MediaStream Processing API

W3C Working Group Note 31 May 2012

This version:

http://www.w3.org/TR/2012/NOTE-streamproc-20120531/

Latest published version:

http://www.w3.org/TR/streamproc/

Latest editor's draft:

https://dvcs.w3.org/hg/audio/raw-file/tip/streams/StreamProcessing.html

Previous version:

http://www.w3.org/TR/2011/WD-streamproc-20111215/

Public Comments:

public-audio@w3.org

Working Group:

Audio WG

Editor:

Robert O'Callahan, Mozilla Corporation <robert@ocallahan.org>

Copyright © 2012 W3C^® (MIT, ERCIM, Keio), All Rights Reserved. W3C liability, trademark and document use rules apply.

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Table of Contents

1. 1. Introduction
   1. 1.1. Scenarios
2. 2. MediaStreams
   1. 2.1. The Semantics Of MediaStreams
   2. 2.2. Media Formats
   3. 2.3. MediaStream Extensions
3. 3. Media Elements
   1. 3.1. Media Element Extensions
   2. 3.2. Audio Element Extensions
4. 4. Stream Mixing And Processing
   1. 4.1. Time-varying Attributes
   2. 4.2. ProcessedMediaStream
   3. 4.3. MediaInput
   4. 4.4. Worker Processing
5. 5. Built-In Processing Engines
   1. 5.1. Default Processing Engine
   2. 5.2. "LastInput" Processing Engine
6. 6. Media Graph Considerations
7. 7. Canvas Recording
8. 8. Implementation Considerations
9. 9. Examples

1. Introduction

The ideas here build on Ian Hickson's proposal for HTML Streams, adding features partly inspired by the Mozilla audio API and the Chrome audio API. Unlike previous audio API proposals, the API presented here integrates with proposed API for media capture from local devices, integrates with proposed API for peer-to-peer media streaming, handles audio and video in a unified framework, incorporates Worker-based Javascript audio processing, and specifies synchronization across multiple media sources and effects. The API presented here does not include a library of "native" effects; those should be added as new "named effects" (described below), perhaps as a "level 2" spec.

The work here is nascent. Until a prototype implementation exists, this proposal is likely to be incomplete and possibly not even implementable.

1.1. Scenarios

These are concrete usage scenarios that have helped guide the design of the API. They are higher-level than use-cases.

1. Play video with processing effect applied to the audio track (e.g. high-pass filter)
2. Play video with processing effects mixing in out-of-band audio tracks (in sync) (e.g. mixing in an audio commentary with audio ducking)
3. Capture microphone input and stream it out to a peer with a processing effect applied to the audio (e.g. XBox 360 chat with voice distortion)
4. Capture microphone input and visualize it as it is being streamed out to a peer and recorded (e.g. Internet radio broadcast)
5. Capture microphone input, visualize it, mix in another audio track and stream the result to a peer and record (e.g. Internet radio broadcast)
6. Receive audio streams from peers, mix them with spatialization effects, and play (e.g. live chat with spatial feature)
7. Seamlessly chain from the end of one input stream to another (e.g. playlists, audio/video editing)
8. Seamlessly switch from one input stream to another (e.g. adaptive streaming)
9. Synthesize samples from JS data (e.g. game emulators or MIDI synthesizer)
10. Trigger a sound sample to be played through the effects graph ASAP but without causing any blocking (e.g. game sound effects)
11. Trigger a sound sample to be played through the effects graph at a given time (e.g. game sound effects)
12. Capture video from a camera and analyze it (e.g. face recognition)
13. Capture video and audio, record it to a file and upload the file (e.g. Youtube upload)
14. Capture video from a canvas element, record it and upload (e.g. Screencast/"Webcast", or composite multiple video sources with effects into a single canvas then record)

2. MediaStreams

2.1. The Semantics Of MediaStreams

The description of MediaStreams here extends and must remain compatible with

Each MediaStream DOM object has an underlying media stream. The underlying media streams form a graph;

To avoid interruptions due to script execution, script execution can overlap with media stream processing;

• Changes to the MediaStream and MediaInput DOM objects are batched together between stable states (as defined by the HTML spec, while script is not running), and propagate as an atomic change to the media stream graph to take effect after some delay. This ensures that incomplete changes to the media stream graph during script execution do not result in transient glitches in the output, and that scripted graph changes do not interfere with concurrent media processing.
  Thus the following script would never cause an interruption in audio output, since no stable state occurs between the two volume changes:

    stream.inputs[0].volume = 0;
    if (needToPlay()) {
      stream.inputs[0].volume = 1.0;
    }

  Specify exactly which attributes (and methods) are subject to this regime, possibly extending to attributes and methods already defined in HTML for media elements etc.

• State changes from the media stream graph are only propagated back to the MediaStream DOM objects during a stable state. This ensures that between stable states (usually, during the execution of a script event handler), MediaStream DOM APIs always reflect a consistent snapshot of the state of the media stream graph.

In this spec, references to MediaStreams and MediaInputs refer to the DOM-visible state, and references to media streams and input ports refer to the underlying real-time media stream graph.

A stream is an abstraction of a time-varying video and/or audio signal. At a given point in time, a media stream can be blocked, that is, not playing for some reason. All non-blocked streams play at the same constant rate --- real time. Streams cannot be seeked or played at a rate other than real time. For convenience we define a stream's "current time" as the duration it has played since it was created, but (unless blocking is involved) streams do not have independent timelines; they are synchronized.

At the implementation level, and when processing media data with Workers, we assume that each stream has a buffered window of media data available, containing the sample it is currently playing (or that it will play the next time it unblocks). This buffer defines the stream's contents into the future.

A stream can be in the finished state. A finished stream is always blocked and can

A stream that has no consumers must block. Stream consumers defined in this specification are media elements and ProcessedMediaStreams (see below). This avoids situations where streams that aren't connected

A media stream contains video and audio tracks. Tracks can start and end at any time. Each track

2.2 Media Formats

This spec mostly treats the formats used for stream audio and video data

Built-in audio processing filters guarantee that if all the audio inputs constantly have the same uncompressed format

When samples are exposed to a Worker for processing, the user-agent chooses a fixed uncompressed audio format (sample rate and channel configuration) for its inputs and outputs; see section 4.4.

However, suggested resampling algorithms will be provided in an appendix.

2.3 MediaStream Extensions

partial interface MediaStream {
  readonly attribute double currentTime;

  ProcessedMediaStream createProcessor(optional in DOMString namedEffect);
  ProcessedMediaStream createWorkerProcessor(in Worker worker);
};

The currentTime attribute returns the amount of time that this MediaStream has played since it was created.

The createProcessor(namedEffect) method returns a new ProcessedMediaStream with this MediaStream as its sole input.

The createWorkerProcessor(worker) method returns a new ProcessedMediaStream with this MediaStream as its sole input.

Add event handlers or callback functions for all finished and blocking state changes?

3. Media Elements

3.1 Media Element Extensions

We extend HTML media elements to produce and consume streams. When an HTML media element

partial interface HTMLMediaElement {
  readonly attribute MediaStream stream;
 
  MediaStream captureStream();
  MediaStream captureStreamUntilEnded();
  readonly attribute boolean audioCaptured;

  attribute any src;
};

The stream attribute returns a stream which always plays whatever the element is playing. The stream is blocked while the media element is not playing. It is never finished, even when playback ends

Currently the HTML media element spec says that playing would fire on an element that is able to play except that a downstream MediaController is blocked. This is incompatible with the above. I think that part of the HTML media spec should be changed so that only elements that are actually going to play fire playing.

The captureStream() method returns a new MediaStream that plays the same audio and video as stream.

The captureStreamUntilEnded() method returns a new MediaStream that plays the same audio and video as stream, until the element next reaches

While the media element is playing a resource whose origin is not the same as the media element itself, MediaStreams for the element do not play the content; they block.

This prevents leaking of media contents to scripts in the page. In the future we could relax this and allow the streams to play as long as there's no scripted processing of the data downstream, but that's trickier.

While the audioCaptured attribute is true, the element does not produce direct audio output.

The src attribute is extended to allow it to be set to a MediaStream. The element

The URL.createObjectURL(stream) method defined for HTML MediaStreams can create a URL to be

3.2 Audio Element Extensions

We add an Audio constructor taking a MediaStream as a parameter.

[NamedConstructor=Audio(MediaStream src)]
partial interface HTMLAudioElement {
}

4. Stream Mixing And Processing

4.1 Time-varying Attributes

To enable precise control over the timing of attribute changes, many attributes can be set using a

Using a timed setter method never changes the observed attribute value immediately. Setter method changes always take effect after the next stable state, as described in section 2.1. Setting the attribute value changes the observed attribute value immediately, but the change to the underlying media stream will still not take effect until after

Multiple pending changes to an attribute are allowed. Calling the setter method with

4.2 ProcessedMediaStream

A ProcessedMediaStream combines zero or more input streams and applies some processing to

[Constructor(),
 Constructor(in Worker worker, in optional long audioSampleRate, in optional short audioChannels)]
interface ProcessedMediaStream : MediaStream {
  readonly attribute MediaInput[] inputs;
  MediaInput addInput(in MediaStream inputStream, in optional double outputStartTime, in optional double inputStartTime);

  attribute any params;
  void setParams(in any params, in optional double startTime);

  readonly attribute boolean autofinish;
};

The constructors create a new ProcessedMediaStream with no inputs.

Specify valid values for audioChannels and audioSampleRate.

The inputs attribute returns an array of MediaInputs, one for

A MediaInput represents an input port. An input port is active while it is enabled (see below) and the input stream is not finished.

The addInput(inputStream, outputStartTime, inputStartTime) method adds a new MediaInput to the end of the

More precisely, when the addInput call takes effect (see section 2.1),

1. If the outputStartTime was omitted, set it to the output stream's current time.
2. If the inputStartTime was omitted, set it to the input stream's current time.
3. Compute the deadline miss delay: the maximum of
   • The input stream's curent time minus inputStartTime
   • The output stream's current time minus outputStartTime
4. If the deadline miss delay is greater than zero, add it to the inputStartTime and outputStartTime. (This would be a good place for user-agents to emit a developer-accessible warning.)
5. While the input stream's current time is less than inputStartTime, or the output stream's current time is less than outputStartTime:
   • Whenever the input stream's current time is equal to inputStartTime, block the input stream.
   • Whenever the output stream's current time is equal to outputStartTime, block the output stream.
6. Enable the input port.

  var p = inputStream.createProcessor();
  p.addInput(inputStream, 5);

The params attribute and the setParams(params, startTime) timed setter method set the paramters for this stream. On setting, a structured clone of this object is made. The clone is sent to

When an input stream finishes, at the next stable state any MediaInputs for that

When the autofinish attribute is true, then when all stream inputs are finished (including if there are no inputs), the stream will automatically enter the finished state and never produce any more output (even if new inputs are attached).

4.3 MediaInput

A MediaInput object controls how an input stream contributes to the combined stream.

interface MediaInput {
  readonly attribute MediaStream stream;

  MediaInput addFollowing(MediaStream inputStream);

  attribute double volume;
  void setVolume(in double volume, in optional double startTime, in optional double fadeTime);

  attribute any params;
  void setParams(in any params, in optional double startTime);

  attribute boolean blockInput;
  attribute boolean blockOutput;

  void remove(in optional double time);
};

The stream attribute returns the MediaStream connected to this input.

The addFollowing(inputStream) method adds a new MediaInput to

Sequencing media resources using media element durations is possible, e.g. processor.addInput(video1.captureStreamUntilEnded()); processor.addInput(video2.captureStreamUntilEnded(),video1.duration);. However, media element durations may not be perfectly reliable due to limitations of resource formats or commonly-used encoders. For example, WebM resources often have a slight mismatch between the number of audio samples and the duration metadata. For other resources, such as live streaming sources, the duration may not be knowable in advance. Thus, using the duration is often unreliable and additional API is warranted.

Add additional API to select particular tracks.

The volume volume attribute and the setVolume timed setter method

Specify the exact transition function. Tim says "w=cos((pi/2)*t)^2 for t=0...1".

The params attribute and the setParams(params, startTime) timed setter method set the paramters for this input. On setting, a structured clone of this object is made. The clone is sent to

For the timed setter methods of MediaInput, the subject stream is the output stream, so changes take effect when the output stream's current time is equal to startTime.

The blockInput and blockOutput attributes control

Need to look again at these. It's not clear we have use cases for both attributes, and I haven't implemented them yet and they could be hard to implement.

The remove(time) method removes this MediaInput from the inputs array of its owning

4.4 Worker Processing

A ProcessedMediaStream with a worker computes its output by dispatching a sequence of onprocessmedia callbacks to the worker, passing each a ProcessMediaEvent parameter. A ProcessMediaEvent provides audio sample buffers for each input stream. Each sample buffer for a given ProcessMediaEvent has the same duration, so the inputs presented to the worker are always in sync. (Inputs may be added or removed between ProcessMediaEvents, however.) The sequence of buffers provided for an input stream is the audio data to be played by that input stream. The user-agent will precompute data for the input streams as necessary.

For example, if a Worker computes the output sample for time T as a function of the [T - 1s, T + 1s] interval of an input stream, then initially the Worker would simply refuse to output anything until it has received at least 1s of input stream data, forcing the user-agent to precompute the input stream at least 1s ahead of the current time. (Note that large Worker latencies will increase the latency of changes to the media graph.)

Note that Workers do not have access to most DOM API objects. In particular, Workers have no direct access to MediaStreams.

Note that a ProcessedMediaStream's worker cannot be a SharedWorker. This ensures that the worker can run in the same process as the page in multiprocess browsers, so media streams can be confined to a single process.

Currently ProcessMediaEvent does not offer access to video data. This should be added later.

partial interface DedicatedWorkerGlobalScope {
  attribute Function onprocessmedia;
};

The onprocessmedia attribute is the function to be called whenever stream data needs to be processed.

interface ProcessMediaEvent : Event {
  readonly attribute double inputTime;

  readonly attribute any params;
  readonly attribute double paramsStartTime;

  readonly attribute MediaInputBuffer inputs[];
  readonly attribute long audioSampleRate;
  readonly attribute short audioChannels;
  reaodnly attribute long audioLength;

  void writeAudio(in Float32Array data);

  void finish();
};

The inputTime attribute returns the duration of the input that has been consumed by the

The params attribute provides a structured clone of the parameters object set by

The paramsStartTime attribute returns the first time (measured in duration of input consumed for this stream) that this params object was set.

Note that the parameters objects are constant over the duration of the inputs presented in the event. Frequent changes to parameters will reduce the length of the input buffers that can be presented to the worker.

inputs provides access to MediaStreamBuffers for each active input stream

audioSampleRate and audioChannels represent the format of the input and

audioLength is the duration of the input(s) multiplied by the sample rate. If there are no inputs,

writeAudio(data) writes audio data to the stream output.

It is permitted to write less audio than the duration of the inputs (including none). This indicates latency in the filter. Normally the user-agent will dispatch another event to provide

A synthesizer with no inputs can output as much data as it wants; the UA will buffer data and fire events as necessary. Filters that misbehave, e.g. by always writing zero-length buffers, will cause the stream to block due to an underrun.

If writeAudio is not called during the event handler, then the input audio buffers are added together and written to the output.

If writeAudio is called outside the event handler, the call is ignored.

Calling finish() puts the stream into the finished state (once any previously buffered output has been consumed). The event callback will never be called again. finish() can be called at any time, inside or outside the event handler.

The output video track is computed as if there was no worker (see above).

This will change when we add video processing.

interface MediaInputBuffer {
  readonly attribute any params;
  readonly attribute double paramsStartTime;

  readonly attribute Float32Array audioSamples;
};

The params attribute provides a structured clone of the parameters object set by

The paramsStartTime attribute returns the first time (measured in duration of input consumed for this stream) that this params object was set.

audioSamples gives access to the audio samples for each input stream. The array length will be event.audioLength multiplied by event.audioChannels. The samples are floats ranging from -1 to 1, laid out non-interleaved, i.e. consecutive segments of audioLength samples each. The durations of the input buffers for the input streams will be equal. The audioSamples object will be a fresh object in each event. For inputs with no audio track, audioSamples will be all zeroes.

5. Built-In Processing Engines

5.1. Default Processing Engine

A ProcessedMediaStream with the default processing engine produces output as follows:

• If no active input has an audio track, the output has no audio track. Otherwise, the output has a single audio track whose metadata (id, kind, label, and language) is equal to that of the audio track for the last active input that has an audio track. The output audio track is produced by adding the samples of the audio tracks of the active inputs together.
• If no active input has a video track, the output has no video track. Otherwise, the output has a single video track whose metadata (id, kind, label, and language) is equal to that of the video track for the last active input that has a video track. The output video track is produced by compositing together all the video frames from the video tracks of the active inputs, with the video frames from higher-numbered inputs on top of the video frames from lower-numbered inputs; each video frame is letterboxed to the size of the video frame for the last active input that has a video track.

  This means if the last input's video track is opaque, the video output is simply the video track of the last input.

5.2. "LastInput" Processing Engine

A ProcessedMediaStream with the "LastInput" processing engine simply produces the last active input stream as output. If there are no active input streams, it produces the same output as the default processing engine.

6. Media Graph Considerations

6.1. Cycles

While a ProcessedMediaStream has itself as a direct or indirect input stream (considering only active inputs), it is blocked.

6.2. Blocking

At each moment, every stream should not be blocked except as explicitly required by this specification.

7. Canvas Recording

To enable video synthesis and some easy kinds of video effects we can record the contents of a canvas:

partial interface HTMLCanvasElement {
  readonly attribute MediaStream stream;
};

The stream attribute is a stream containing a video track with the "live" contents of the canvas as video frames whose size is the size of the canvas, and no audio track. It always returns the same stream for a given element.

8. Implementation Considerations

Here will be some non-normative implementation suggestions.

9. Examples

Add Worker scripts for these examples.

1. Play video with processing effect applied to the audio track

   <video src="foo.webm" id="v" controls></video>
   <audio id="out" autoplay></audio>
   <script>
   document.getElementById("out").src =
      document.getElementById("v").captureStream().createWorkerProcessor(new Worker("effect.js"));
   </script>

2. Play video with processing effects mixing in out-of-band audio tracks (in sync)

   <video src="foo.webm" id="v"></video>
   <audio src="back.webm" id="back"></audio>
   <audio id="out" autoplay></audio>
   <script>
     var mixer = document.getElementById("v").captureStream().createWorkerProcessor(new Worker("audio-ducking.js"));
     mixer.addInput(document.getElementById("back").captureStream());
     document.getElementById("out").src = mixer;
     function startPlaying() {
       document.getElementById("v").play();
       document.getElementById("back").play();
     }
     // MediaController is a more convenient API because it ties together control of the elements,
     // but using streams is more flexible (e.g. they can be seeked to different offsets).
   </script>

3. Capture microphone input and stream it out to a peer with a processing effect applied to the audio

   <script>
     navigator.getUserMedia('audio', gotAudio);
     function gotAudio(stream) {
       peerConnection.addStream(stream.createWorkerProcessor(new Worker("effect.js")));
     }
   </script>

4. Capture microphone input and visualize it as it is being streamed out to a peer and recorded

   <canvas id="c"></canvas>
   <script>
     navigator.getUserMedia('audio', gotAudio);
     var streamRecorder;
     function gotAudio(stream) {
       var worker = new Worker("visualizer.js");
       var processed = stream.createWorkerProcessor(worker);
       worker.onmessage = function(event) {
         drawSpectrumToCanvas(event.data, document.getElementById("c"));
       }
       streamRecorder = processed.record();
       peerConnection.addStream(processed);
     }
   </script>

5. Capture microphone input, visualize it, mix in another audio track and stream the result to a peer and record

   <canvas id="c"></canvas>
   <audio src="back.webm" id="back"></audio>
   <script>
     navigator.getUserMedia('audio', gotAudio);
     var streamRecorder;
     function gotAudio(stream) {
       var worker = new Worker("visualizer.js");
       var processed = stream.createWorkerProcessor(worker);
       worker.onmessage = function(event) {
         drawSpectrumToCanvas(event.data, document.getElementById("c"));
       }
       var mixer = processed.createProcessor();
       mixer.addInput(document.getElementById("back").captureStream());
       streamRecorder = mixer.record();
       peerConnection.addStream(mixer);
     }
   </script>

6. Receive audio streams from peers, mix them with spatialization effects, and play

   <script>
     var worker = new Worker("spatializer.js");
     var spatialized = stream.createWorkerProcessor(worker);
     peerConnection.onaddstream = function (event) {
       spatialized.addInput(event.stream).params = {x:..., y:..., z:...};
     };
     (new Audio(spatialized)).play();
   </script>

7. Seamlessly chain from the end of one input stream to another

   This method requires that you know each stream's duration, which is a bit suboptimal. To get around that we'd need new API, perhaps a new kind of ProcessedMediaStream that plays streams in serial.

   <audio src="in1.webm" id="in1" preload></audio>
   <audio src="in2.webm" id="in2"></audio>
   <script>
     var in1 = document.getElementById("in1");
     in1.onloadedmetadata = function() {
       var mixer = in1.captureStreamUntilEnded().createProcessor();
       var in2 = document.getElementById("in2");
       mixer.addInput(in2.captureStreamUntilEnded(), in1.duration);
       (new Audio(mixer)).play();
       in1.play();
     }
   </script>

8. Seamlessly switch from one input stream to another, e.g. to implement adaptive streaming

   There are two ways to implement seamless switching: seek the second resource to before the current time and then run the decoder faster than real-time to catch up to the first resource's play point, or seek the second resource to after the current time and enable it when the first resource catches up to the seek point. The first is more robust if the seek takes unexpectedly long, but the second is less demanding on the decoder. Only the second method is currently implementable with this API (since by design there is no way to drive MediaStreams faster than real-time). If we want to support the first method as well, the right way would be to add API to media elements to let them seek to synchronize with a given MediaStream.

   <audio src="in1.webm" id="in1" preload></audio>
   <audio src="in2.webm" id="in2"></audio>
   <audio id="out" autoplay></audio>
   <script>
     var stream1 = document.getElementById("in1").captureStream();
     var mixer = stream1.createProcessor("LastInput");
     document.getElementById("out").src = mixer;
     function switchStreams() {
       var in2 = document.getElementById("in2");
       in2.currentTime = in1.currentTime + 10; // arbitrary, but we should be able to complete the seek within this time
       mixer.addInput(in2.captureStream(), mixer.currentTime + 10);
       in2.play();
       // in2 will be blocked until the input port is enabled
       in2.onplaying = function() { mixer.inputs[0].remove(); };
     }
   </script>

9. Synthesize samples from JS data

   <audio id="out" autoplay></audio>
   <script>
     document.getElementById("out").src =
       new ProcessedMediaStream(new Worker("synthesizer.js"));
   </script>

10. Trigger a sound sample to be played through the effects graph ASAP but without causing any blocking

    <script>
      var effectsMixer = ...;
      function playSound(src) {
        var audio = new Audio(src);
        audio.oncanplaythrough = function() {
          var stream = audio.captureStreamUntilEnded();
          var port = effectsMixer.addInput(stream);
          port.blockOutput = false;
          audio.play();
        }
      }
    </script>

11. Trigger a sound sample to be played through the effects graph in five seconds

    <script>
      var effectsMixer = ...;
      var audio = new Audio(...);
      function triggerSound() {
        var sound = audio.clone();
        var stream = sound.captureStreamUntilEnded();
        sound.play();
        effectsMixer.addInput(stream, effectsMixer.currentTime + 5);
      }
    </script>

12. Capture video from a camera and analyze it (e.g. face recognition)

    <script>
      navigator.getUserMedia('video', gotVideo);
      function gotVideo(stream) {
        stream.createWorkerProcessor(new Worker("face-recognizer.js"));
      }
    </script>

13. Capture video, record it to a file and upload the file (e.g. Youtube)

    <script>
      navigator.getUserMedia('video', gotVideo);
      var streamRecorder;
      function gotVideo(stream) {
        streamRecorder = stream.record();
      }
      function stopRecording() {
        streamRecorder.getRecordedData(gotData);
      }
      function gotData(blob) {
        var x = new XMLHttpRequest();
        x.open('POST', 'uploadMessage');
        x.send(blob);
      }
    </script>

14. Capture video from a canvas, record it to a file then upload

    <canvas width="640" height="480" id="c"></canvas>
    <script>
      var canvas = document.getElementById("c");  
      var streamRecorder = canvas.stream.record();
      function stopRecording() {
        streamRecorder.getRecordedData(gotData);
      }
      function gotData(blob) {
        var x = new XMLHttpRequest();
        x.open('POST', 'uploadMessage');
        x.send(blob);
      }
      var frame = 0;
      function updateCanvas() {
        var ctx = canvas.getContext("2d");
        ctx.clearRect(0, 0, 640, 480);
        ctx.fillText("Frame " + frame, 0, 200);
        ++frame;
      }
      setInterval(updateCanvas, 30);
    </script>

Related Work

• W3C-RTC charter (Harald et. al.)
• WhatWG proposal (Ian Hickson et. al.)
• Chrome audio API
• Mozilla audio API
• WhatWG MediaController API