Sequencer (for electronic music)

A sequencer used for music is a device that sequences musical events for predictable playback. For early electronic instruments these sequencers often take the form of a collection of switches that are cued in series and which trigger some other device like an VCA. The switches may also be used to control a variable voltage source which controls the pitch of an oscillator. The sequencer most commonly has a number of switches that is a multiple of 2 like 4, 8, 12, 16, and has a looping function so that the series of switching may restart as soon as the last switch in series is triggered.

Image source http://owyheesound.com/sequencer.php

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