dancer.js is a high-level audio API, usable with both Mozilla's Audio Data API and Webkit's Web Audio API, designed to make sweet visualizations.

http://jsantell.github.com/dancer.js

v0.1.0 (6/3/2012)

• Use real-time audio frequency data and map it to any arbitrary visualization
• Leverage beat detection into your visualizations
• Simple API to time callbacks and events to any section of a song
• Extensible framework supporting plugins and custom behaviours

All controls return this.

• play() plays the audio and begins the dance.
• stop() stops the madness.

• getTime() returns the current time.
• getSpectrum() returns the frequency data array.
• getFrequency( freq [, endFreq ] ) returns the magnitude of a frequency or average over a range of frequencies.
• isLoaded() returns a boolean value for the dancer instance's song load state.
• isPlaying() returns a boolean value indicating whether the dancer instance's song is currently playing or not.

All section methods return this (CHAIN IT UP) and callbacks executed with this referencing the dancer instance.

• after( t, callback ) fires callback on every frame after time t.
• before( t, callback ) fires callback on every frame before time t.
• between( t0, t1, callback ) fires callback on every frame between time t0 and t1.
• onceAt( t, callback ) fires callback once at time t.

Basic pub/sub to tie into the dancer instance. update and loaded are predefined events called within the framework that are published on every frame (update) and on audio file load (loaded). All callbacks executed with this referencing the dancer instance.

• bind( name, callback ) subscribes a callback of name. Can call this method several times to bind several callbacks of the same name.
• unbind( name ) unsubscribes all callbacks of name.
• trigger( name ) calls all callbacks of name.

Beats are detected when the amplitude (normalized values between 0 and 1) of a specified frequency, or the max amplitude over a range, is greater than the minimum threshold, as well as greater than the previously registered beat's amplitude, which is decreased by the decay rate per frame.

• createBeat( options ) creates a new beat instance tied to the dancer instance, with an options object passed as an argument. Options listed below.
  • frequency the frequency (element of the spectrum) to check for a spike. Can be a single frequency (number) or a range (2 element array) that uses the frequency with highest amplitude. Default: [ 0, 10 ]
  • threshold the minimum amplitude of the frequency range in order for a beat to occur. Default: 0.3
  • decay the rate that the previously registered beat's amplitude is reduced by on every frame. Default: 0.02
  • onBeat the callback to be called when a beat is detected.
  • offBeat the callback to be called when there is no beat on the current frame.

• addPlugin( name, fn ) registers a plugin of name with initiation function fn -- described in more detail below
• isSupported() returns either true or false indicating whether the browser supports the Web Audio API or Audio Data API

These methods can be called on a beat instance to turn on and off the registered callbacks

• on() turns on the beat instance's callbacks and detections
• off() turns off the beat instance's callbacks and detections

  var
    dancer = new Dancer( "sickjams.ogg" ),
    beat = dancer.createBeat({
      onBeat: function ( mag ) {
        console.log('Beat!');
      },
      offBeat: function ( mag ) {
        console.log('no beat :(');
      }
    });

  // Let's turn this beat on right away
  beat.on();

  dancer.onceAt( 10, function() {
    // Let's set up some things once at 10 seconds
  }).between( 10, 60, function() {
    // After 10s, let's do something on every frame for the first minute
  }).after( 60, function() {
    // After 60s, let's get this real and map a frequency to an object's y position
    // Note that the instance of dancer is bound to "this"
    object.y = this.frequency( 400 );
  }).onceAt( 120, function() {
    // After 120s, we'll turn the beat off as another object's y position is still being mapped from the previous "after" method
    beat.off();
  });

  dancer.play();

You can extend the Dancer prototype by calling the static method addPlugin( name, fn ), which extends the Dancer prototype. A Dancer instance then can call the function provided in its context and subscribe to a preexisting event like update, or make your own. Look in the plugins/ directory for examples.

This project uses smoosh to build and jasmine for testing. A CLI for testing would be awesome, but Mozilla and WebKit implementations differ greatly -- go to spec/index.html in Mozilla/WebKit browsers to test.