web-audio-api 

> Node.js implementation of Web audio API

This library implements the Web Audio API specification (also know as WAA) on Node.js.

What's Implemented

- [x] AudioContext (partially)
- [x] AudioParam (almost there)
- [x] AudioBufferSourceNode
- [x] ScriptProcessorNode
- [x] GainNode
- [ ] OscillatorNode
- [ ] DelayNode

Installation

``shell
npm install --save web-audio-api
`

Demo

Get ready, this is going to blow up your mind:

`
npm install
npm run test-speaker
`

Audio Output

By default, web-audio-api doesn't play back the sound it generates. In fact, an AudioContext has no default output, and you need to give it a writable node stream to which it can write raw PCM audio. After creating an AudioContext, set its output stream like this : audioContext.outStream = writableStream.

$3

This is probably the simplest way to play back audio. Install node-speaker with npm install speaker, then do something like this :

`javascript
const { AudioContext } = require('web-audio-api')
const Speaker = require('speaker')

const context = new AudioContext

context.outStream = new Speaker({
channels: context.format.numberOfChannels,
bitDepth: context.format.bitDepth,
sampleRate: context.sampleRate
})

// Create some audio nodes here to make some noise ...
`

$3

Linux users can play back sound from web-audio-api by piping its output to aplay. For this, simply send the generated sound straight to stdout like this :

`javascript
const { AudioContext } = require('web-audio-api')
const context = new AudioContext()

context.outStream = process.stdout

// Create some audio nodes here to make some noise ...
`

Then start your script, piping it to aplay like so :

`
node myScript.js | aplay -f cd
`

$3

icecast is a open-source streaming server. It works great, and is very easy to setup. icecast accepts connections from different source clients which provide the sound to encode and stream. ices is a client for icecast which accepts raw PCM audio from its standard input, and you can send sound from web-audio-api to ices (which will send it to icecast) by simply doing :

`javascript
const { spawn } = require('child_process')
const { AudioContext } = require('web-audio-api')
const context = new AudioContext()

var ices = spawn('ices', ['ices.xml'])
context.outStream = ices.stdin
`

A live example is available on Sébastien's website

Using Gibber

Gibber is a great audiovisual live coding environment for the browser made by Charlie Roberts. For audio, it uses Web Audio API, so you can run it on web-audio-api. First install gibber with npm :

npm install gibber.audio.lib

Then to you can run the following test to see that everything works:

npm test gibber.audio.lib

Overall view of implementation

Each time you create an `AudioNode` (like for instance an `AudioBufferSourceNode` or a `GainNode`), it inherits from `DspObject` which is in charge of two things:
- register schedule events with `_schedule`
- compute the appropriate digital signal processing with `_tick`

Each time you connect an `AudioNode` using `source.connect(destination, output, input)` it connects the relevant `AudioOutput` instances of `source` node to the relevant `AudioInput` instance of the `destination` node.

To instantiate all of these `AudioNode`, you needed an overall `AudioContext` instance. This latter has a `destination` property (where the sound will flow out), instance of `AudioDestinationNode`, which inherits from `AudioNode`. The `AudioContext` instance keeps track of connections to the `destination`. When that happens, it triggers the audio loop, calling `_tick` infinitely on the `destination`, which will itself call `_tick` on its input ... and so forth go up on the whole audio graph.

Running the debugger

Right now everything runs in one process, so if you set a break point in your code, there's going to be a lot of buffer underflows, and you won't be able to debug anything.

One trick is to kill the AudioContext right before the break point, like this:

`javascript
context[Symbol.dispose]()
debugger
`

that way the audio loop is stopped, and you can inspect your objects in peace.

Alternatives

* web-audio-engine − JS implementation of web-audio-api.
* web-audio-js − fork of web-audio-engine with extra nodes.
* lab-sound
* node-audio
* WAAPISim
* web-audio-api-rs

License

MIT

🕉