WAAClock.js

WAAClock is a small library to help you schedule things in time with Web Audio API.

var clock = new WAAClock(audioContext)
clock.start()

Schedule custom events

// prints 'wow!' at context.currentTime = 13
var event = clock.callbackAtTime(function() { console.log('wow!') }, 13)
// prints 'wow!' in 13 seconds
var event = clock.setTimeout(function() { console.log('wow!') }, 13)

Set events to repeat periodically

var event = clock.callbackAtTime(function() { console.log('wow!') }, 3).repeat(2)

Cancel an event

// Start an oscillator node at context.currentTime = 13
var event = clock.callbackAtTime(function() { oscNode.start(13) }, 13)
// ... but change your mind and cancel that
event.clear()

Change the tempo of a group of events

var event1 = clock.callbackAtTime(function() { console.log('wow!') }, 1).repeat(2)
  , event2 = clock.callbackAtTime(function() { console.log('what?') }, 2).repeat(2)

// in 10 seconds, the tempo will be multiplied by 2
clock.setTimeout(function() {
  clock.timeStretch(context.currentTime, [event1, event2], 0.5)
}, 10)

note : this library uses current Web Audio API specification. Some older browsers still use prefixed / deprecated function names. You can use Chris Wilson's AudioContext-MonkeyPatch if you want to support those older browsers as well.

Downloads

You can download the latest stable release of WAAClock from dist/.

Examples

• tempo change
• basic drum machine.

More infos about scheduling

WAAClock implements the technique explained in Chris Wilson's article A Tale of Two Clocks, providing it as a reusable library and adding extra control and features. By default, events are triggered by a combination of ScriptProcessorNode and setTimeout, but you can also implement a custom tickMethod to better control event triggering precision and performance.

In short, WAAClock merely executes your callback slightly before the given deadline, so you would have time to schedule things exactly using Web Audio API primitives. For example :

var osc = audioContext.createOscillator()
osc.connect(audioContext.destination)

var startEvent = clock.callbackAtTime(function(event) {
  osc.start(event.deadline)
}, 100)

Each event created with WAAClock has a tolerance zone [deadline - early, deadline + late] in which it must be executed. The event is executed as soon as the clock enters this tolerance zone. On the other hand, if the event hasn't been executed when the clock gets out of the tolerance zone, the event will be dropped (but in practice this shouldn't happen).

You can change the tolerance of an event by calling Event.tolerance, but be wise about it : a too tight upper bound late, and the event could be dropped abusively, a too loose lower bound early, and the event will be executed too early.

API

WAAClock(context, opts)

WAAClock handles all the scheduling work. It is the only object you need to create directly.

You can set the default tolerance of events with the options toleranceLate and toleranceEarly.

You can also pass a tickMethod option of "manual" to disable the built-in ScriptProcessorNode method of triggering scheduled events, and instead call clock.tick() yourself on a regular interval.

start()

Starts the clock. This will also erase all the events that were previously scheduled.

stop()

Stops the clock.

callbackAtTime(func, deadline)

Schedules func to run before deadline in seconds, and returns an Event object.

setTimeout(func, delay)

Schedules func to run after delay seconds, and returns an Event object.

timeStretch(tRef, events, ratio)

Stretch time and repeat time of events by ratio, keeping their relative distance, and taking tRef as a reference . In fact this is equivalent to changing the tempo.

tick()

Executes any events that are due since the last tick() call. This method does not need to be called unless you are using tickMethod: 'manual'. If you are, this method must be called regularly during playback to trigger events. For example, to use worker-timers for consistent scheduling when the page is in the background:

import WAAClock from "waaclock";
import * as wt from "worker-timers";

const clock = new WAAClock(audioContext, { tickMethod: 'manual' });
clock.start();

// Event precision is limited by how often clock.tick() is called.
const interval = wt.setInterval(() => clock.tick(), 10);

// ...later...
clock.stop()
wt.clearInterval(interval);

Event

Every scheduling method returns an event object. All methods from Event return the calling event, so that you can chain them.

deadline

The deadline of the event.

schedule(deadline)

Reschedule the deadline of an event, deadline is the absolute time as given by context.currentTime.

tolerance(values)

Sets the event's tolerance, values is on object that can have keys late and early. See WAAClock for a detailed explanation. Example :

// The following executes `cb` before time 11. However, `cb` can be executed as early as
// time 10.9, and if something happends that prevent the event to be executed early enough,
// after time 12 the event will be dropped.
var clock.callbackAtTime(cb, 11)
  .tolerance({ early: 0.1, late: 1 })

repeat(time)

Sets the event to repeat every time seconds. If you want to remove the repeat you can pass null. Note that even if an event is dropped because it expired, subsequent "repeats" of the event will still be executed.

clear()

Cancels the event execution. This will work only if the event hasn't been scheduled yet (see WAAClock for more infos).

Event: 'expired'

This message is emitted when the clock fell out of the event tolerance zone. You can listen to it by calling on :

event.onexpired = function(event) { console.log('oooh :(!') }

Running the tests

Tests are written with mocha. Just install mocha globally and run mocha from the root directory. Integration with node-web-audio-api is tested manually running node test/node-web-audio-api-test.js.

Building

Build with browserify to dist/WAAClock-latest.js by running npm run build.

License

Released under MIT license

Change log

0.5.3

• bug fixes.

0.5.2

• bug fixes.

0.5.1

• bug fixes.

0.5.0

• removed support for prefixed AudioContext
• removed underscore dependency
• changed Event.tolerance API
• renamed Event.time to Event.deadline
• added tRef argument to timeStretch
• removed executed event and EventEmitter
• expired event to callback

0.4.0

• made WAAClock.start method public, and start needs to be called explicitely
• WAAClock.stop method
• removed web audio API monkey-patching
• removed support for old web audio API functions

0.3.2

• bug fix

0.3.1

• made schedule method of Event public.

0.3.0

• added support for node-web-audio-api

0.2.0

• changed the tick method from setInterval to ScriptProcessorNode
• added event's toleranceEarly and toleranceLate
• removed clock tickTime and lookAheadTime options
• added support for old Web Audio API names

0.1.2

• added callbackAtTime
• bug fixes