0;
        const unsigned short SCHEDULED_STATE = 1;
        const unsigned short PLAYING_STATE = 2;
        const unsigned short FINISHED_STATE = 3;

        readonly attribute unsigned short playbackState;

        // Playback this in-memory audio asset     
        // Many sources can share the same buffer     
        attribute AudioBuffer buffer;

        attribute AudioParam playbackRate; 
        attribute boolean loop;

        void noteOn(in double when);
        void noteGrainOn(in double when, in double grainOffset, in double grainDuration);
        void noteOff(in double when);

    }
    

4.10.2. Methods and Parameters

The noteOn method

Schedules a sound to playback at an exact time.

The when parameter describes at what time (in seconds) the sound should start playing. It is in the same time coordinate system as AudioContext.currentTime. If 0 is passed in for this value or if the value is less than currentTime, then the sound will start playing immediately. Either noteOn or noteGrainOn (but not both) may only be called one time and must be called before noteOff is called or an exception will be thrown.

The noteGrainOn method

Schedules a portion of a sound to playback at an exact time.

The when parameter describes at what time (in seconds) the sound should start playing. It is in the same time coordinate system as AudioContext.currentTime. If 0 is passed in for this value or if the value is less than currentTime, then the sound will start playing immediately.

The grainOffset parameter describes the offset in the buffer (in seconds) for the portion to be played.

The grainDuration parameter describes the duration of the portion (in seconds) to be played. Either noteOn or noteGrainOn (but not both) may only be called one time and must be called before noteOff is called or an exception will be thrown.

The noteOff method

Schedules a sound to stop playback at an exact time.

The when parameter describes at what time (in seconds) the sound should stop playing. It is in the same time coordinate system as AudioContext.currentTime. If 0 is passed in for this value or if the value is less than currentTime, then the sound will stop playing immediately. noteOff must only be called one time and only after a call to noteOn or noteOff, or an exception will be thrown.

4.11. The MediaElementAudioSourceNode Interface

This interface represents an audio source from an audio or video element.

    numberOfInputs  : 0
    numberOfOutputs : 1
    

 

    interface MediaElementAudioSourceNode : AudioSourceNode {

    }
    

4.12. The JavaScriptAudioNode Interface

This interface is an AudioNode which can generate, process, or analyse audio directly using JavaScript.

    numberOfInputs  : 1
    numberOfOutputs : 1
    

The JavaScriptAudioNode is constructed with a bufferSize which must be one of the following values: 256, 512, 1024, 2048, 4096, 8192, 16384. This value controls how frequently the onaudioprocess event handler is called and how many sample-frames need to be processed each call. Lower numbers for bufferSize will result in a lower (better) latency. Higher numbers will be necessary to avoid audio breakup and glitches. The value chosen must carefully balance between latency and audio quality.

numberOfInputChannels and numberOfOutputChannels determine the number of input and output channels. It is invalid for both numberOfInputChannels and numberOfOutputChannels to be zero.

    var node = context.createJavaScriptNode(bufferSize, numberOfInputChannels, numberOfOutputChannels);
    

 

    interface JavaScriptAudioNode : AudioNode {

        attribute EventListener onaudioprocess;
        
        readonly attribute long bufferSize;

    }
    

4.13. The AudioProcessingEvent Interface

This interface is a type of Event which is passed to the onaudioprocess event handler used by JavaScriptAudioNode.

The event handler processes audio from the input (if any) by accessing the audio data from the inputBuffer attribute. The audio data which is the result of the processing (or the synthesized data if there are no inputs) is then placed into the outputBuffer.

 

    interface AudioProcessingEvent : Event {

        JavaScriptAudioNode node;
        readonly attribute float playbackTime;
        readonly attribute AudioBuffer inputBuffer;
        readonly attribute AudioBuffer outputBuffer; 

    }
    

4.14. The AudioPannerNode Interface

This interface represents a processing node which positions / spatializes an incoming audio stream in three-dimensional space. The spatialization is in relation to the AudioContext's AudioListener (listener attribute).

    numberOfInputs  : 1
    numberOfOutputs : 1

The audio stream from the input will be either mono or stereo, depending on the connection(s) to the input.

The output of this node is hard-coded to stereo (2 channels) and currently cannot be configured.

 

    interface AudioPannerNode : AudioNode {

        // Panning model          
        const unsigned short EQUALPOWER = 0;
        const unsigned short HRTF = 1;
        const unsigned short SOUNDFIELD = 2;
        
        // Distance model 
        const unsigned short LINEAR_DISTANCE = 0;
        const unsigned short INVERSE_DISTANCE = 1;
        const unsigned short EXPONENTIAL_DISTANCE = 2;

        // Default for stereo is HRTF 
        attribute unsigned short panningModel;

        // Uses a 3D cartesian coordinate system 
        void setPosition(in float x, in float y, in float z);
        void setOrientation(in float x, in float y, in float z);
        void setVelocity(in float x, in float y, in float z);

        // Distance model and attributes          
        attribute unsigned short distanceModel;
        attribute float refDistance;
        attribute float maxDistance;
        attribute float rolloffFactor;

        // Directional sound cone          
        attribute float coneInnerAngle;
        attribute float coneOuterAngle;
        attribute float coneOuterGain;

        // Dynamically calculated gain values          
        readonly attribute AudioGain coneGain;
        readonly attribute AudioGain distanceGain;

    };
    

4.14.1. Constants

EQUALPOWER

A simple and efficient spatialization algorithm using equal-power panning.

HRTF

A higher quality spatialization algorithm using a convolution with measured impulse responses from human subjects. This panning method renders stereo output.

SOUNDFIELD

An algorithm which spatializes multi-channel audio using sound field algorithms.

LINEAR_DISTANCE

A linear distance model which calculates distanceGain according to:

1 - rolloffFactor * (distance - refDistance) / (maxDistance - refDistance)
    

INVERSE_DISTANCE

An inverse distance model which calculates distanceGain according to:

refDistance / (refDistance + rolloffFactor * (distance - refDistance))
    

EXPONENTIAL_DISTANCE

An exponential distance model which calculates distanceGain according to:

pow(distance / refDistance, -rolloffFactor)
  

4.14.2. Attributes

listener

Represents the listener whose position and orientation is used together with the panner's position and orientation to determine how the audio will be spatialized.

panningModel

Determines which spatialization algorithm will be used to position the audio in 3D space. See the constants for the available choices. The default is HRTF.

distanceModel

Determines which algorithm will be used to reduce the volume of an audio source as it moves away from the listener.

refDistance

A reference distance for reducing volume as source move further from the listener.

maxDistance

The maximum distance between source and listener, after which the volume will not be reduced any further.

rolloffFactor

Describes how quickly the volume is reduced as source moves away from listener.

coneInnerAngle

A parameter for directional audio sources, this is an angle, inside of which there will be no volume reduction.

coneOuterAngle

A parameter for directional audio sources, this is an angle, outside of which the volume will be reduced to a constant value of coneOuterGain.

coneOuterGain

A parameter for directional audio sources, this is the amount of volume reduction outside of the coneOuterAngle.

4.15. The AudioListener Interface

This interface represents the position and orientation of the person listening to the audio scene. All AudioPannerNode objects spatialize in relation to the AudioContext's listener. See this section for more details about spatialization.

 

    interface AudioListener {

        // same as OpenAL (default 1)          
        attribute float dopplerFactor;

        // in meters / second (default 343.3)          
        attribute float speedOfSound;

        // Uses a 3D cartesian coordinate system 
        void setPosition(in float x, in float y, in float z);
        void setOrientation(in float x, in float y, in float z, in float xUp, in float yUp, in float zUp);
        void setVelocity(in float x, in float y, in float z);

    };
    

4.15.1. Attributes

dopplerFactor

A constant used to determine the amount of pitch shift to use when rendering a doppler effect.

speedOfSound

The speed of sound used for calculating doppler shift. The default value is 343.3 meters / second.

4.16. The ConvolverNode Interface

This interface represents a processing node which applies a linear convolution effect given an impulse response. Normative requirements for multi-channel convolution matrixing are described here.

    numberOfInputs  : 1
    numberOfOutputs : 1
    

 

    interface ConvolverNode : AudioNode {

        attribute AudioBuffer buffer;
        attribute boolean normalize;

    };
    

4.16.1. Attributes

buffer

A mono, stereo, or 4-channel AudioBuffer containing the (possibly multi-channel) impulse response used by the ConvolverNode. At the time when this attribute is set, the buffer and the state of the normalize attribute will be used to configure the ConvolverNode with this impulse response having the given normalization.

normalize

Controls whether the impulse response from the buffer will be scaled by an equal-power normalization when the buffer atttribute is set. Its default value is true in order to achieve a more uniform output level from the convolver when loaded with diverse impulse responses. If normalize is set to false, then the convolution will be rendered with no pre-processing/scaling of the impulse response. Changes to this value do not take effect until the next time the buffer attribute is set.

If the normalize attribute is false when the buffer attribute is set then the ConvolverNode will perform a linear convolution given the exact impulse response contained within the buffer.

Otherwise, if the normalize attribute is true when the buffer attribute is set then the ConvolverNode will first perform a scaled RMS-power analysis of the audio data contained within buffer to calculate a normalizationScale given this algorithm:

 

float calculateNormalizationScale(buffer)
{
    const float GainCalibration = 0.00125;
    const float GainCalibrationSampleRate = 44100;
    const float MinPower = 0.000125;
  
    // Normalize by RMS power.
    size_t numberOfChannels = buffer->numberOfChannels();
    size_t length = buffer->length();

    float power = 0;

    for (size_t i = 0; i < numberOfChannels; ++i) {
        float* sourceP = buffer->channel(i)->data();
        float channelPower = 0;

        int n = length;
        while (n--) {
            float sample = *sourceP++;
            channelPower += sample * sample;
        }

        power += channelPower;
    }

    power = sqrt(power / (numberOfChannels * length));

    // Protect against accidental overload.
    if (isinf(power) || isnan(power) || power < MinPower)
        power = MinPower;

    float scale = 1 / power;

    // Calibrate to make perceived volume same as unprocessed.
    scale *= GainCalibration;

    // Scale depends on sample-rate.
    if (buffer->sampleRate())
        scale *= GainCalibrationSampleRate / buffer->sampleRate();

    // True-stereo compensation.
    if (buffer->numberOfChannels() == 4)
        scale *= 0.5;

    return scale;
}
          

During processing, the ConvolverNode will then take this calculated normalizationScale value and multiply it by the result of the linear convolution resulting from processing the input with the impulse response (represented by the buffer) to produce the final output. Or any mathematically equivalent operation may be used, such as pre-multiplying the input by normalizationScale, or pre-multiplying a version of the impulse-response by normalizationScale.

4.17. The RealtimeAnalyserNode Interface

This interface represents a node which is able to provide real-time frequency and time-domain analysis information. The audio stream will be passed un-processed from input to output.

    numberOfInputs  : 1
    numberOfOutputs : 1    Note that this output may be left unconnected.
    

 

    interface RealtimeAnalyserNode : AudioNode {

        // Real-time frequency-domain data         
        void getFloatFrequencyData(in Float32Array array);
        void getByteFrequencyData(in Uint8Array array);

        // Real-time waveform data         
        void getByteTimeDomainData(in Uint8Array array);

        attribute unsigned long fftSize;
        readonly attribute unsigned long frequencyBinCount;

        attribute float minDecibels;
        attribute float maxDecibels;

        attribute float smoothingTimeConstant;

    };
    

4.17.1. Attributes

fftSize

The size of the FFT used for frequency-domain analysis. This must be a power of two.

frequencyBinCount

Half the FFT size.

minDecibels

The minimum power value in the scaling range for the FFT analysis data for conversion to unsigned byte values.

maxDecibels

The maximum power value in the scaling range for the FFT analysis data for conversion to unsigned byte values.

smoothingTimeConstant

A value from 0 -> 1 where 0 represents no time averaging with the last analysis frame.

4.18. The AudioChannelSplitter Interface

The AudioChannelSplitter is for use in more advanced applications and would often be used in conjunction with AudioChannelMerger.

    numberOfInputs  : 1
    numberOfOutputs : Variable N (defaults to 6) // number of "active" (non-silent) outputs is determined by number of channels in the input
    

This interface represents an AudioNode for accessing the individual channels of an audio stream in the routing graph. It has a single input, and a number of "active" outputs which equals the number of channels in the input audio stream. For example, if a stereo input is connected to an AudioChannelSplitter then the number of active outputs will be two (one from the left channel and one from the right). There are always a total number of N outputs (determined by the numberOfOutputs parameter to the AudioContext method createChannelSplitter()), The default number is 6 if this value is not provided. Any outputs which are not "active" will output silence and would typically not be connected to anything.

Example:

Please note that in this example, the splitter does not interpret the channel identities (such as left, right, etc.), but simply splits out channels in the order that they are input.

One application for AudioChannelSplitter is for doing "matrix mixing" where individual gain control of each channel is desired.

 

    interface AudioChannelSplitter : AudioNode {

    };
    

4.19. The AudioChannelMerger Interface

The AudioChannelMerger is for use in more advanced applications and would often be used in conjunction with AudioChannelSplitter.

    numberOfInputs  : Variable N (default to 6)  // number of connected inputs may be less than this
    numberOfOutputs : 1
    

This interface represents an AudioNode for combining channels from multiple audio streams into a single audio stream. It has a variable number of inputs (defaulting to 6), but not all of them need be connected. There is a single output whose audio stream has a number of channels equal to the sum of the numbers of channels of all the connected inputs. For example, if an AudioChannelMerger has two connected inputs (both stereo), then the output will be four channels, the first two from the first input and the second two from the second input. In another example with two connected inputs (both mono), the output will be two channels (stereo), with the left channel coming from the first input and the right channel coming from the second input.

Example:

Please note that in this example, the merger does not interpret the channel identities (such as left, right, etc.), but simply combines channels in the order that they are input.

Be aware that it is possible to connect an AudioChannelMerger in such a way that it outputs an audio stream with a large number of channels greater than the maximum supported by the audio hardware. In this case where such an output is connected to the AudioContext .destination (the audio hardware), then the extra channels will be ignored. Thus, the AudioChannelMerger should be used in situations where the number of channels is well understood.

 

    interface AudioChannelMerger : AudioNode {

    };
    

4.20. The DynamicsCompressorNode Interface

DynamicsCompressorNode is an AudioNode processor implementing a dynamics compression effect.

Dynamics compression is very commonly used in musical production and game audio. It lowers the volume of the loudest parts of the signal and raises the volume of the softest parts. Overall, a louder, richer, and fuller sound can be achieved. It is especially important in games and musical applications where large numbers of individual sounds are played simultaneous to control the overall signal level and help avoid clipping (distorting) the audio output to the speakers.

    numberOfInputs  : 1
    numberOfOutputs : 1
    

    			 

    interface DynamicsCompressorNode : AudioNode {

        readonly attribute AudioParam threshold; // in Decibels
        readonly attribute AudioParam knee; // in Decibels
        readonly attribute AudioParam ratio; // unit-less
        readonly attribute AudioParam reduction; // in Decibels
        readonly attribute AudioParam attack; // in Seconds
        readonly attribute AudioParam release; // in Seconds

    }
        
      

4.21. The BiquadFilterNode Interface

BiquadFilterNode is an AudioNode processor implementing very common low-order filters.

Low-order filters are the building blocks of basic tone controls (bass, mid, treble), graphic equalizers, and more advanced filters. Multiple BiquadFilterNode filters can be combined to form more complex filters. The filter parameters such as "frequency" can be changed over time for filter sweeps, etc. Each BiquadFilterNode can be configured as one of a number of common filter types as shown in the IDL below. The default filter type is LOWPASS

    numberOfInputs  : 1
    numberOfOutputs : 1
    

 

    interface BiquadFilterNode : AudioNode {

        // Filter type.
        const unsigned short LOWPASS = 0;
        const unsigned short HIGHPASS = 1;
        const unsigned short BANDPASS = 2;
        const unsigned short LOWSHELF = 3;
        const unsigned short HIGHSHELF = 4;
        const unsigned short PEAKING = 5;
        const unsigned short NOTCH = 6;
        const unsigned short ALLPASS = 7;

        attribute unsigned short type;
        readonly attribute AudioParam frequency; // in Hertz
        readonly attribute AudioParam Q; // Quality factor
        readonly attribute AudioParam gain; // in Decibels

        void getFrequencyResponse(in Float32Array frequencyHz,
                                  in Float32Array magResponse,
                                  in Float32Array phaseResponse);

    }
    

4.21.1 LOWPASS

A lowpass filter allows frequencies below the cutoff frequency to pass through and attenuates frequencies above the cutoff. LOWPASS implements a standard second-order resonant lowpass filter with 12dB/octave rolloff.

frequency

The cutoff frequency above which the frequencies are attenuated

Q

Controls how peaked the response will be at the cutoff frequency. A large value makes the response more peaked.

gain

Not used in this filter type

4.21.2 HIGHPASS

A highpass filter is the opposite of a lowpass filter. Frequencies above the cutoff frequency are passed through, but frequencies below the cutoff are attenuated. HIGHPASS implements a standard second-order resonant highpass filter with 12dB/octave rolloff.

frequency

The cutoff frequency below which the frequencies are attenuated

Q

Controls how peaked the response will be at the cutoff frequency. A large value makes the response more peaked.

gain

Not used in this filter type

4.21.3 BANDPASS

A bandpass filter allows a range of frequencies to pass through and attenuates the frequencies below and above this frequency range. BANDPASS implements a second-order bandpass filter.

frequency

The center of the frequency band

Q

Controls the width of the band. The width becomes narrower as the Q value increases.

gain

Not used in this filter type

4.21.4 LOWSHELF

The lowshelf filter allows all frequencies through, but adds a boost (or attenuation) to the lower frequencies. LOWSHELF implements a second-order lowshelf filter.

frequency

The upper limit of the frequences where the boost (or attenuation) is applied.

Q

Not used in this filter type.

gain

The boost, in dB, to be applied. If the value is negative, the frequencies are attenuated.

4.21.5 HIGHSHELF

The highshelf filter is the opposite of the lowshelf filter and allows all frequencies through, but adds a boost to the higher frequencies. HIGHSHELF implements a second-order highshelf filter

frequency

The lower limit of the frequences where the boost (or attenuation) is applied.

Q

Not used in this filter type.

gain

The boost, in dB, to be applied. If the value is negative, the frequencies are attenuated.

4.21.6 PEAKING

The peaking filter allows all frequencies through, but adds a boost (or attenuation) to a range of frequencies.

frequency

The center frequency of where the boost is applied.

Q

Controls the width of the band of frequencies that are boosted. A large value implies a narrow width.

gain

The boost, in dB, to be applied. If the value is negative, the frequencies are attenuated.

4.21.7 NOTCH

The notch filter (also known as a band-stop or band-rejection filter) is the opposite of a bandpass filter. It allows all frequencies through, except for a set of frequencies.

frequency

The center frequency of where the notch is applied.

Q

Controls the width of the band of frequencies that are attenuated. A large value implies a narrow width.

gain

Not used in this filter type.

4.21.8 ALLPASS

An allpass filter allows all frequencies through, but changes the phase relationship between the various frequencies. ALLPASS implements a second-order allpass filter

frequency

The frequency where the center of the phase transition occurs. Viewed another way, this is the frequency with maximal group delay.

Q

Controls how sharp the phase transition is at the center frequency. A larger value implies a sharper transition and a larger group delay.

gain

Not used in this filter type.

4.21.9. Methods

The getFrequencyResponse method

Given the current filter parameter settings, calculates the frequency response for the specified frequencies.

The frequencyHz parameter specifies an array of frequencies at which the response values will be calculated.

The magResponse parameter specifies an output array receiving the linear magnitude response values.

The phaseResponse parameter specifies an output array receiving the phase response values in radians.

4.22. The WaveShaperNode Interface

WaveShaperNode is an AudioNode processor implementing non-linear distortion effects.

Non-linear waveshaping distortion is commonly used for both subtle non-linear warming, or more obvious distortion effects. Arbitrary non-linear shaping curves may be specified.

    numberOfInputs  : 1
    numberOfOutputs : 1
    

 

    interface WaveShaperNode : AudioNode {

        attribute Float32Array curve;

    }
    

4.23. The Oscillator Interface

Oscillator represents an audio source generating a periodic waveform. It can be set to a few commonly used waveforms. Additionally, it can be set to an arbitrary periodic waveform through the use of a WaveTable object.

Oscillators are common foundational building blocks in audio synthesis. An Oscillator will start emitting sound at the time specified by the noteOn() method.

Mathematically speaking, a continuous-time periodic waveform can have very high (or infinitely high) frequency information when considered in the frequency domain. When this waveform is sampled as a discrete-time digital audio signal at a particular sample-rate, then care must be taken to discard (filter out) the high-frequency information higher than the Nyquist frequency (half the sample-rate) before converting the waveform to a digital form. If this is not done, then aliasing of higher frequencies (than the Nyquist frequency) will fold back as mirror images into frequencies lower than the Nyquist frequency. In many cases this will cause audibly objectionable artifacts. This is a basic and well understood principle of audio DSP.

There are several practical approaches that an implementation may take to avoid this aliasing. But regardless of approach, the idealized discrete-time digital audio signal is well defined mathematically. The trade-off for the implementation is a matter of implementation cost (in terms of CPU usage) versus fidelity to achieving this ideal.

It is expected that an implementation will take some care in achieving this ideal, but it is reasonable to consider lower-quality, less-costly approaches on lower-end hardware.

    numberOfInputs  : 0
    numberOfOutputs : 1
    

 

    interface Oscillator : AudioSourceNode {

        // Type constants.
        const unsigned short SINE = 0;
        const unsigned short SQUARE = 1;
        const unsigned short SAWTOOTH = 2;
        const unsigned short TRIANGLE = 3;
        const unsigned short CUSTOM = 4;

        attribute unsigned short type;
        
        const unsigned short UNSCHEDULED_STATE = 0;
        const unsigned short SCHEDULED_STATE = 1;
        const unsigned short PLAYING_STATE = 2;
        const unsigned short FINISHED_STATE = 3;

        readonly attribute unsigned short playbackState;

        readonly attribute AudioParam frequency; // in Hertz
        readonly attribute AudioParam detune; // in Cents

        void noteOn(in double when);
        void noteOff(in double when);
        void setWaveTable(in WaveTable waveTable);

    }
    

4.23.2. Methods and Parameters

The setWaveTable method

Sets an arbitrary custom periodic waveform given a WaveTable.

The noteOn method

defined as in AudioBufferSourceNode.

The noteOff method

defined as in AudioBufferSourceNode.

4.24. The WaveTable Interface

WaveTable represents an arbitrary periodic waveform to be used with an Oscillator. Please see createWaveTable() and setWaveTable() and for more details.

 

    interface WaveTable {

    }
    

4.25. The MediaStreamAudioSourceNode Interface

This interface represents an audio source from a MediaStream. The first AudioMediaStreamTrack from the MediaStream will be used as a source of audio.

    numberOfInputs  : 0
    numberOfOutputs : 1
    

 

    interface MediaStreamAudioSourceNode : AudioSourceNode {

    }
    

5. Integration with the audio and video elements

A MediaElementAudioSourceNode can be created from an HTMLMediaElement using an AudioContext method.

 
var mediaElement = document.getElementById('mediaElementID');
var sourceNode = context.createMediaElementSource(mediaElement);
sourceNode.connect(filterNode);

6. Mixer Gain Structure

Background

One of the most important considerations when dealing with audio processing graphs is how to adjust the gain (volume) at various points. For example, in a standard mixing board model, each input bus has pre-gain, post-gain, and send-gains. Submix and master out busses also have gain control. The gain control described here can be used to implement standard mixing boards as well as other architectures.

The inputs to AudioNodes have the ability to accept connections from multiple outputs. The input then acts as a unity gain summing junction with each output signal being added with the others:

In cases where the channel layouts of the outputs do not match, an up-mix will occur to the highest number of channels.

Gain Control

But many times, it's important to be able to control the gain for each of the output signals. The AudioGainNode gives this control:

Using these two concepts of unity gain summing junctions and AudioGainNodes, it's possible to construct simple or complex mixing scenarios.

Example: Mixer with Send Busses

In a routing scenario involving multiple sends and submixes, explicit control is needed over the volume or "gain" of each connection to a mixer. Such routing topologies are very common and exist in even the simplest of electronic gear sitting around in a basic recording studio.

Here's an example with two send mixers and a main mixer. Although possible, for simplicity's sake, pre-gain control and insert effects are not illustrated:

This diagram is using a shorthand notation where "send 1", "send 2", and "main bus" are actually inputs to AudioNodes, but here are represented as summing busses, where the intersections g2_1, g3_1, etc. represent the "gain" or volume for the given source on the given mixer. In order to expose this gain, an AudioGainNode is used:

Here's how the above diagram could be constructed in JavaScript:

 

var context = 0;
var compressor = 0;
var reverb = 0;
var delay = 0;
var s1 = 0;
var s2 = 0;

var source1 = 0;
var source2 = 0;
var g1_1 = 0;
var g2_1 = 0;
var g3_1 = 0;
var g1_2 = 0;
var g2_2 = 0;
var g3_2 = 0;

// Setup routing graph 
function setupRoutingGraph() {
    context = new AudioContext();

    compressor = context.createDynamicsCompressor();

    // Send1 effect 
    reverb = context.createConvolver();
    // Convolver impulse response may be set here or later 

    // Send2 effect 
    delay = context.createDelayNode();

    // Connect final compressor to final destination 
    compressor.connect(context.destination);

    // Connect sends 1 & 2 through effects to main mixer 
    s1 = context.createGainNode();
    reverb.connect(s1);
    s1.connect(compressor);
    
    s2 = context.createGainNode();
    delay.connect(s2);
    s2.connect(compressor);

    // Create a couple of sources 
    source1 = context.createBufferSource();
    source2 = context.createBufferSource();
    source1.buffer = manTalkingBuffer;
    source2.buffer = footstepsBuffer;

    // Connect source1 
    g1_1 = context.createGainNode();
    g2_1 = context.createGainNode();
    g3_1 = context.createGainNode();
    source1.connect(g1_1);
    source1.connect(g2_1);
    source1.connect(g3_1);
    g1_1.connect(compressor);
    g2_1.connect(reverb);
    g3_1.connect(delay);

    // Connect source2 
    g1_2 = context.createGainNode();
    g2_2 = context.createGainNode();
    g3_2 = context.createGainNode();
    source2.connect(g1_2);
    source2.connect(g2_2);
    source2.connect(g3_2);
    g1_2.connect(compressor);
    g2_2.connect(reverb);
    g3_2.connect(delay);

    // We now have explicit control over all the volumes g1_1, g2_1, ..., s1, s2 
    g2_1.gain.value = 0.2;  // For example, set source1 reverb gain 

     // Because g2_1.gain is of type "AudioGain" which is an "AudioParam", 
     // an automation curve could also be attached to it. 
     // A "mixing board" UI could be created in canvas or WebGL controlling these gains. 
}

 

8. Channel Layouts

It's important to define the channel ordering (and define some abbreviations) for different layouts.

The channel layouts are clear:

  Mono
    0: M: mono
    
  Stereo
    0: L: left
    1: R: right
  

A more advanced implementation can handle channel layouts for quad and 5.1:

  Quad
    0: L:  left
    1: R:  right
    2: SL: surround left
    3: SR: surround right

  5.1
    0: L:   left
    1: R:   right
    2: C:   center
    3: LFE: subwoofer
    4: SL:  surround left
    5: SR:  surround right
  

Other layouts can also be considered.

9. Channel up-mixing and down-mixing

Up Mixing

Consider what happens when converting an audio stream with a lower number of channels to one with a higher number of channels. This can be necessary when mixing several outputs together where the channel layouts differ. It can also be necessary if the rendered audio stream is played back on a system with more channels.

Mono up-mix:
    
    1 -> 2 : up-mix from mono to stereo
        output.L = input;
        output.R = input;

    1 -> 4 : up-mix from mono to quad
        output.L = input;
        output.R = input;
        output.SL = 0;
        output.SR = 0;

    1 -> 5.1 : up-mix from mono to 5.1
        output.L = 0;
        output.R = 0;
        output.C = input; // put in center channel
        output.LFE = 0;
        output.SL = 0;
        output.SR = 0;

Stereo up-mix:

    2 -> 4 : up-mix from stereo to quad
        output.L = input.L;
        output.R = input.R;
        output.SL = 0;
        output.SR = 0;

    2 -> 5.1 : up-mix from stereo to 5.1
        output.L = input.L;
        output.R = input.R;
        output.C = 0;
        output.LFE = 0;
        output.SL = 0;
        output.SR = 0;

Quad up-mix:

    4 -> 5.1 : up-mix from stereo to 5.1
        output.L = input.L;
        output.R = input.R;
        output.C = 0;
        output.LFE = 0;
        output.SL = input.SL;
        output.SR = input.SR;

Down Mixing

A down-mix will be necessary, for example, if processing 5.1 source material, but playing back stereo.

  
Mono down-mix:

    2 -> 1 : stereo to mono
        output = 0.5 * (input.L + input.R);

    4 -> 1 : quad to mono
        output = 0.25 * (input.L + input.R + input.SL + input.SR);

    5.1 -> 1 : 5.1 to mono
        ???


Stereo down-mix:

    4 -> 2 : quad to stereo
        output.L = 0.5 * (input.L + input.SL);
        output.R = 0.5 * (input.R + input.SR);

    5.1 -> 2 : 5.1 to stereo
        ???

10. Event Scheduling

• Audio events such as start/stop play and volume fades can be scheduled to happen in a rhythmically perfect way (sample-accurate scheduling)
• Allows sequencing applications such as drum-machines, digital-dj mixers. Ultimately, it may be useful for DAW applications.
• Allows rhythmically accurate segueways from one section of music to another (as is possible with the FMOD engine).
• Allows scheduling of sound "grains" for granular synthesis effects.

11. Spatialization / Panning

Background

A common feature requirement for modern 3D games is the ability to dynamically spatialize and move multiple audio sources in 3D space. Game audio engines such as OpenAL, FMOD, Creative's EAX, Microsoft's XACT Audio, etc. have this ability.

Using an AudioPannerNode, an audio stream can be spatialized or positioned in space relative to an AudioListener. An AudioContext will contain a single AudioListener. Both panners and listeners have a position in 3D space using a right-handed cartesian coordinate system. AudioPannerNode objects (representing the source stream) have an orientation vector representing in which direction the sound is projecting. Additionally, they have a sound cone representing how directional the sound is. For example, the sound could be omnidirectional, in which case it would be heard anywhere regardless of its orientation, or it can be more directional and heard only if it is facing the listener. AudioListener objects (representing a person's ears) have an orientation and up vector representing in which direction the person is facing. Because both the source stream and the listener can be moving, they both have a velocity vector representing both the speed and direction of movement. Taken together, these two velocities can be used to generate a doppler shift effect which changes the pitch.

During rendering, the AudioPannerNode calculates an azimuth and elevation. These values are used internally by the implementation in order to render the spatialization effect. See the Panning Algorithm section for details of how these values are used.

The following algorithm must be used to calculate the azimuth and elevation:

 
// Calculate the source-listener vector.
vec3 sourceListener = source.position - listener.position;

if (sourceListener.isZero()) {
    // Handle degenerate case if source and listener are at the same point.
    azimuth = 0;
    elevation = 0;
    return;
}

sourceListener.normalize();

// Align axes.
vec3 listenerFront = listener.orientation;
vec3 listenerUp = listener.up;
vec3 listenerRight = listenerFront.cross(listenerUp);
listenerRight.normalize();

vec3 listenerFrontNorm = listenerFront;
listenerFrontNorm.normalize();

vec3 up = listenerRight.cross(listenerFrontNorm);

float upProjection = sourceListener.dot(up);

vec3 projectedSource = sourceListener - upProjection * up;
projectedSource.normalize();

azimuth = 180 * acos(projectedSource.dot(listenerRight)) / PI;

// Source in front or behind the listener.
double frontBack = projectedSource.dot(listenerFrontNorm);
if (frontBack < 0)
    azimuth = 360 - azimuth;

// Make azimuth relative to "front" and not "right" listener vector.
if ((azimuth >= 0) && (azimuth <= 270))
    azimuth = 90 - azimuth;
else
    azimuth = 450 - azimuth;

elevation = 90 - 180 * acos(sourceListener.dot(up)) / PI;

if (elevation > 90)
    elevation = 180 - elevation;
else if (elevation < -90)
    elevation = -180 - elevation;

Panning Algorithm

mono->stereo and stereo->stereo panning must be supported. mono->stereo processing is used when all connections to the input are mono. Otherwise stereo->stereo processing is used.

The following algorithms must be implemented:

• Equal-power (Vector-based) panning

  This is a simple and relatively inexpensive algorithm which provides basic, but reasonable results. It is commonly used when panning musical sources.

  The elevation value is ignored in this panning algorithm.

  The following steps are used for processing:

  1. The azimuth value is first contained to be within the range -90 <= azimuth <= +90 according to:

         // Clamp azimuth to allowed range of -180 -> +180.
         azimuth = max(-180, azimuth);
         azimuth = min(180, azimuth);
     
         // Now wrap to range -90 -> +90.
         if (azimuth < -90)
             azimuth = -180 - azimuth;
         else if (azimuth > 90)
             azimuth = 180 - azimuth;
         

  2. A 0 -> 1 normalized value x is calculated from azimuth for mono->stereo as:

         x = (azimuth + 90) / 180    
         

     Or for stereo->stereo as:

         if (azimuth <= 0) { // from -90 -> 0
             // inputL -> outputL and "equal-power pan" inputR as in mono case
             // by transforming the "azimuth" value from -90 -> 0 degrees into the range -90 -> +90.
             x = (azimuth + 90) / 90;
         } else { // from 0 -> +90
             // inputR -> outputR and "equal-power pan" inputL as in mono case
             // by transforming the "azimuth" value from 0 -> +90 degrees into the range -90 -> +90.
             x = azimuth / 90;
         }
         

  3. Left and right gain values are then calculated:

         gainL = cos(0.5 * PI * x);
         gainR = sin(0.5 * PI * x);    
         

  4. For mono->stereo, the output is calculated as:

         outputL = input * gainL
         outputR = input * gainR
         

     Else for stereo->stereo, the output is calculated as:

         if (azimuth <= 0) { // from -90 -> 0
             outputL = inputL + inputR * gainL;
             outputR = inputR * gainR;
         } else { // from 0 -> +90
             outputL = inputL * gainL;
             outputR = inputR + inputL * gainR;
         }
         

• HRTF panning (stereo only)

  This requires a set of HRTF impulse responses recorded at a variety of azimuths and elevations. There are a small number of open/free impulse responses available. The implementation requires a highly optimized convolution function. It is somewhat more costly than "equal-power", but provides a more spatialized sound.

  

Distance Effects

Sounds which are closer are louder, while sounds further away are quieter. Exactly how a sound's volume changes according to distance from the listener depends on the distanceModel attribute.

During audio rendering, a distance value will be calculated based on the panner and listener positions according to:

v = panner.position - listener.position

distance = sqrt(dot(v, v))

distance will then be used to calculate distanceGain which depends on the distanceModel attribute. See the Constants section for details of how this is calculated for each distance model.

As part of its processing, the AudioPannerNode scales/multiplies the input audio signal by distanceGain to make distant sounds quieter and nearer ones louder.

Sound Cones

The listener and each sound source have an orientation vector describing which way they are facing. Each sound source's sound projection characteristics are described by an inner and outer "cone" describing the sound intensity as a function of the source/listener angle from the source's orientation vector. Thus, a sound source pointing directly at the listener will be louder than if it is pointed off-axis. Sound sources can also be omni-directional.

Doppler Shift

• Introduces a pitch shift which can realistically simulate moving sources.
• Depends on: source / listener velocity vectors, speed of sound, doppler factor.

12. Linear Effects using Convolution

Background

Convolution is a mathematical process which can be applied to an audio signal to achieve many interesting high-quality linear effects. Very often, the effect is used to simulate an acoustic space such as a concert hall, cathedral, or outdoor amphitheater. It can also be used for complex filter effects, like a muffled sound coming from inside a closet, sound underwater, sound coming through a telephone, or playing through a vintage speaker cabinet. This technique is very commonly used in major motion picture and music production and is considered to be extremely versatile and of high quality.

Each unique effect is defined by an impulse response. An impulse response can be represented as an audio file and can be recorded from a real acoustic space such as a cave, or can be synthetically generated through a great variety of techniques.

Motivation for use as a Standard

A key feature of many game audio engines (OpenAL, FMOD, Creative's EAX, Microsoft's XACT Audio, etc.) is a reverberation effect for simulating the sound of being in an acoustic space. But the code used to generate the effect has generally been custom and algorithmic (generally using a hand-tweaked set of delay lines and allpass filters which feedback into each other). In nearly all cases, not only is the implementation custom, but the code is proprietary and closed-source, each company adding its own "black magic" to achieve its unique quality. Each implementation being custom with a different set of parameters makes it impossible to achieve a uniform desired effect. And the code being proprietary makes it impossible to adopt a single one of the implementations as a standard. Additionally, algorithmic reverberation effects are limited to a relatively narrow range of different effects, regardless of how the parameters are tweaked.

A convolution effect solves these problems by using a very precisely defined mathematical algorithm as the basis of its processing. An impulse response represents an exact sound effect to be applied to an audio stream and is easily represented by an audio file which can be referenced by URL. The range of possible effects is enormous.

Implementation Guide

Linear convolution can be implemented efficiently. Here are some notes describing how it can be practically implemented.

Reverb Effect (with matrixing)

This section is normative.

In the general case the source has N input channels, the impulse response has K channels, and the playback system has M output channels. Thus it's a matter of how to matrix these channels to achieve the final result.

The subset of N, M, K below must be implemented (note that the first image in the diagram is just illustrating the general case and is not normative, while the following images are normative). Without loss of generality, developers desiring more complex and arbitrary matrixing can use multiple ConvolverNode objects in conjunction with an AudioChannelMerger.

Single channel convolution operates on a mono audio input, using a mono impulse response, and generating a mono output. But to achieve a more spacious sound, 2 channel audio inputs and 1, 2, or 4 channel impulse responses will be considered. The following diagram, illustrates the common cases for stereo playback where N and M are 1 or 2 and K is 1, 2, or 4.

Recording Impulse Responses

This section is informative.

The most modern and accurate way to record the impulse response of a real acoustic space is to use a long exponential sine sweep. The test-tone can be as long as 20 or 30 seconds, or longer.
Several recordings of the test tone played through a speaker can be made with microphones placed and oriented at various positions in the room. It's important to document speaker placement/orientation, the types of microphones, their settings, placement, and orientations for each recording taken.

Post-processing is required for each of these recordings by performing an inverse-convolution with the test tone, yielding the impulse response of the room with the corresponding microphone placement. These impulse responses are then ready to be loaded into the convolution reverb engine to re-create the sound of being in the room.

Tools

Two command-line tools have been written:
generate_testtones generates an exponential sine-sweep test-tone and its inverse. Another tool convolve was written for post-processing. With these tools, anybody with recording equipment can record their own impulse responses. To test the tools in practice, several recordings were made in a warehouse space with interesting acoustics. These were later post-processed with the command-line tools.

% generate_testtones -h
Usage: generate_testtone
	[-o /Path/To/File/To/Create] Two files will be created: .tone and .inverse
	[-rate <sample rate>] sample rate of the generated test tones
	[-duration <duration>] The duration, in seconds, of the generated files
	[-min_freq <min_freq>] The minimum frequency, in hertz, for the sine sweep

% convolve -h
Usage: convolve input_file impulse_response_file output_file

Recording Setup

The Warehouse Space

13. JavaScript Synthesis and Processing

This section is informative.

The Mozilla project has conducted Experiments to synthesize and process audio directly in JavaScript. This approach is interesting for a certain class of audio processing and they have produced a number of impressive demos. This specification includes a means of synthesizing and processing directly using JavaScript by using a special subtype of AudioNode called JavaScriptAudioNode.

Here are some interesting examples where direct JavaScript processing can be useful:

Custom DSP Effects

Unusual and interesting custom audio processing can be done directly in JS. It's also a good test-bed for prototyping new algorithms. This is an extremely rich area.

Educational Applications

JS processing is ideal for illustrating concepts in computer music synthesis and processing, such as showing the de-composition of a square wave into its harmonic components, FM synthesis techniques, etc.

JavaScript Performance

JavaScript has a variety of performance issues so it is not suitable for all types of audio processing. The approach proposed in this document includes the ability to perform computationally intensive aspects of the audio processing (too expensive for JavaScript to compute in real-time) such as multi-source 3D spatialization and convolution in optimized C++ code. Both direct JavaScript processing and C++ optimized code can be combined due to the APIs modular approach.

15. Performance Considerations

15.1. Latency: What it is and Why it's Important

For web applications, the time delay between mouse and keyboard events (keydown, mousedown, etc.) and a sound being heard is important.

This time delay is called latency and is caused by several factors (input device latency, internal buffering latency, DSP processing latency, output device latency, distance of user's ears from speakers, etc.), and is cummulative. The larger this latency is, the less satisfying the user's experience is going to be. In the extreme, it can make musical production or game-play impossible. At moderate levels it can affect timing and give the impression of sounds lagging behind or the game being non-responsive. For musical applications the timing problems affect rhythm. For gaming, the timing problems affect precision of gameplay. For interactive applications, it generally cheapens the users experience much in the same way that very low animation frame-rates do. Depending on the application, a reasonable latency can be from as low as 3-6 milliseconds to 25-50 milliseconds.

15.2. Audio Glitching

Audio glitches are caused by an interruption of the normal continuous audio stream, resulting in loud clicks and pops. It is considered to be a catastrophic failure of a multi-media system and must be avoided. It can be caused by problems with the threads responsible for delivering the audio stream to the hardware, such as scheduling latencies caused by threads not having the proper priority and time-constraints. It can also be caused by the audio DSP trying to do more work than is possible in real-time given the CPU's speed.

15.3. Hardware Scalability

The system should gracefully degrade to allow audio processing under resource constrained conditions without dropping audio frames.

First of all, it should be clear that regardless of the platform, the audio processing load should never be enough to completely lock up the machine. Second, the audio rendering needs to produce a clean, un-interrupted audio stream without audible glitches.

The system should be able to run on a range of hardware, from mobile phones and tablet devices to laptop and desktop computers. But the more limited compute resources on a phone device make it necessary to consider techniques to scale back and reduce the complexity of the audio rendering. For example, voice-dropping algorithms can be implemented to reduce the total number of notes playing at any given time.

Here's a list of some techniques which can be used to limit CPU usage:

15.3.1. CPU monitoring

In order to avoid audio breakup, CPU usage must remain below 100%.

The relative CPU usage can be dynamically measured for each AudioNode (and chains of connected nodes) as a percentage of the rendering time quantum. In a single-threaded implementation, overall CPU usage must remain below 100%. The measured usage may be used internally in the implementation for dynamic adjustments to the rendering. It may also be exposed through a cpuUsage attribute of AudioNode for use by JavaScript.

In cases where the measured CPU usage is near 100% (or whatever threshold is considered too high), then an attempt to add additional AudioNodes into the rendering graph can trigger voice-dropping.

15.3.2. Voice Dropping

Voice-dropping is a technique which limits the number of voices (notes) playing at the same time to keep CPU usage within a reasonable range. There can either be an upper threshold on the total number of voices allowed at any given time, or CPU usage can be dynamically monitored and voices dropped when CPU usage exceeds a threshold. Or a combination of these two techniques can be applied. When CPU usage is monitored for each voice, it can be measured all the way from the AudioSourceNode through any effect processing nodes which apply uniquely to that voice.

When a voice is "dropped", it needs to happen in such a way that it doesn't introduce audible clicks or pops into the rendered audio stream. One way to achieve this is to quickly fade-out the rendered audio for that voice before completely removing it from the rendering graph.

When it is determined that one or more voices must be dropped, there are various strategies for picking which voice(s) to drop out of the total ensemble of voices currently playing. Here are some of the factors which can be used in combination to help with this decision:

• Older voices, which have been playing the longest can be dropped instead of more recent voices.
• Quieter voices, which are contributing less to the overall mix may be dropped instead of louder ones.
• Voices which are consuming relatively more CPU resources may be dropped instead of less "expensive" voices.
• An AudioNode can have a priority attribute to help determine the relative importance of the voices.

15.3.3. Simplification of Effects Processing

Most of the effects described in this document are relatively inexpensive and will likely be able to run even on the slower mobile devices. However, the convolution effect can be configured with a variety of impulse responses, some of which will likely be too heavy for mobile devices. Generally speaking, CPU usage scales with the length of the impulse response and the number of channels it has. Thus, it is reasonable to consider that impulse responses which exceed a certain length will not be allowed to run. The exact limit can be determined based on the speed of the device. Instead of outright rejecting convolution with these long responses, it may be interesting to consider truncating the impulse responses to the maximum allowed length and/or reducing the number of channels of the impulse response.

In addition to the convolution effect. The AudioPannerNode may also be expensive if using the HRTF panning model. For slower devices, a cheaper algorithm such as EQUALPOWER can be used to conserve compute resources.

15.3.4. Sample Rate

For very slow devices, it may be worth considering running the rendering at a lower sample-rate than normal. For example, the sample-rate can be reduced from 44.1KHz to 22.05KHz. This decision must be made when the AudioContext is created, because changing the sample-rate on-the-fly can be difficult to implement and will result in audible glitching when the transition is made.

15.3.5. Pre-flighting

It should be possible to invoke some kind of "pre-flighting" code (through JavaScript) to roughly determine the power of the machine. The JavaScript code can then use this information to scale back any more intensive processing it may normally run on a more powerful machine. Also, the underlying implementation may be able to factor in this information in the voice-dropping algorithm.

TODO: add specification and more detail here

15.3.6. Authoring for different user agents

JavaScript code can use information about user-agent to scale back any more intensive processing it may normally run on a more powerful machine.

15.3.7. Scalability of Direct JavaScript Synthesis / Processing

Any audio DSP / processing code done directly in JavaScript should also be concerned about scalability. To the extent possible, the JavaScript code itself needs to monitor CPU usage and scale back any more ambitious processing when run on less powerful devices. If it's an "all or nothing" type of processing, then user-agent check or pre-flighting should be done to avoid generating an audio stream with audio breakup.

15.4. JavaScript Issues with real-time Processing and Synthesis:

While processing audio in JavaScript, it is extremely challenging to get reliable, glitch-free audio while achieving a reasonably low-latency, especially under heavy processor load.

• JavaScript is very much slower than heavily optimized C++ code and is not able to take advantage of SSE optimizations and multi-threading which is critical for getting good performance on today's processors. Optimized native code can be on the order of twenty times faster for processing FFTs as compared with JavaScript. It is not efficient enough for heavy-duty processing of audio such as convolution and 3D spatialization of large numbers of audio sources.
• setInterval() and XHR handling will steal time from the audio processing. In a reasonably complex game, some JavaScript resources will be needed for game physics and graphics. This creates challenges because audio rendering is deadline driven (to avoid glitches and get low enough latency).
• JavaScript does not run in a real-time processing thread and thus can be pre-empted by many other threads running on the system.
• Garbage Collection (and autorelease pools on Mac OS X) can cause unpredictable delay on a JavaScript thread.
• Multiple JavaScript contexts can be running on the main thread, stealing time from the context doing the processing.
• Other code (other than JavaScript) such as page rendering runs on the main thread.
• Locks can be taken and memory is allocated on the JavaScript thread. This can cause additional thread preemption.

The problems are even more difficult with today's generation of mobile devices which have processors with relatively poor performance and power consumption / battery-life issues.

16. Example Applications

This section is informative.

Please see the demo page for working examples.

Here are some of the types of applications a web audio system should be able to support:

Basic Sound Playback

Simple and low-latency playback of sound effects in response to simple user actions such as mouse click, roll-over, key press.

3D Environments and Games

An HTML5 version of Quake has already been created. Audio features such as 3D spatialization and convolution for room simulation could be used to great effect.

3D environments with audio are common in games made for desktop applications and game consoles. Imagine a 3D island environment with spatialized audio, seagulls flying overhead, the waves crashing against the shore, the crackling of the fire, the creaking of the bridge, and the rustling of the trees in the wind. The sounds can be positioned naturally as one moves through the scene. Even going underwater, low-pass filters can be tweaked for just the right underwater sound.

Box2D is an interesting open-source library for 2D game physics. It has various implementations, including one based on Canvas 2D. A demo has been created with dynamic sound effects for each of the object collisions, taking into account the velocities vectors and positions to spatialize the sound events, and modulate audio effect parameters such as filter cutoff.

A virtual pool game with multi-sampled sound effects has also been created.

Musical Applications

Many music composition and production applications are possible. Applications requiring tight scheduling of audio events can be implemented and can be both educational and entertaining. Drum machines, digital DJ applications, and even timeline-based digital music production software with some of the features of GarageBand can be written.

Music Visualizers

When combined with WebGL GLSL shaders, realtime analysis data can be presented in entertaining ways. These can be as advanced as any found in iTunes.

Educational Applications

A variety of educational applications can be written, illustrating concepts in music theory and computer music synthesis and processing.

Artistic Audio Exploration

There are many creative possibilites for artistic sonic environments for installation pieces.

17. Security Considerations

This section is informative.

18. Privacy Considerations

This section is informative. When giving various information on available AudioNodes, the Web Audio API potentially exposes information on characteristic features of the client (such as audio hardware sample-rate) to any page that makes use of the AudioNode interface. Additionally, timing information can be collected through the RealtimeAnalyzerNode or JavaScriptAudioNode interface. The information could subsequently be used to create a fingerprint of the client.

Currently audio input is not specified in this document, but it will involve gaining access to the client machine's audio input or microphone. This will require asking the user for permission in an appropriate way, probably via the getUserMedia() API.

19. Requirements and Use Cases

Please see Example Applications

B.Acknowledgements

Special thanks to the W3C Audio Working Group. Members of the Working Group are (at the time of writing, and by alphabetical order):
Berkovitz, Joe (public Invited expert);Cardoso, Gabriel (INRIA);Carlson, Eric (Apple, Inc.);Gregan, Matthew (Mozilla Foundation);Jägenstedt, Philip (Opera Software);Kalliokoski, Jussi (public Invited expert);Lowis, Chris (British Broadcasting Corporation);MacDonald, Alistair (W3C Invited Experts);Michel, Thierry (W3C/ERCIM);Noble, Jer (Apple, Inc.);O'Callahan, Robert(Mozilla Foundation);Paradis, Matthew (British Broadcasting Corporation);Raman, T.V. (Google, Inc.);Rogers, Chris (Google, Inc.);Schepers, Doug (W3C/MIT);Shires, Glen (Google, Inc.);Smith, Michael (W3C/Keio);Thereaux, Olivier (British Broadcasting Corporation);Wei, James (Intel Corporation);Wilson, Chris (Google, Inc.);

C. Web Audio API Change Log

date:        Tue Jun 26 15:56:31 2012 -0700
* add MediaStreamAudioSourceNode

date:        Mon Jun 18 13:26:21 2012 -0700
* minor formatting fix

date:        Mon Jun 18 13:19:34 2012 -0700
* Add details for azimuth/elevation calculation

date:        Fri Jun 15 17:35:27 2012 -0700
* Add equal-power-panning details

date:        Thu Jun 14 17:31:16 2012 -0700
* Add equations for distance models

date:        Wed Jun 13 17:40:49 2012 -0700
* Bug 17334: Add precise equations for AudioParam.setTargetValueAtTime()

date:        Fri Jun 08 17:44:26 2012 -0700
* fix small typo

date:        Fri Jun 08 16:54:04 2012 -0700
* Bug 17413: AudioBuffers' relationship to AudioContext

date:        Fri Jun 08 16:05:45 2012 -0700
* Bug 17359: Add much more detail about ConvolverNode

date:        Fri Jun 08 12:59:29 2012 -0700
* minor formatting fix

date:        Fri Jun 08 12:57:11 2012 -0700
* Bug 17335: Add much more technical detail to setValueCurveAtTime()

date:        Wed Jun 06 16:34:43 2012 -0700
*Add much more detail about parameter automation, including an example

date:        Mon Jun 04 17:25:08 2012 -0700
* ISSUE-85: Oscillator folding considerations

date:        Mon Jun 04 17:02:20 2012 -0700
* ISSUE-45: AudioGain scale underdefined

date:        Mon Jun 04 16:40:43 2012 -0700
* ISSUE-41: AudioNode as input to AudioParam underdefined

date:        Mon Jun 04 16:14:48 2012 -0700
* ISSUE-20: Relationship to currentTime

date:        Mon Jun 04 15:48:49 2012 -0700
* ISSUE-94: Dynamic Lifetime

date:        Mon Jun 04 13:59:31 2012 -0700
* ISSUE-42: add more detail about AudioParam sampling and block processing

date:        Mon Jun 04 12:28:48 2012 -0700
* fix typo - minor edits

date:        Thu May 24 18:01:20 2012 -0700
* ISSUE-69: add implementors guide for linear convolution

date:        Thu May 24 17:35:45 2012 -0700
* ISSUE-49: better define AudioBuffer audio data access

date:        Thu May 24 17:15:29 2012 -0700
* fix small typo

date:        Thu May 24 17:13:34 2012 -0700
* ISSUE-24: define circular routing behavior

date:        Thu May 24 16:35:24 2012 -0700
* ISSUE-42: specify a-rate or k-rate for each AudioParam

date:        Fri May 18 17:01:36 2012 -0700
* ISSUE-53: noteOn and noteOff interaction

date:        Fri May 18 16:33:29 2012 -0700
* ISSUE-34: Remove .name attribute from AudioParam

date:        Fri May 18 16:27:19 2012 -0700
* ISSUE-33: Add maxNumberOfChannels attribute to AudioDestinationNode

date:        Fri May 18 15:50:08 2012 -0700
* ISSUE-19: added more info about AudioBuffer - IEEE 32-bit

date:        Fri May 18 15:37:27 2012 -0700
* ISSUE-29: remove reference to webkitAudioContext

date:        Fri Apr 27 12:36:54 2012 -0700
* fix two small typos reported by James Wei

date:        Tue Apr 24 12:27:11 2012 -0700
* small cleanup to AudioChannelSplitter and AudioChannelMerger

date:        Tue Apr 17 11:35:56 2012 -0700
* small fix to createWaveTable()

date:        Tue Apr 13 2012
* Cleanup AudioNode connect() and disconnect() method descriptions.
* Add AudioNode connect() to AudioParam method.

date:        Tue Apr 13 2012
* Add Oscillator and WaveTable
* Define default values for optional arguments in createJavaScriptNode(), createChannelSplitter(), createChannelMerger()
* Define default filter type for BiquadFilterNode as LOWPASS

date:        Tue Apr 11 2012
* add AudioContext .activeSourceCount attribute
* createBuffer() methods can throw exceptions
* add AudioContext method createMediaElementSource()
* update AudioContext methods createJavaScriptNode() (clean up description of parameters)
* update AudioContext method createChannelSplitter() (add numberOfOutputs parameter)
* update AudioContext method createChannelMerger() (add numberOfInputs parameter)
* update description of out-of-bounds AudioParam values (exception will not be thrown)
* remove AudioBuffer .gain attribute
* remove AudioBufferSourceNode .gain attribute
* remove AudioListener .gain attribute
* add AudioBufferSourceNode .playbackState attribute and state constants
* RealtimeAnalyserNode no longer requires its output be connected to anything
* update AudioChannelMerger section describing numberOfOutputs (defaults to 6 but settable in constructor)
* update AudioChannelSplitter section describing numberOfInputs (defaults to 6 but settable in constructor)
* add note in Spatialization sections about potential to get arbitrary convolution matrixing

date:        Tue Apr 10 2012
* Rebased editor's draft document based on edits from Thierry Michel (from 2nd public working draft).

date:        Tue Mar 13 12:13:41 2012 -0100
*  fixed all the HTML errors
*  added ids to all Headings
*  added alt attribute to all img
*  fix broken anchors
*  added a new status of this document section
*  added mandatory spec headers
*  generated a new table of content
*  added a Reference section
*  added an Acknowledgments section
*  added a Web Audio API Change Log 

date:        Fri Mar 09 15:12:42 2012 -0800
* add optional maxDelayTime argument to createDelayNode()
* add more detail about playback state to AudioBufferSourceNode
* upgrade noteOn(), noteGrainOn(), noteOff() times to double from float

date:        Mon Feb 06 16:52:39 2012 -0800
* Cleanup JavaScriptAudioNode section
* Add distance model constants for AudioPannerNode according to the OpenAL spec
* Add .normalize attribute to ConvolverNode
* Add getFrequencyResponse() method to BiquadFilterNode
* Tighten up the up-mix equations

date:        Fri Nov 04 15:40:58 2011 -0700
summary:     Add more technical detail to BiquadFilterNode description (contributed by Raymond Toy)

date:        Sat Oct 15 19:08:15 2011 -0700
summary:     small edits to the introduction

date:        Sat Oct 15 19:00:15 2011 -0700
summary:     initial commit

date:        Tue Sep 13 12:49:11 2011 -0700
summary:     add convolution reverb design document

date:        Mon Aug 29 17:05:58 2011 -0700
summary:     document the decodeAudioData() method

date:        Mon Aug 22 14:36:33 2011 -0700
summary:     fix broken MediaElementAudioSourceNode link

date:        Mon Aug 22 14:33:57 2011 -0700
summary:     refine section describing integration with HTMLMediaElement

date:        Mon Aug 01 12:05:53 2011 -0700
summary:     add Privacy section

date:        Mon Jul 18 17:53:50 2011 -0700
summary:     small update - tweak musical applications thumbnail images

date:        Mon Jul 18 17:23:00 2011 -0700
summary:     initial commit of Web Audio API specification