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https://github.com/itsjoesullivan/envelope-generator

Simple ADSR envelope generator for web audio
https://github.com/itsjoesullivan/envelope-generator

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Simple ADSR envelope generator for web audio

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README 

        
## Envelope Generator ![Test status](https://api.travis-ci.org/itsjoesullivan/envelope-generator.svg)



Basic ADSR envelope generator for web audio. A demo is running [here](http://joesul.li/van/envelope-generator/).



- The release stage exists as a separate `GainNode`, so the envelope doesn't need to keep track of its output gain internally.

- Uses the [voltage](https://github.com/mmckegg/adsr/blob/master/index.js#L126) idea from [mmckegg/adsr](https://github.com/mmckegg/adsr).



### Example



```bash

npm install --save envelope-generator

```



```javascript

import Envelope from 'envelope-generator';



let context = new AudioContext();

let osc = context.createOscillator();



let gain = context.createGain();



let env = new Envelope(context, {

  attackTime: 0.1,

  decayTime: 3,

  sustainLevel: 0.4,

  releaseTime: 0.1

});



env.connect(gain.gain);



var startAt = context.currentTime;



var releaseAt = startAt + 0.5;



osc.start(startAt);

env.start(startAt);



env.release(releaseAt);



let stopAt = env.getReleaseCompleteTime();

osc.stop(stopAt);

env.stop(stopAt);

```



### Usage



#### Constructor



The constructor accepts two arguments: an AudioContext and a settings object. All settings are optional, but you will probably want to set at least `attackTime`, `decayTime`, `sustainLevel`, and `releaseTime`.



- All `...Time` properties are in seconds

- All `curve`, `attackCurve`, `decayCurve`, and `releaseCurve` properties default to `"linear"`, with `"exponential"` the alternative.

- attackCurve, decayCurve, and releaseCurve override their respective curves

- Passing an `initialValueCurve` will determine the shape of the curve usually covered by the attack and decay sections of an envelope, overriding any other curve values.

- Passing a `releaseValueCurve` will determine the shape of the release, overriding any other release curve value.

- Both initialValueCurve and releaseValueCurve are expected to be normalized, i.e. not extending outside of the bounds [0, 1]. This is relatively intuitive for the `initialValueCurve`, but ensure that your `releaseValueCurve` also starts at a value of 1 to avoid any jumps in the sound. The reason for this is that these two curves are applied in series.

- The sampleRate property applies to initialValueCurve and releaseValueCurve, allowing them to be expressed in a sampleRate different from that of the context.



```javascript

let context = new AudioContext();

let settings = {

  curve: "linear",

  attackCurve: "linear",

  decayCurve: "linear",

  releaseCurve: "linear",

  initialValueCurve: Float32Array,

  releaseValueCurve: Float32Array,

  sampleRate: 44100,

  delayTime: 0,

  startLevel: 0,

  maxLevel: 1,

  attackTime: 0.1,

  holdTime: 0,

  decayTime: 0,

  sustainLevel: 0.5,

  releaseTime: 1

};

let env = new Envelope(context, settings)

```



#### connect



The `connect` method should be attached directly to `AudioParam`s:



```javascript

let osc = context.createOscillator();

let gainNode = context.createGain();

let env = new Envelope(context, settings);

env.connect(gainNode.gain);

```



#### start



The `start` method triggers the attack and decay stages of the envelope:



```javascript

let osc = context.createOscillator();

let gainNode = context.createGain();

let env = new Envelope(context, settings);

env.connect(gainNode.gain);



osc.start(context.currentTime);

env.start(context.currentTime);

```



#### release



The `release` method triggers the release stage of the envelope:



```javascript

let osc = context.createOscillator();

let gainNode = context.createGain();

let env = new Envelope(context, settings);

env.connect(gainNode.gain);



osc.start(context.currentTime);

env.start(context.currentTime);



// Release the envelope after 1 second

env.release(context.currentTime + 1);

```



#### getReleaseCompleteTime



Releasing the envelope isn't the same as stopping the sound source. Once release has been called, `getReleaseCompleteTime()` will return the time that the envelope finishes its release stage. If this is an amp envelope, and the startLevel (i.e., where the envelope will release to) is 0, `getReleaseCompleteTime()` is when your sound source is guaranteed to be silent and can be stopped:



```javascript

let osc = context.createOscillator();

let gainNode = context.createGain();

let env = new Envelope(context, settings);

env.connect(gainNode.gain);



osc.start(context.currentTime);

env.start(context.currentTime);



env.release(context.currentTime + 1);



// Stop the oscillator once the envelope has completed.

osc.stop(env.getReleaseCompleteTime());

```



#### stop



Because they are generating a signal, envelopes need to be stopped as well as released. This should coincide with when the actual sound source is stopped.



```javascript

let osc = context.createOscillator();

let gainNode = context.createGain();

let env = new Envelope(context, settings);

env.connect(gainNode.gain);



osc.start(context.currentTime);

env.start(context.currentTime);



env.release(context.currentTime + 1);



// Stop the oscillator once the envelope has completed.

let stopAt = env.getReleaseCompleteTime();

osc.stop(stopAt);

env.stop(stopAt);

```