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A 5 stage audio envelope generator
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README.md

audio-contour npm

js-standard-style license

A 5 stage audio envelope generator. You can see the demo here:

var Contour = require('audio-contour')
var ac = new AudioContext()

var vca = ac.createGain()
var osc = ac.createOscillator()
osc.connect(vca)

var env = Contour(ac, { t1: 0.2, t4: 0.5 })
env.connect(vca.gain)

env.start()
env.onstart = function (when) { osc.start(when) }
env.onended = function () { osc.stop() }
env.stop(ac.currentTime + 3)

This module implements a alpha-juno style envelope generator:

Envelope Diagram from audiorealism.se

If you want to learn more about envelope generators, read this

There are a lot of envelope generator implementations. Here are the standalone ones I know (there are several audio libraries that implements them):

Why choose this library over the others:

  • Unlike others, it implements a 5 stage envelope (and can be reduced to a standard ADSR envelope)
  • It supports onstart and onended events
  • Can specify gate duration (for sequencer style)
  • It's small (2.5Kb minified)

Why don't choose this library:

  • It's very young project, still in development and not battle tested.
  • Other libraries are great too!

Installation

Via npm: npm i --save audio-contour

Usage

Create an envelope

To create an envelope use the Contour function:

var ac = new AudioContext()
var Contour = require('audio-contour')
var env = Contour(ac)

You can pass options to that function:

var env = Contour(ac, { t1: 1 })

or change them on the object before start:

var env = Contour(ac)
env.t1 = 1
env.t4 = 0.5

Apply the envelope

To apply the envelope, you have to connect it to something. For example, you can create a vca (voltage controlled amplifier) connecting it to a gain's gain param:

var vca = ac.createGain()
env.connect(vca.gain)

Or create a vcf (voltage controlled filter) ocnnecting it to a filter frequency param:

var vcf = ac.createBiquadFilter()
env.connect(vcf.frequency)

Start and stop the envelope

You can use start and stop function to the envelope:

var now = ac.currentTime
env.start(now)
// suppose your audio source is an oscillator
osc.start(now)
var finish = env.stop(now + 1)

The stop function returns the time when the release phase ended. Can be used to stop the audio sources:

osc.start(finish)

Remeber that if duration is not Infinity, the envelope will stop automatically:

var env = Contour(ac)
env.duration = 1
env.start() // => it will automatically stop after 1 second

Events

Two events are supported: onstart and onended. The onstart event handler will be trigger at same time as the start function of the envelope, so it receives a time parameter. The onended event handler will be called when the envelope effectively stops:

env.duration = 1
env.onstart = function (when) { osc.start(when) }
env.onended = function () { osc.stop(ac.currentTime) }
env.start() // since duration is not Infinity, both envent handlers will be called

Create a standard ADSR

When t3 is 0, the audio-contour behaves like a normal ADSR envelope.

Additionally, you can use the standard attack, decay, sustain and release parameters in the constructor to build the envelope:

var env = Contour(ac, { attack: 0.1, decay: 0.2, sustain: 0.8, release: 0.5 })
env.t1 // => 0.1 (the attack)
env.t2 // => 0.2 (the decay)
env.t3 // => 0
env.t4 // => 0.5 (the release)

Run tests and examples

To run the tests, clone this repo and: npm install && npm test.

To run the example you need watchify installed: npm install -g watchify. Then, move to examples directory and type: npm install && npm start

License

MIT License

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