Interval

This page details the Interval class and all its methods.

Creating Intervals

There are three main ways to create Interval vectors from scratch. Directly, using the class constructor; using strings via the static methods Interval.fromName or Interval.fromSPN, or from two Pitch vectors via Interval.between.

Class Constructor

constructor(w: number, h: number);

Internally an interval is defined as some number of whole steps and some number of half steps from the unison. You can directly initialise an Interval like so:

let m = new Interval(5, 2); // coordinates for an octave

m.w; // 5
m.h; // 2

fromName

static fromName(name: string): interval;

It’s often more convenient to initialise an Interval from a standard interval name as a string. Interval.fromSPN parses such a string:

let m = Interval.fromName("P8");

m.w; // 5
m.h; // 2

fromSPN

static fromSPN(ps: string, qs: string): interval;

Initialises an interval from two note names specified as SPN strings. Measures the distance from the first Pitch to the second.

let m = Interval.fromSPN("C4", "C5");

m.w; // 5
m.h; // 2

between

static between(p: pitch, q: pitch): interval;

Measures the distance from the first Pitch to the second pitch. That is, Interval.between(p, q) returns the vector $q - p$:

let p = Note.fromSPN("C4");
let q = Note.fromSPN("C5");
let m = Interval.between(p, q);

m.w; // 5
m.h; // 2

Common Queries

The following methods extract information from an Interval vector.

chroma

get chroma(): number;

The signed distance of an Interval from the unison in perfect fifths:

let m = Interval.fromName("P5");
m.chroma; // 1

m = Interval.fromName("M3");
m.chroma; // 4

m = Interval.fromName("m3");
m.chroma; // -3

isDiatonic

get isDiatonic(): boolean;

Affirms whether a given interval is capable of occurring diatonically (i.e. is found within the major scale).

let m = Interval.fromName("P5");
m.isDiatonic; // true

m = Interval.fromName("A4");
m.isDiatonic; // true

m = Interval.fromName("A6");
m.isDiatonic; // false

quality

get quality(): number;

The quality component of an Interval vector’s standard name, expressed as a signed integer.

• 0 means perfect.
• +1 means major.
• -1 means minor.
• +2 means augmented.
• -2 means diminished.
• etc. for any arbitrary quality.

let m = Interval.fromName("P5");
m.quality; // 0

m = Interval.fromName("M3");
m.quality; // 1

m = Interval.fromName("m3");
m.quality; // -1

m = Interval.fromName("A6");
m.quality; // 2

stepspan

get stepspan(): number;

The “stepspan” of an Interval is the number of diatonic steps it contains, where 0 is the unison, 1 is a generic second and so on. Essentially one less than the number conventionally used to describe the generic size of intervals.

let m = Interval.fromName("M6");
m.stepspan; // 5

m = Interval.fromName("P8");
m.stepspan; // 7

pc7

get pc7(): number;

The 0-indexed 7-tone pitch class of an Interval. Essentially its size in steps, modulo 7.

let m = Interval.fromName("M6");
m.pc7; // 5

m = Interval.fromName("M13");
m.pc7; // also 5

pc12

get pc12(): number;

The 12-tone pitch class interval of an Interval vector.

let m = Interval.fromName("P5");
m.pc12; // 7

name

get name(): string;

The standard name of an Interval rendered as a string.

let m = Interval.fromName("d5");
m.name; // "d5"

simple

get simple(): interval;

The simple

negative

get negative(): interval;

The flipped version of an Interval. An ascending major 3rd will become a descending major 3rd. Or for vertical intervals, voices will exchange places.

let m = Interval.fromName("M3");
let p = Note.fromSPN("C4");

p.transposeReal(m); // E4
p.transposeReal(m.negative); // Ab3

isEqual

isEqual(m: interval): boolean;

Returns true if and only if the passed in Interval vector and the current vector are identical. Note that this does not mean “are enharmonically equivalent in 12TET”, or “have the same chroma”: they must literally hold the same coordinates.

let m = Pitch.fromName("A4");
let n = Pitch.fromName("D5");

m.isEqual(n); // false

n = Pitch.fromName("A11");
m.isEqual(n); // false

n = Pitch.fromName("A4");
m.isEqual(n); // true

isEnharmonic

isEnharmonic(m: interval, edo?: number): boolean;

Returns true if the passed in Interval is enharmonic to the current Interval in the passed in EDO tuning. If no second argument is passed this method defaults to 12TET. Intervals will still register as enharmonic if one is simple and the other is compound.

let m = Interval.fromName("m7");
let n = Interval.fromName("A6");

m.isEnharmonic(n); // true (in 12TET)
m.isEnharmonic(n, 31); // false (in 31EDO)

m = Interval.fromName("AAAA6");
n = Interval.fromName("P8");

m.isEnharmonic(n); // false (in 12TET)
m.isEnharmonic(n, 31); // true (in 31EDO)

Transformations

The following methods produce a new vector from the current Interval. None of them mutate the original object.

add

add(m: interval): interval;

Adds two Interval vectors together.

let m = Interval.fromName("M3");
let n = Interval.fromName("m3");

m,add(n); // P5

subtract

subtract(m: interval): interval;

Subtracts two Interval vectors.

let m = Interval.fromName("P5");
let n = Interval.fromName("m3");

m.subtract(n); // M3

Ranges

range.diatonic

diatonic(start?: Interval, end?: Interval): Generator<Interval, void, unknown>;

Generates a range of Interval vectors between the two boundary Intervals passed-in, such that all intervals found diatonically occur.

If no boundary Intervals are specified, defaults to P1 - P8.

let m = Interval.fromName("P1");
let n = Interval.fromName("M9");
Interval.range.diatonic(m, n).forEach(k => console.log(k.name));
// P1 m2 M2 M3 P4 d5 A4 P5 m6 M6 m7 M7 P8 m9 M9

range.melodic

melodic(): Generator<Interval, void, unknown>;

Generates all the intervals traditionally considered “easily singable”. Essentially no dissonant leaps or augmented/diminished intervals, nor leaps bigger than an octave.

Interval.range.melodic().forEach(m => console.log(m.name));
// P1 m2 M2 M3 P4 P5 m6 M6 P8