An arpeggiator is a device that automatically steps one note at a time through a group of notes that are held down or latched together . Some will go up or down or up and down through the notes that you hold down . Some you can specify the order in some random ways . It can be very useful in dance music , it's almost like having an analogue sequencer in that while the notes are playing - you can open and close the filter for a very electronic feel . That is if your synth is cool enough to have real time controls .