Instant Audio Processing with Web Audio

Instant Audio Processing with Web Audio

By : Chris Khoo

Buy this Book

Instant Audio Processing with Web Audio

By: Chris Khoo

Buy this Book

Overview of this book

Web Audio is an upcoming industry standard for web audio processing. Using the API, developers today can develop web games and applications with real-time audio effects to rival their desktop counterparts. Instant Audio Processing with Web Audio is your hands-on guide to the Web Audio API. Using clear, step-by-step exercises, this book explores the API and how to apply it to produce real-time audio effects such as audio stitching, audio ducking, and audio equalization. This book is an in-depth study of the Web Audio API. Through a series of practical, step-by-step exercises, this book will guide you through the basics of playing audio all the way to the task of building a 5-band audio equalizer. Along the way, we'll learn how to utilize Web Audio's scripting functionality to build real-time audio effects such as audio stitching and ducking. Then, we'll use this knowledge to build a basic audio layer step-by-step which can be used in our web applications/games. With its in-depth coverage of the Web Audio API and its practical advice on various audio implementation scenarios, Instant Audio Processing with Web Audio How-to is your ultimate guide to Web Audio.

Instant Audio Processing with Web Audio

Credits

About the Author

About the Reviewer

www.PacktPub.com

Preface

Free Chapter

Instant Audio Processing with Web Audio

Setting up a web host (Simple)

Playing audio in a loop (Simple)

Setting the volume (Simple)

Automating the audio parameters (Intermediate)

Building an equalizer using BiquadFilterNode (Advanced)

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Setting the volume (Simple)

Now that we've wrapped our heads around playing audio, it's time to look at controlling the sound volume. In this recipe, we'll build an audio player with real-time volume controls. We'll use a piano sound loop instead of the choir loop this time around to keep things fresh.

Getting ready

The complete source code for this recipe is available in the code bundle at recipes/Recipe4_1a.

How to do it...

Start with a clean copy of the base framework template. The template bundle is located at tools/RecipeFramework in the code bundle.
Open index.html with a text editor.

In the HTML section, we'll add the piano loop toggle and the volume control widget:

<div id="appwindow">

<h2>Setting The Volume</h2>
<form>
    <input type="checkbox" id="piano" />
    <label for="piano">Piano Loop</label>
    <span>VOLUME</span>
    <span id="pianovol" style="display: inline-block; 
          width: 300px;"></span>
</form>

</div>

Like the previous recipes, we'll need the Web Audio initialization and audio file loading routines; we'll add these to the JavaScript section:

function loadAudioFromUrl( url, loadedCallbackFn, 
                           callbackContext ) {
    var request = new XMLHttpRequest();
    request.open("GET", url, true);
    request.responseType = "arraybuffer";

    request.onload = function() {
        consoleout( "Loaded audio '" + url + "'" );
        later( 0, loadedCallbackFn, callbackContext, 
               request.response );
    };

    request.onerror = function() {
        consoleout( "ERROR: Failed to load audio from " 
                    + url );
    };

    request.send();
}

WebAudioApp.prototype.initWebAudio = function() {
    var audioContextClass = window.webkitAudioContext 
                            || window.AudioContext;

    if( audioContextClass == null )
        return false;

    this.audioContext = new audioContextClass();
    return true;
};

Let's take a first stab at tidying up the boilerplate code for controlling node playback; we'll wrap the start and stop audio playback functionalities into the functions stopNode() and startNode(), respectively. It's a marginal improvement, but an improvement nonetheless:

function stopNode( node, stopSecs ) {
    if( node.stop instanceof Function )
        node.stop( stopSecs );
    else if( node.noteOff instanceof Function )
        node.noteOff( stopSecs );
}

function startNode( node, startSecs,loop ) {
    // Turn on looping if necessary
    if( loop )
        node.loop = true;

    // Start playback at a predefined time
    if( node.start instanceof Function )
        node.start( startSecs );
    else if( node.noteOn instanceof Function )
        node.noteOn( startSecs );

    return node;
}

We'll create the function WebAudioApp.initMusicControls() to wrap all the sound manipulation logic:
```
WebAudioApp.prototype.initMusicControls = 
function( elemId, audioSrc, elemVolId ) {
}
```

We'll start by adding the logic for loading the audio data into WebAudioApp.initMusicControls() and initializing the sound toggle:

// Initialize the button and disable it by default
var jqButton = $( elemId ).button({ disabled: true });

// Load the audio
var audioBuffer;
loadAudioFromUrl( audioSrc, function(audioData){
    // Decode the audio data into an audio buffer
    this.audioContext.decodeAudioData(
        audioData,
        function( audioBufferIn ) {
            consoleout( "Decoded audio for '"
                        + audioSrc + "'" );

            // Cache the audio buffer
            audioBuffer = audioBufferIn;

            // Enable the button once the audio is ready
            jqButton.button( "option","disabled",false );
        }
    );
}, this );

It's time to enhance our node graph with a volume controller. In Web Audio, volume is controlled using a separate GainNode node instance. We'll instantiate a GainNode instance and attach it to our node graph; the instance will serve as our application's volume controller:

// Create the volume control gain node
var context = this.audioContext,
    volNode;

if( context.createGain instanceof Function )
    volNode = context.createGain();
else if( context.createGainNode instanceof Function )
    volNode = context.createGainNode();

// Connect the volume control the the speaker
volNode.connect( context.destination );

Next, we'll initialize our HTML slider widget and link the slider changes to the GainNode instance, of our volume controller:

// Create the volume control
$( elemVolId ).slider({
    min: volNode.gain.minValue,
    max: volNode.gain.maxValue,
    step: 0.01,

    value: volNode.gain.value,

    // Add a callback function when the user
    // moves the slider
    slide: function( event, ui ) {
        // Set the volume directly
        volNode.gain.value = ui.value;

        consoleout( "Adjusted music volume: " + ui.value );
    }
});

We complete the WebAudioApp.initMusicControls() function by implementing the sound toggle event handler for starting and stopping the audio loop:

var me = this;
var activeNode;
jqButton.click(function( event ) {

    // Stop the active source node
    if( activeNode != null ) {
        stopNode( activeNode, 0 );
        activeNode = null;

        consoleout( "Stopped music loop '" + audioSrc + "'" );
    }

    // Start a new sound on button activation
    if($(this).is(':checked')) {
        // Start the loop playback
        activeNode = me.audioContext.createBufferSource();
        activeNode.buffer = audioBuffer;
        startNode( activeNode, 0, true );

        // Connect it to the volume control
        activeNode.connect( volNode );

        consoleout( "Played music loop '" + audioSrc + "'" );
    }
});

Finally, in WebAudioApp.start(), we initialize Web Audio and the music controls:

WebAudioApp.prototype.start = function() {
  if( !this.initWebAudio() ) {
    consoleout( "Browser does not support WebAudio" );
    return;
  }

  this.initMusicControls(
    "#piano",
    "assets/looperman-morpheusd-dreamworld-fullpiano-120-bpm.wav",
    "#pianovol" );
};

Launch the application test URL in a web browser (http://localhost/myaudiomixer) to see the end result. The following is a screenshot of what we should see in the browser:

How it works...

We add in volume control support by inserting a GainNode instance between the audio source node and the speaker output node, as shown in the following diagram:

We control the output volume from a GainNode instance by adjusting its gain audio parameter. The gain audio parameter specifies the multiplier applied to the GainNode input signal when producing its output signal.

Let's take a deeper look at WebAudioApp.initLoopToggle() and its implementation to support volume control:

We create GainNode using the method createGain() or createGainNode() of AudioContext and cache the instance volNode:
```
    var context = this.audioContext;

    var volNode;
    if( context.createGain instanceof Function )
        volNode = context.createGain();
    else if( context.createGainNode instanceof Function )
        volNode = context.createGainNode();
```
New GainNode instances have their gain audio parameter automatically set to 1.
Note
We check for the AudioBufferSourceNode methods createGainNode() and createGain(). createGainNode() is the old name for the method createGain(). Again, W3C recommends supporting both function names for backward compatibility.
We connect the GainNode instance to the context's AudioDestinationNode instance:
```
    volNode.connect( context.destination );
```

Whenever we start playing sounds, we instantiate and activate a new AudioBufferSourceNode instance as before, except that we connect its output to the volume controller, volNode, instead of directly connecting it to the speaker node. Notice how we're reusing the same GainNode instance for all sounds played:

    var me = this;
    var activeNode;
    jqButton.click(function( event ) {

        // Stop the active source node
        if( activeNode != null ) {
            me.stopNode( activeNode, 0 );
            activeNode = null;

            ...
        }

        // Start a new sound on button activation
        if($(this).is(':checked')) {
            // Start the loop playback
            activeNode = me.startNode( audioBuffer, 0, true );

            // Connect it to the volume control
            activeNode.connect( volNode );

            ...
        }
    });

The GainNode instances are reusable unlike the AudioBufferSourceNode instances. The GainNode instance will remain alive so long as we maintain an active JavaScript reference through volNode.

We initialize the volume control element and configure the range and starting values using the GainNode instance's gain audio parameter settings:

    $( elemVolId ).slider({
        min: volNode.gain.minValue,
        max: volNode.gain.maxValue,
        step: 0.01,

        value: volNode.gain.value,

When the slider is moved, we set the GainNode instance's gain.value attribute to the slider value:

        slide: function( event, ui ) {

            volNode.gain.value = ui.value;

            ...

        }
    });

Let's take a look at the Web Audio API methods/attributes introduced in this recipe:

AudioContext.createGain() instantiates and returns a GainNode instance:
```
interface AudioContext {

    function createGain():GainNode;

};
```
The GainNode instance multiplies its input signal values by a gain multiplier. The GainNode instance stores the gain multiplier in an AudioParam instance accessible through the gain attribute:
```
interface GainNode : AudioNode {

    var gain:AudioParam;

};
```
AudioParam is Web Audio's standard audio parameter interface. It allows applications to retrieve the parameter configuration and to tweak its value. The interface also allows applications to script the parameter behavior.

We'll look at its basic attributes for the moment and explore the scripting functionality in the next recipe:

The AudioParam.value attribute stores the applied parameter value of the instance. Applications write to this attribute to change the parameter value directly:
```
interface AudioParam {

    var value:Number;
```
The minValue and maxValue attributes store the parameter's minimum and maximum values, respectively.
```
var minValue:Number;        // readonly
    var maxValue:Number;        // readonly
```
Note
AudioParam does not clamp the value attribute to minValue and maxValue. When an audio parameter is out of range, the resulting behavior is undefined—results are browser specific and may vary depending on the AudioNode class type.
The AudioParam instance's default value is stored in defaultValue:
```
    var defaultValue:Number;    // readonly
};
```

There's more...

Encapsulating the audio playback logic into the functions startNode() and stopNode() improved the code's readability, but only marginally. In addition, it's tiring having to reimplement the same boilerplate code for initializing Web Audio and for handling audio file loads. It's time to take the next step—we'll integrate the Web Audio boilerplate functionality into the framework template:

We'll make the Web Audio initialization and sound loading logic a permanent part of the framework.
We'll wrap the node graph construction and sound playback logic into an AudioLayer class.

This is the framework template we'll be using for the rest of the recipes.

The following are the class definitions for the new AudioLayer class and the updated WebAudioApp class:

The AudioLayer constructor constructs the basic Node Graph:

class AudioLayer {

    function AudioLayer( context:AudioContext );

AudioLayer exposes the volume gain audio parameter through its gain attribute:
```
    var gain:AudioParam;
```

AudioLayer.playAudioBuffer() implements the audio buffer playback logic:

    function playAudioBuffer( 
               audioBuffer:AudioBuffer,
               startSecs:Number,
               loop:Boolean ):AudioBufferSourceNode
};

WebAudioApp.initWebAudio() contains the WebAudio initialization logic:

class WebAudioApp {

    function start(); 
    function initWebAudio();

WebAudioApp.loadAudio() contains the audio buffer loading logic:

    function loadAudio(
                 audioSrc:String,
                 callbackFn:function(buffer:AudioBuffer),
                 context:*? ); 
};

We'll also clean up the logic for handling backwards compatibility.

Previously, we searched for the appropriate method name at the time of execution:

    // Start playback at a predefined time
    // Call the appropriate playback start function
    // on the AudioBufferSourceNode
    if( sourceNode.start instanceof Function )
        sourceNode.start( startSecs );
    if( sourceNode.noteOn instanceof Function )
        sourceNode.noteOn( startSecs );

This approach is tedious and error prone.

A better approach is to ensure that the new method name always exists. If it does not exist, we'll map the old method to its new name as shown in the following code:

    // Make sure that an AudioBufferSourceNode instance 
    // always has the new method name. This code occurs after 
    // the AudioBufferSourceNode is constructed
    if( sourceNode.start == null )
        sourceNode.start = sourceNode.noteOn;

    // The rest of the code can now assume that the start() 
    // function always exists...

The updated template framework is found in the tools/RecipeFrameworkV2 subdirectory in the code bundle.

The directory recipe/Recipe4_1b contains this recipe's source code after it's adapted to the new template framework.

Instant Audio Processing with Web Audio

By : Chris Khoo

Instant Audio Processing with Web Audio

By: Chris Khoo

Overview of this book

Related Content you might be interested in

Current Title:

Instant Audio Processing with Web Audio

Setting the volume (Simple)

Getting ready

How to do it...

How it works...

Note

Note

There's more...