A synthesizer specially designed to produce drum sounds, and usually incorporating a sequencer capable of producing combinations of typical drum patterns (repeating sequences of usually 4 or 8 bars of drumming, programmed in the manner in which a drummer might play a certain style of music). Two vastly different schools of though regarding drum machines have developed over the years: analog machines which use all-analog circuitry to simulate drum and percussion sounds, and sample playback machines which are usually loaded with samples of real drum sounds. Some of the latter are romplers, which have little capability beyond reproducing canned sounds. Most modern drum machines are MIDI controlled. Certain drum sounds are mapped to note numbers, so that a note on message with the proper number will play the drum sound. Unfortunately, there is no standard for how this should be done, and some machines are notorious for their seemingly random note assignments.
Drum machines have their own subculture within the synth scene. In particular, the Roland TR-series of analog drum machines, originally manufacturered in the '80s, became popular with hip-hop artists who discovered that by modifying the machines and tweaking the calibration settings, they could produce the types of sounds popular in hip-hop, such as the earth-shaking, nearly subsonic kick drum popular within the genre.