tonegenerator

ToneGenerator for node.js

This thing generates raw PCM data, specified by a frequency and length in seconds.

Generating Tones:

tone(frequency, lengthInSeconds, volume = 30, sampleRate = 44100)

volume and sampleRate are optional, the default is shown above. If you want to specify sampleRate, you have to specify volume!

var tone = require('tonegenerator');
var A440 = tone(440, 20, 30); // get PCM data for a 440hz A, 20 seconds, volume 30
var A440_low_sample = tone(440, 20, 30, 22050); // this array has lower sample rate and will only be half as long

The data is returned as a normal array, so you can do operations on it.

Combining notes

// An A-major chord
var tone1 = tone(440, 2, 60)
var tone2 = tone(554.37, 2, 30)
var tone3 = tone(659.26, 2, 30)

// "playing" one tone at the time
// note that at this time, our sound is just an array
// of gain values. By appending the raw PCM data for one after another,
// we can play them in a sequence
var res = tone1.concat(tone2, tone3)

// By adding values of the tones for each sample,
// we play them simultaneously, as a chord
for(var i = 0; i < tone1.length; i++) {
  res.push(tone1[i] + tone2[i] + tone3[i])
}

Volume on 8-bit and 16-bit PCM data

The meaning of the 'volume' value depends on whether you're creating 8-bit or 16-bit data. For 8-bit data, the max volume to avoid distortion is 128. For 16-bit data, the max volume is 32768. Those values are available as require('tonegenerator').MAX_8 and require('tonegenerator').MAX_16 respectively.

Writing 8-bit data to a Wave File

Before writing your PCM data to a file, you need to convert it to a buffer of UInt8 values. 8-bit wave data goes from 0-255, so we need to add 128 to each value:

var tone = require('tonegenerator');
// Use this package to write a header for the wave file
// https://www.npmjs.org/package/waveheader
var header = require('waveheader');
var fs = require('fs');

var file = fs.createWriteStream('8bit-example.wav')
var samples = tone(440, 2, tone.MAX_8)

file.write(header(samples.length, {
  bitDepth: 8
}))

// Convert -128 -> 127 range into 0 -> 255
var data = Uint8Array.from(samples, function (val) {
  return val + 128
})

if (Buffer.from) { // Node 5+
  buffer = Buffer.from(data)
} else {
  buffer = new Buffer(data)
}
file.write(buffer)
file.end()

Writing 16-bit data to a Wave file

16-bit data requires a little bit more work, since we need to take Endianess into account. Unlike 8-bit data, the volumes does not start at 0, but at -32768.

All the references to data length need to be doubled.

var tone = require('tonegenerator');
var header = require('waveheader');
var fs = require('fs');

var file = fs.createWriteStream('16bit-example.wav')
var samples = tone(440, 2, tone.MAX_16)

file.write(header(samples.length * 2, {
  bitDepth: 16
}))

var data = Int16Array.from(samples)

var size = data.length * 2 // 2 bytes per sample
if (Buffer.allocUnsafe) { // Node 5+
  buffer = Buffer.allocUnsafe(size)
} else {
  buffer = new Buffer(size)
}

data.forEach(function (value, index) {
  buffer.writeInt16LE(value, index * 2)
})

file.write(buffer)
file.end()

Writing stereo sounds

In stereo wave data, the sample for each channel comes right after each other. The principle looks like [sample0-1 sample0-2 sample1-1 sample1-2]. So we need to first generate the data for each channel, then interleave them.

var tone = require('tonegenerator');
var header = require('waveheader');
var fs = require('fs');

var file = fs.createWriteStream('16bit-stereo.wav')
// A loud A for channel 1
var channel1 = tone(440, 2, tone.MAX_16)
// A not so loud C for channel 2
var channel2 = tone(554.37, 2, tone.MAX_16 / 4)

// create an array where the 2 channels are interleaved:
var samples = []
for (var i = 0; i < channel1.length; i++) {
  samples.push(channel1[i])
  samples.push(channel2[i])
}

file.write(header(samples.length * 2, {
  channels: 2,
  bitDepth: 16
}))

var data = Int16Array.from(samples)

var size = data.length * 2 // 2 bytes per sample
if (Buffer.allocUnsafe) { // Node 5+
  buffer = Buffer.allocUnsafe(size)
} else {
  buffer = new Buffer(size)
}

data.forEach(function (value, index) {
  buffer.writeInt16LE(value, index * 2)
})

file.write(buffer)
file.end()

Reading:

• Wave PCM SoundFile format - make sure to read the 'Notes' section
• ABC of Uncompressed digital audio