Drum machines are sequencers with a synthesizer component that is tailored to the MIDI note numbers specified for drums. The General MIDI specification reserves MIDI channel 10 for this purpose. They are specialized for the creation of rhythms by playing synthesized or sampled drum sounds in a predetermined order.
The original drum machines were referred to as rhythm machines because they only played preprogrammed rhythms such as mambo, tango, etc. About 1980 user-programmable drum machines appeared, allowing musicians to create any rhythm they wanted. The Roland TR-808 was one of the first and most popular of the programmable drum machines and the sounds that are particular to that machine have become pop music clichés, heard on countless recordings. Early examples such as the TR-series used a method of synchronization called DIN-synch, or synch-24. Some of these machines also output analog voltages CV/Gate that could be used to synchronize or control analog synthesizers and other music equipment.
Drum machines are typically programmed by specifying which sixteenth notes of a bar a given drum will sound on. By stringing differently-programmed bars together, fills, breaks, rhythmic changes, and longer phrases can be created. Drum machine controls typically include Tempo, Start and Stop, volume control of individual sounds, keys to trigger individual drum sounds, and storage locations for a number of different rhythms. Most drum machines can also be controlled via MIDI.
Stand-alone drum machines had become less common by the year 2000, being partly supplanted by samplers, computer software-based sequencing with virtual drum machines, and workstation synthesizers that have drum sequencing built in. TR-808 and other digitized drum machine sounds can be found on archives on the Internet. However, drum machines are still being made by companies such as Roland Corporation (under the name Boss), Zoom, and Alesis, whose SR16 drum machine has remained popular since the early 1990s.