A MIDI controller is any hardware or software that generates and transmits Musical Instrument Digital Interface (MIDI) data to MIDI-enabled devices, typically to trigger sounds and control parameters of an electronic music performance.
MIDI controllers usually do not create or produce musical sounds by themselves. MIDI controllers typically have some type of interface which the performer presses, strikes, blows or touches. This action generates MIDI data (e.g. notes played and their intensity), which can then be transmitted to a MIDI-compatible sound module or synthesizer using a MIDI cable. The sound module or synthesizer in turn produces a sound which is amplified through a loudspeaker.
The most commonly used MIDI controller is the electronic musical keyboard MIDI controller. When the keys are played, the MIDI controller sends MIDI data about the pitch of the note, how hard the note was played and its duration. Other common MIDI controllers are wind controllers, which a musician blows into and presses keys to transmit MIDI data, and electronic drums.
The MIDI controller can be populated with any number of sliders, knobs, buttons, pedals and other sensors, and may or may not include a piano keyboard. Many Audio control surfaces are MIDI-based and so are essentially MIDI controllers.
While the most common use of MIDI controllers is to trigger musical sounds and play musical instruments, MIDI controllers are also used to control other MIDI-compatible devices, such as stage lights, digital audio mixers and complex guitar effects units.
MIDI was designed with keyboards in mind, and any controller that is not a keyboard is considered an "alternative" controller. This was seen as a limitation by composers who were not interested in keyboard-based music, but the standard proved flexible, and MIDI compatibility was introduced to other types of controllers, including guitars, wind instruments and drum machines.
Keyboards are by far the most common type of MIDI controller. These are available in sizes that range from 25-key, 2-octave models, to full-sized 88-key instruments. Some are keyboard-only controllers, though many include other real-time controllers such as sliders, knobs, and wheels. Commonly, there are also connections for sustain and expression pedals. Most keyboard controllers offer the ability to split the playing area into "zones", which can be of any desired size and can overlap with each other. Each zone can respond to a different MIDI channel and a different set of performance controllers, and can be set to play any desired range of notes. This allows a single playing surface to target a number of different devices. MIDI capabilities can also be built into traditional keyboard instruments, such as grand pianos and Rhodes pianos. Pedal keyboards can operate the pedal tones of a MIDI organ, or can drive a bass synthesizer such as the revived Moog Taurus.
Wind controllers allow MIDI parts to be played with the same kind of expression and articulation that is available to players of wind and brass instruments. They allow breath and pitch glide control that provide a more versatile kind of phrasing, particularly when playing sampled or physically modeled wind instrument parts. A typical wind controller has a sensor that converts breath pressure to volume information, and may allow pitch control through a lip pressure sensor and a pitch-bend wheel. Some models include a configurable key layout that can emulate different instruments' fingering systems. Examples of such controllers include Akai's Electronic Wind Instrument (EWI) and Electronic Valve Instrument (EVI). The EWI uses a system of keypads and rollers modeled after a traditional woodwind instrument, while the EVI is based on an acoustic brass instrument, and has three switches that emulate a trumpet's valves. Simpler breath controllers are also available: unlike wind controllers, they do not trigger notes and are intended for use in conjunction with a keyboard or synthesizer. Examples of breath controllers are the Yamaha BC3 (now discontinued) and the TEControl USB-MIDI Breath Controller.
Drum and percussion controllers
Keyboards can be used to trigger drum sounds, but are impractical for playing repeated patterns such as rolls, due to the length of key travel. After keyboards, drum pads are the next most significant MIDI performance controllers. Drum controllers may be built into drum machines, may be standalone control surfaces, or may emulate the look and feel of acoustic percussion instruments. The pads built into drum machines are typically too small and fragile to be played with sticks, and are played with fingers. Dedicated drum pads such as the Roland Octapad or the DrumKAT are playable with the hands or with sticks, and are often built in the form of a drum kit. There are also percussion controllers such as the vibraphone-style MalletKAT, and Don Buchla's Marimba Lumina. MIDI triggers can also be installed into acoustic drum and percussion instruments. Pads that can trigger a MIDI device can be homemade from a piezoelectric sensor and a practice pad or other piece of foam rubber.
Stringed instrument controllers
A guitar can be fit with special pickups that digitize the instrument's output, and allow it to play a synthesizer's sounds. These assign a separate MIDI channel for each string, and may give the player the choice of triggering the same sound from all six strings, or playing a different sound from each. Some models, such as Yamaha's G10, dispense with the traditional guitar body and replace it with electronics. Other systems, such as Roland's MIDI pickups, are included with or can be retrofitted to a standard instrument. Max Mathews designed a MIDI violin for Laurie Anderson in the mid-1980s, and MIDI-equipped violas, cellos, contrabasses, and mandolins also exist.
Specialized performance controllers
DJ digital controllers may be standalone units such as the Faderfox or the Allen & Heath Xone 3D, or may be integrated with a specific piece of software, such as Traktor or Scratch Live. These typically respond to MIDI clock sync, and provide control over mixing, looping, effects, and sample playback.
MIDI triggers attached to shoes or clothing are sometimes used by stage performers. The Kroonde Gamma wireless sensor can capture physical motion as MIDI signals. Sensors built into a dance floor at the University of Texas at Austin convert dancers' movements into MIDI messages, and David Rokeby's Very Nervous System art installation created music from the movements of passers-through. Software applications exist which enable the use of iOS devices as gesture controllers.
Numerous experimental controllers exist which abandon traditional musical interfaces entirely. These include the gesture-controlled Buchla Thunder, sonomes such as the C-Thru Music Axis, which rearrange the scale tones into an isometric layout, and Haken Audio's keyless, touch-sensitive Continuum playing surface. Experimental MIDI controllers may be created from unusual objects, such as an ironing board with heat sensors installed, or a sofa equipped with pressure sensors. GRIDI is a large scale physical MIDI sequencer with embedded LEDs developed by Yuvi Gerstein in 2015, which uses balls as inputs. The Eigenharp controller is a combination of a breath controller, a configurable series of multi-dimensional control keys, and ribbon controllers designed to control its own virtual instrument software.