um-sequencer

A simple and generic JavaScript music sequencer library with no dependencies.

Installation

yarn add um-sequencer or npm install um-sequencer --save

Example

import Sequencer from 'um-sequencer'

let audioContext = new window.AudioContext()
let sequencer = Sequencer(() => audioContext.currentTime)

sequencer.play([
  { time: 0, callback: Beep(440) },
  { time: 1, callback: Beep(440) },
  { time: 2, callback: Beep(440) },
  { time: 3, callback: Beep(880) }
], {
  tempo: 140,
  loopLength: 4
})

setTimeout(sequencer.stop, 5000)

function Beep(freq) {
  return (time) => {
    let osc = audioContext.createOscillator()
    osc.frequency.value = freq
    osc.connect(audioContext.destination)
    osc.start(time)
    osc.stop(time + 0.1)
  }
}

API

Sequencer(getCurrentTime, [opts])

Creates a sequencer object with the given clock source & timing options.

getCurrentTime is a function which should return the current time in seconds. The most typical source for this would be the Web Audio API AudioContext currentTime property, but it could be any high-resolution clock.

opts is an optional config object with the attributes interval, to set the time between clock ticks, and lookahead, to set the window of time after a tick in which found events will be scheduled. If you do not understand these parameters, don't mess with them. For more information see A Tale Of Two Clocks.

sequencer.play(events, [opts])

Begins playback of a sequence of events with the given options.

events is an array of event objects each of which should have two attributes, time, the time in quarter-notes from the beginning of the sequence at which the event should fire, and callback, the function to be called at that time.

opts is an optional config object with the attributes tempo, the tempo in bpm at which to play the sequence, and loopLength, a length in quarter-notes at which to loop the sequence. If no loopLength is specified, the sequence will not loop.

sequencer.stop()

Stops playback of the sequence.

TODO

• Tests
• Change tempo during playback
• Optionally run timer in a web worker
• Handle changing sequence during playback (same length)
• Handle changing sequence during playback (different length)