simplejack

JACK is a platform that enables sound applications to easily send and receive realtime sound from other applications or hardware.

This module enables you to register simple JACK clients:

// Create the client
var simplejack = require("simplejack");
var client = simplejack("my-client");

// Register audio callback
client.callback(function(t) {
  return Math.sin(2*Math.PI * 440 * t);
});

// Start the client!
client.activate();

This will create a JACK client named my-client, with 1 audio port that outputs a 440Hz tone. The t passed to the callback is the time in seconds since the client was last activated, or the callback was last changed.

Install

First make sure you have JACK v2 development files installed. On Debian/Ubuntu and derivatives, you can do it with:

sudo apt-get install libjack-jackd2-dev

Then install the module as usual, e.g.

npm install simplejack

Options

You can pass an object to simplejack() in order to customize options:

var client = simplejack({ name: "foo", forceName: true, ports: 2 });

The available options (and their default values) are:

{
  // Name of the client.
  name: "simplejack",

  // If enabled and a client with the same name exists, creation
  // will fail. Otherwise a unique name will be created.
  forceName: false,

  // Outgoing audio ports. If this is an array of strings,
  // the strigs are the names of the created ports.
  // If an integer N is passed, then N ports will be
  // created with names "out-0", "out-1", etc.
  ports: 1,

  // Mark the ports as "terminal", meaning the audio in these
  // ports is "original" and not available on any other port.
  // Synthesizers, hardware interfaces, etc. should set this flag.
  terminal: false,

  // Mark the ports as "physical", meaning these ports correspond
  // to some kind of physical I/O connector.
  physical: false,

  // Explicitely connect to the server with that name.
  // Otherwise JACK will try to guess or use "default".
  server: null,

  // If the server isn't started, fail instead of starting it.
  noStartServer: null
}

Multiple ports

If you open more than one port, the second argument passed to the callback is the index of the port for which the sample is being produced.

So, to produce a 500Hz tone on the first port and white noise on the other, do:

var client = simplejack({ ports: 2 });

client.callback(function(t, port) {
  if (port === 0)
    return Math.sin(2*Math.PI * 500 * t);
  else
    return 2 * Math.random() - 1;
});

Implementation & limitations

Disclaimer: yes I know Node.JS is the worst platform to do realtime sound processing because there's a single event loop and blah, blah, blah...

A buffer is kept (always with the correct size), on which samples are precalculated and saved. When the "process" callback gets called for the first time, zeros are returned, and we queue UV to calculate the next samples by calling the JS callback and save them on the buffer.

The next time the "process" callback gets called the calculated samples on the buffer are simply copied to JACK's output buffer, and the next samples calculation is scheduled, and so on. If for some reason the samples aren't ready when the "process" callback is called, zeros are returned instead.

When the client is deactivated the buffer is zeroed. FIXME

One of the problems with JS is that there aren't fixed integers, it's all floating point numbers. And I decided to pass time in seconds instead of sample number, which makes it worse. Consequence: you will loose a bit of precision in the time, and thus in the generated samples. I don't think it's noticeable though.