beatsPerMeasure

beatsPerMeasure is how many beats fit in one measure. It is the only input to a measure's length:

measureLength = beatsPerMeasure / tempo * 60      # seconds

It is a rational number, not an integer — (7/2) and (3/2) are valid — and it is a count of beats, not the numerator of a time signature. One beat is always 60 / tempo seconds.

Aliases

Write	Meaning
bpm	canonical short form — what the app writes and what the widget shows
beatsPerMeasure	accepted, rewritten to bpm on save

base.bpm and [3].bpm are both valid.

Defaults

Situation	Value
BaseNote in a module created from scratch	4
BaseNote in defaultModule.json	4
A note with no beatsPerMeasure expression	inherits the BaseNote's value
Nothing resolvable at all	4 (silent fallback)

beatsPerMeasure is one of the three inheriting properties, along with tempo and measureLength. [5].bpm on a note that defines none of its own yields the BaseNote's.

Where you edit it

For the whole composition: click the BaseNote (the circle to the left of time zero), edit the beatsPerMeasure row, press Save.

For one measure: click a measure bar's triangle to open the measure widget. It has two rows: a startTime row, and a Measure Duration row. The Measure Duration row is a single Raw: expression field with a Save button (the button appears once you type).

The field is pre-filled with the measure's own beatsPerMeasure expression, or — when the measure has none — with the BaseNote's, shown in DSL. Saving writes the expression you typed to that measure, creating a per-measure override.

The value is a beat count, not a time signature

beatsPerMeasure is used literally as a number of beats. Entering (7/8) gives a measure of 0.875 beats, not a seven-eight bar. For a bar of seven eighth-notes with a quarter-note beat, enter (7/2).

Expression examples

On the BaseNote

4        # four beats to the bar
3        # three
2        # two
(7/2)    # seven eighth-notes, if a beat is a quarter

Legacy JavaScript syntax

new Fraction(4)
new Fraction(3)
new Fraction(7, 2)

Reading it

base.bpm         # the BaseNote's beats per measure
[5].bpm          # note 5's (or the BaseNote's, if note 5 defines none)

Legacy JavaScript syntax

module.baseNote.getVariable('beatsPerMeasure')

Deriving a measure length

measure(base)              # the BaseNote's measure length in seconds; stored as base.ml
beat(base) * base.bpm      # the same value, written out
measure(base) * 2          # two measures
measure(base) * (1/2)      # half a measure

beat(base) * base.bpm is exactly what the shipped scale-system modules put in the BaseNote's measureLength.

Legacy JavaScript syntax

module.findMeasureLength(module.baseNote)
module.findMeasureLength(module.baseNote).mul(new Fraction(2))

Common meters

Measure length is a beat count, so a compound meter has to be expressed in the unit the tempo defines. With a quarter-note beat:

Feel	beatsPerMeasure	Measure length at 120 BPM
4/4	4	2 s
3/4	3	1.5 s
2/4	2	1 s
5/4	5	2.5 s
6/8 (six eighths)	3	1.5 s
7/8 (seven eighths)	(7/2)	1.75 s

Measure bars and mixed meters

A note with a startTime but no duration and no frequency is a measure bar. It is drawn as a vertical dashed line, never sounds, and is where a per-measure beatsPerMeasure lives. Measure bars are chained through startTime, each one starting a measure length after the previous:

{ "id": 1, "startTime": "base.t" },
{ "id": 2, "startTime": "[1].t + measure([1])" },
{ "id": 3, "startTime": "[2].t + measure([2])", "beatsPerMeasure": "3" },
{ "id": 4, "startTime": "[3].t + measure([3])" }

Measure 3 is three beats long; measures 1, 2 and 4 inherit 4 beats from the BaseNote. Because measure 4 starts at [3].t + measure([3]), shortening measure 3 pulls measure 4 — and everything anchored after it — earlier. That is the whole mixed-meter mechanism; there is no separate time-signature object.

TIP

A measure bar's length is derived from its own beatsPerMeasure and its own tempo (falling back to the BaseNote's for either). Regular notes do not get a derived measure length: for them, [N].ml resolves to the BaseNote's measure length unless they define measureLength explicitly.

To add measures, select the BaseNote — the row is labelled Add New Measure Chain — or the last measure of an existing chain, where it is labelled Add Measure. Then press Add. The row does not appear on a regular note, or on a measure in the middle of a chain.

Dependencies

Changing the BaseNote's beatsPerMeasure re-evaluates every measure length derived from it, and therefore every measure bar downstream in a chain, and therefore every note anchored to those measures. Referencing another note's bpm ([5].bpm) creates a real dependency on note 5.

In the workspace

Vertical dashed lines are the measure bars, one per measure, spaced by each measure's own length. They move when tempo or beatsPerMeasure changes. (The horizontal dotted lines are the octave guides — see frequency.)

Each measure bar carries a triangle handle at the top; drag it to move the measure, click it to open the measure widget.

See also

• tempo
• duration
• startTime
• Working with Measures