node-alsa
=========

ALSA bindings for Node.js

Requirements
------------

You must have a system on which ALSA is supported with the libasound2-dev package installed:

``sh
$ sudo apt-get install libasound2-dev
`

You'll also probably want to have an audio device for either playback or capture.


Installation
------------

Install with npm:

`sh
$ npm install alsa
`

or via git:

`sh
$ npm install git+https://github.com/xdissent/node-alsa.git
`


Usage
-----

The alsa module exports 2 high-level classes for playback and recording, a simple wrapper class around PCM operations, and a subset of the low-level libasound2 functions.

`js
var alsa = require('alsa'),

// The following variables represent the defaults for the Playback and Capture constructors.
device = 'default', // ALSA default device
channels = 2, // Stereo
rate = 44100, // Sample rate
format = alsa.libasound.PCM_FORMAT_S16_LE, // PCM format (signed 16 bit LE int)
access = alsa.libasound.PCM_ACCESS_RW_INTERLEAVED, // Access mode
async = false; // Async-mode - YOU DO NOT WANT THIS!

// The Capture class is a stream.Readable subclass.
var capture = new alsa.Capture(device, channels, rate, format, access, async);
capture.pipe(process.stdout); // Treat it like any other readable stream.

// The Playback class is a stream.Writable subclass.
var playback = new alsa.Playback(device, channels, rate, format, access, async);
process.stdin.pipe(playback); // Treat it like any other writable stream.

// The Pcm class is a wrapper around common libasound2 functions.
var pcm = new alsa.Pcm(device, alsa.libasound.PCM_STREAM_CAPTURE); // or PCM_STREAM_PLAYBACK

// Open PCM device, set parameters, and prepare (chainable).
pcm.open().access(access).format(format).channels(channels).rate(rate).prepare();

// PCM parameters are combo getters and setters.
console.log('Channels: ' + pcm.channels());

// Calculate some sizes and create a buffer.
var bytesPerFrame = pcm.bytesPerFrame(),
framesPerBuffer = 1024,
bufferSize = framesPerBuffer * bytesPerFrame,
buffer = new Buffer(bufferSize);

// Read frames from PCM device into the buffer, then get the raw frames.
var readFrames = pcm.readi(buffer, framesPerBuffer),
frames = buffer.slice(0, readFrames * bytesPerFrame);

// Low level libasound2 api access works too (pcm.pcm is a libasound2 snd_pcm_t pointer).
console.log(alsa.libasound.snd_pcm_name(pcm.pcm));
`


Examples
--------

Downsample audio captured from an ALSA device to a sample rate of 11025, in mono, encoded to 32 bit float PCM data and output to stdout:

`coffeescript
alsa = require 'alsa'
stream = require 'stream'

class Downer extends stream.Transform
constructor: -> stream.Transform.call this

# Naively drop 3 out of 4 samples, combine the interleaved left and right into
# a single sample, then convert to 32 bit float PCM data.
_transform: (chunk, encoding, callback) ->

# This is the naive part. Should be checking for partial samples.
floats = new Buffer chunk.length / 4
f = 0
for i in [0...chunk.length] by 16 # 4 bytes per frame downsampled by 4.
l16 = chunk.readInt16LE i # Read left channel sample.
r16 = chunk.readInt16LE i + 2 # Read right channel sample.
# Sum samples and normalize as a float
floats.writeFloatLE (l16 + r16) / 65536, f
f += 4 # Skip ahead 4 bytes in buffer (32 bit float).
@push floats # Write transformed samples to stream...
callback() # And we're done.

Create a Capture and a Downer and chain them to stdout.


capture = new alsa.Capture
downer = new Downer
capture.pipe(downer).pipe(process.stdout)
``