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

• AudioContext (partially)
• AudioParam (almost there)
• AudioBufferSourceNode
• ScriptProcessorNode
• GainNode
• OscillatorNode
• DelayNode

Installation

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.

Example: Playing back sound with node-speaker

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

import { AudioContext } from 'web-audio-api'
import Speaker from '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 ...

Example : playing back sound with aplay

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 :

import { AudioContext } from '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

Example : creating an audio stream with icecast2

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 :

import { spawn } from 'child_process'
import { AudioContext } from '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:

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
• node-web-audio-api

License

MIT

🕉