music-machine

Create music by defining a musical style. Describe the style's syntax with a generative context-free grammar.

Example

A music-machine guide will give you a list of possible next notes at each step. As an example, here is an in-progress guide which is using a grammar that follows counterpoint rules.

// guide is in-progress. This is what we have built so far:
guide.construction()    =>
[ 'D3', 'E3', 'F3', 'C3', 'D3', 'F3', 'E3', 'G3']
 
                           // these are the choices for the next note
guide.choices()         => [ 'A3', 'F3', 'E3', 'Bb2', 'C3', 'D3' ]
guide.choose('Bb2')
 
// after a large leap of a sixth from D3 -> Bb2, the grammar limits our choices
guide.choices()         => [ 'C3', 'D3' ]
guide.choose('C3')
 
guide.choices()         => [ 'D3', 'E3', 'F3', 'G3', 'Bb2' ]
guide.choose('F3')
 
guide.choices()         => [ 'E3', 'D3' ]
guide.choose('E3')
 
guide.choices()         => [ 'F3', 'G3', 'C3', 'D3' ]
guide.choose('D3')
 
guide.construction()    =>
[ 'D3', 'E3', 'F3', 'C3', 'D3', 'F3', 'E3', 'G3', 'Bb2', 'C3', 'F3', 'E3', 'D3']

Overview

This library is an extension of grammar-graph which enables music patterns to be applied in any key or mode. If you instead want to define a musical grammar in terms of literal notes, like the theme from Mozart's Jupiter Symphony, JupiterTheme: C D F E, you should use grammar-graph directly instead of this library. In music-machine, the JupiterTheme pattern would look like this:

JupiterTheme: 2  3  -2

This means to move up a second, then up a third, and then down a second. Note that the interval numbers here are defining the distance between notes, which is why only three intervals are needed to describe a four note pattern.

The JupiterTheme could then be applied to any key:

JupiterTheme in C minor:
  +2   +3   -2
C    D    F    Eb

JupiterTheme in D major:
  +2   +3   -2
D    E    G    F#

Good music is often made up of just a few small patterns which are combined to make something bigger. For example, repeating JupiterTheme twice makes a nice sequence, so we might define a larger pattern with it. I will also add a final descending fifth at the end, just for fun:

JupiterPattern: JupiterTheme  JupiterTheme  -5

JupiterPattern in C major:
 +2  +3  -2   +2   +3   -2   -5
C   D   F   E    F    A    G    C

Music Grammar

A music grammar is a list of rules in an object. Let's examine a grammar that uses the Jupiter theme:

var jupiterGrammar = {
  InfinitePhrase: [ 'JupiterTheme InfinitePhrase',
                    'SecondMotive InfinitePhrase' ],
    JupiterTheme: [ '2  3  -2' ],
    SecondMotive: [ '4  StepDown' ],
        StepDown: [ '-2',
                    '-2  StepDown']
}

Rules

A rule simply means to replace a symbol with its definition. If we are making music and come across the symbol JupiterTheme, we will replace it with its definition: 2 3 -2. Some rules have multiple options, such as StepDown which can be rewritten as -2 or -2 StepDown.

More formally, a grammar is an object consisting of key-value pairs, with each non-terminal symbol pointing to an array of one or more symbol chains choices for this non-terminal. See here for an example of a non-musical grammar in the same format that builds text creatures.

Symbol Chains

2 3 -2 and 4 StepDown are symbol chains. Each symbol is seperated by white-space, so the first symbol chain is made up of three symbols: 2, 3, -2.

Terminal Symbols

If a symbol has no definition in the grammar, it is a terminal. In a music grammar, terminals must be numbers representing intervals. The four terminal symbols in the example are: -2, 2, 3, 4.

Non-terminal Symbols

If a symbol has a definition in the grammar, it is non-terminal and can be broken down further. A non-terminal's definition is an array of one or more symbol chains indicating possible choices for this rule. The four non-terminal symbols in the example are: InfinitePhrase, JupiterTheme, SecondMotive, StepDown.

Recursive definitions

Recursive definitions are what make a grammar interesting and powerful, and ensure that the music will never stop. One recursive definition in our example is: StepDown: [ '-2', '-2 StepDown']. This allows us to either move -2 once if we immediately choose the first option, or to descend infinitely if we always choose the second option. The most conspicuous recursive symbol is: InfinitePhrase: [ 'JupiterTheme InfinitePhrase', 'SecondMotive InfinitePhrase' ]. Both of these definitions lead right back to InfinitePhrase and so will go on and on no matter which option we choose.

Do not define a non-terminal to equal only itself. This will not work: Infinity: ['Infinity']. MusicMachine must be able to reach a non-terminal (interval) from any point in the grammar.

Tutorial

Install the npm module.

npm install music-machine

Create a MusicMachine by passing it a music grammar and the name of a symbol to start at.

var MusicMachine = require('music-machine')
 
var jupiterMachine = new MusicMachine(jupiterGrammar, 'InfinitePhrase')

Use MusicMachine to create a new guide, specifiying a key.

var guide = jupiterMachine.createGuide('C major')

Get the choices for the first note.

guide.choices()    => [ 'C' ]

By default, the guide will start on scale degree 1 of the key you picked. This can be configured when creating the MusicMachine by adding an optional array of scale degree numbers as a third parameter:

var jupiterMachine = new MusicMachine(jupiterGrammar, 'InfinitePhrase', [1, 5])
var guide = jupiterMachine.createGuide('C major')
guide.choices()    => [ 'C', 'G' ]

The first set of choices will be given without an octave number, but you can specify one when you make your choice. If you do not provide on octave number, it will default to octave 4.

// pick C in octave 5
guide.choose('C5')

Looking at our next set of choices, we now have two options:

guide.choices()       => [ 'D5', 'F5' ]

Let's choose D5 and take a peek at our construction so far:

guide.choose('D5')
guide.construction()  => [ 'C5', 'D5' ]

There is only one possible note for the next few choices as we are on the JupiterTheme route in the grammar.

guide.choices()   =>  [ 'F5' ]
guide.choose('F5')

guide.choices()   =>  [ 'E5' ]
guide.choose('E5')

guide.choices()   =>  [ 'F5', 'A5' ]

At any point, you can also check the possible set of raw constructs from the grammar that could have led us to this point:

guide.constructs()    => [ '2 3 -2 2 3 -2 InfinitePhrase',
                           '2 3 -2 4 StepDown InfinitePhrase' ]
guide.construction()  => [ 'C5', 'D5', 'F5', 'E5' ]

You can optionally provide guide.choices() with a number indicating the depth of choices you want. If you request a depth greater than 1, instead of an array of strings, it will return an array of TreeNodes which are each at most nDeep. Each TreeNode is an object with a val property and a next property which points to an array of more TreeNodes.

guide.choices()   => [ 'F5', 'A5' ]
guide.choices(3)  =>
[
  {
    val: 'F5',                        // F5
    next: [
      {
        val: 'A5',                    // F5, A5
        next: [
          {val: 'G5', next: []}       // F5, A5, G5
        ]
      }
    ]
  },
  {
    val: 'A5',                        // A5
    next: [
      {
        val: 'G5',                    // A5, G5
        next: [
          {val: 'A5', next: []},      // A5, G5, A5
          {val: 'C6', next: []},      // A5, G5, C6
          {val: 'F5', next: []}       // A5, G5, F5
        ]
      }
    ]
  }
]

You can also submit multiple choices in an array.

guide.choose(['A5', 'G5', 'F5'])
guide.construction()   =>  [ 'C5', 'D5', 'F5', 'E5', 'A5', 'G5', 'F5' ]

Filters

Filters allow you to further refine a musical style. MusicMachine has some ready-made filter generators. Let's say we want to only allow major, minor, and perfect intervals (excluding diminished and augmented).

guide.choices()        =>  [ 'G5', 'B5', 'E5' ]
// F5 -> B5 is an augmented fourth (A4)
 
var limitQualities = MusicMachine.filter.allowedIntervalQualities('M', 'm', 'P')
guide.addFilter(limitQualities)
 
guide.choices()        =>  [ 'G5', 'E5' ]   // B5 is removed from choices
guide.choose('E5')

Once a filter is applied, it will affect all future choices with this machine. You can also apply filters to a MusicMachine instance directly, and all guides created with that machine will have the filters applied.

See MusicMachine.filter for a list of provided filter generators -- you can use as many filters as you'd like.

Custom Filters

It is easy to create custom filters. Your filter function will be passed the current list of choices and the current construction, and it should return the choices that pass the filter. Here is a filter which does not allow going right back to a note after leaving it:

// do not allow returning to a note right after leaving it
// if construction is [C4, D4], do not allow going back to C4
var avoidPreviousNote = function (choices, construction) {
  if (construction.length < 2) return choices   // filter does not apply
  else {
    var previousNote = construction[construction.length - 2]
    var filtered = choices.filter(function (choice) {
      return choice !== previousNote
    })
    return filtered
  }
}
 
// before adding filter
guide.choices()        => [ 'F5', 'A5', 'D5' ]
guide.construction()   => [ 'C5', 'D5', 'F5', 'E5', 'A5', 'G5', 'F5', 'E5' ]
                                                                 ^ // filter this note
// add the filter
guide.addFilter(avoidPreviousNote)
guide.choices()        => [ 'A5', 'D5' ]

To create more complicated filters, you may wish to use a library like nmusic which can parse pitch strings and analyze pitch relations.

Docs

View the api documentation here.

Development

Clone the git repository and install development dependencies:

git clone https://github.com/jrleszcz/music-machine.git
cd music-machine
npm install

To run eslint and tape tests:

npm test

To generate api documentation for api.md:

npm run docs