MIDI (Musical Instrument Digital Interface) is a technical standard that describes a communications protocol, digital interface, and electrical connectors used to connect a wide variety of electronic musical instruments, computers, and related audio devices. Developed in the early 1980s, MIDI allows devices to communicate performance data (such as note events, control changes, and synchronization signals) without transmitting audio itself. It remains a cornerstone of music production, live performance, and multimedia systems.

1 History

1.1 Origins and development (1981–1983)

The concept of a universal digital interface for synthesizers emerged in 1981, when Sequential Circuits founder Dave Smith proposed a “Universal Synthesizer Interface” at the Audio Engineering Society convention. Collaborating with Roland, Korg, Yamaha, and other manufacturers, the group refined the specifications throughout 1982. The first public demonstration of interconnected synthesizers using this protocol occurred at the 1983 NAMM Show, where a Sequential Circuits Prophet-600 and a Roland Jupiter-6 exchanged note data.

1.2 Standardization and adoption

Following the 1983 debut, the standard was formalized as “MIDI 1.0” and published by the International MIDI Association (IMA). Adoption was rapid: by 1984, nearly all new synthesizers, drum machines, and sequencers included MIDI ports. The standard’s open nature and low implementation cost encouraged widespread use across both professional and consumer-grade equipment.

1.3 Evolution and modern extensions (MIDI 2.0)

For nearly four decades, MIDI 1.0 remained unchanged at its core. In 2020, the MIDI Manufacturers Association (MMA) and the Association of Musical Electronics Industry (AMEI) released MIDI 2.0, a major backward-compatible update. It introduced higher resolution (32-bit instead of 7-bit), bidirectional communication, property exchange for device discovery, and profile configuration. Adoption has gradually increased, with new hardware and software incorporating MIDI 2.0 features while retaining compatibility with the original protocol.

2 Technical specifications

2.1 Hardware interface

2.1.1 Connectors (5‑pin DIN, USB, TRS)

The original MIDI connector is a 5‑pin DIN (Deutsches Institut für Normung) socket, with pins 4 and 5 carrying the current-loop signal and pin 2 a shield. Three types of connectors are defined: MIDI In, MIDI Out, and MIDI Thru (which repeats incoming data). In the 2000s, USB became a common alternative, especially for computer-based controllers. More recently, 3.5 mm TRS (tip‑ring‑sleeve) mini-jacks have been adopted for space‑constrained devices, following a standardized wiring scheme (Type A/B).

2.1.2 Cabling and electrical characteristics

MIDI cables are typically shielded twisted‑pair with a 5‑pin DIN plug at each end. The electrical interface uses a 5 mA current loop, opto‑isolated at the receiver to prevent ground loops. The maximum cable length is specified as 15 meters (50 feet), though longer runs are possible with good shielding. The data rate is 31.25 kbit/s (asynchronous serial).

2.2 Protocol

2.2.1 Message types

MIDI messages are divided into two broad categories: channel messages and system messages. Each message begins with a status byte, followed by one or two data bytes.

2.2.1.1 Channel voice messages (Note On/Off, Aftertouch, Pitch Bend)

Channel voice messages carry performance data for a specific MIDI channel. Note On (velocity 1–127) and Note Off (velocity often ignored) start and stop a note. Aftertouch (channel pressure or key pressure) applies additional expressive force. Pitch Bend uses two data bytes for a 14‑bit range. Other voice messages include Control Change (e.g., modulation wheel, sustain pedal) and Program Change (selects a preset sound).

2.2.1.2 Channel mode messages (Omni, Poly, Mono)

Channel mode messages change how a receiver responds to incoming data. Omni On/Off enables/disables response to all channels (Omni mode) or only the assigned channel. Poly switches to polyphonic operation, Mono to monophonic (one voice per channel). All Notes Off and Reset All Controllers are also channel mode messages.

2.2.1.3 System messages (SysEx, timing clock, start/stop)

System messages are not channel‑specific. System Exclusive (SysEx) allows manufacturer‑specific data, such as patch dumps or firmware updates, using a variable‑length format. System Real-Time messages include Timing Clock (24 pulses per quarter note), Start, Continue, Stop, and Active Sensing. System Common messages include Song Position Pointer, Song Select, and Tune Request.

2.2.2 MIDI channels and routing

MIDI 1.0 defines 16 logical channels (1–16). Each device or virtual track can be assigned to one or more channels. Routing can be simple (one‑to‑one) or complex (daisy‑chaining via MIDI Thru, or using a MIDI merger/patchbay). In MIDI 2.0, the channel count is conceptually extended by using 16 “groups” of 16 channels, but the physical transport remains 16 channels per cable.

2.2.3 Running status byte

To reduce data overhead, MIDI implements “running status”: once a status byte (e.g., Note On) is transmitted, subsequent data pairs for the same message type omit the status byte until a different message type is sent. This reduces bandwidth usage by approximately 30% in typical passages.

2.3 Data format

2.3.1 Standard MIDI File (.mid) structure

Standard MIDI Files (SMF) store performance data as a sequence of timestamped events. They use a chunk‑based binary format.

2.3.1.1 Header chunk

The header chunk (type “MThd”) specifies the file format type (0, 1, or 2), the number of tracks, and the division (ticks per quarter note or SMPTE frame rate). It is always the first chunk in the file.

2.3.1.2 Track chunks and events

Each track chunk (type “MTrk”) contains a sequence of MIDI events (channel voice/system messages), meta‑events (non‑audible data), and SysEx events. Events are preceded by a variable‑length “delta‑time” indicating the time since the previous event. Track chunks can hold any combination of note, controller, and system data.

2.3.1.3 Meta events (tempo, time signature, lyrics)

Meta‑events are embedded in tracks and begin with 0xFF followed by a type byte. Common meta‑events include: Set Tempo (microseconds per quarter note), Time Signature (numerator, denominator power of 2, clocks per tick, 32nd notes per quarter), Key Signature, Lyrics, Cue Point, and End of Track. These events allow SMF files to carry musical instructions beyond pure note data.

2.4 MIDI 2.0 enhancements

2.4.1 Higher resolution and bidirectional communication

MIDI 2.0 replaces the 7‑bit data values of MIDI 1.0 with 32‑bit values for note velocity, aftertouch, pitch bend, and continuous controllers. This enables smooth, high‑precision expression. Bidirectional communication allows devices to discover each other’s capabilities (e.g., available sounds, number of voices) and negotiate settings without manual configuration.

2.4.2 Property exchange and profile configuration

Property Exchange enables devices to share extended metadata (such as patch names, user‑friendly device names, and controller assignments) using JSON‑like structures. Profiles define standardized sets of behavior or sound mapping (e.g., a “Grand Piano” profile could automatically load a specific sound engine configuration). This reduces setup complexity and improves interoperability.

3 Applications and usage

3.1 Music production and sequencing

3.1.1 Digital audio workstations (DAWs) and MIDI tracks

In a DAW (e.g., Ableton Live, Logic Pro, Cubase), MIDI tracks are used to record, edit, and sequence note data. Users draw or play in notes, adjust velocities, apply controllers, and route MIDI to virtual instruments (VST/AU) or external hardware. The MIDI data can be quantized, transposed, and manipulated non‑destructively, making it a flexible composition tool.

3.1.2 Hardware sequencers and grooveboxes

Hardware sequencers (e.g., Elektron Octatrack, Roland TR‑808) and grooveboxes (e.g., Akai MPC) rely on MIDI to trigger internal sound engines or external devices. They often feature step‑based programming and real‑time recording, with built‑in synchronization over MIDI clock.

3.2 Live performance

3.2.1 Controllers (keyboards, pad controllers, wind controllers)

MIDI controllers are input devices that send performance data without generating sound. Keyboard controllers (e.g., Novation Launchkey, Roland A‑Series) are the most common. Pad controllers (e.g., Akai MPC pads) trigger samples or drums. Wind controllers (e.g., Akai EWI) use breath pressure via MIDI aftertouch or controller messages. All connect via USB or traditional DIN ports.

3.2.2 MIDI over USB and wireless (Bluetooth LE MIDI)

USB‑MIDI (class‑compliant) is now standard for connecting controllers to computers and tablets without dedicated drivers. Bluetooth Low Energy (BLE) MIDI, ratified in 2015, enables wireless control over distances up to about 10 meters. BLE MIDI maintains low latency and is widely supported in DAWs and mobile apps.

3.3 Synchronization and show control

3.3.1 MIDI Time Code (MTC)

MTC is a real‑time message set that transmits SMPTE timecode (hours:minutes:seconds:frames) over MIDI. It allows sequencers, DAWs, and video players to lock together. MTC uses quarter‑frame messages (8 messages per frame) plus full‑frame “cueing” messages.

3.3.2 MIDI Machine Control (MMC)

MMC is a protocol based on SysEx that controls transport functions (play, stop, record, rewind) on multitrack recorders and DAWs. It works independently of audio timecode, often used in conjunction with MTC or MIDI clock.

3.3.3 Lighting and theatrical automation

MIDI Show Control (MSC) extends SysEx to control lighting consoles, sound effects, pyro, and other theatre equipment. Simple note‑on messages can trigger cues (e.g., a C3 note increments to the next lighting scene). Many professional lighting desks and media servers accept MSC commands.

4.1 General MIDI (GM) sound set

General MIDI (GM) is a standardized sound mapping defined in 1991. It specifies 128 instrument sounds (e.g., piano, violin, drums) arranged in a fixed order, plus a standard drum map on channel 10. GM ensures that a MIDI file sounds roughly the same on any compliant device. Extended versions (GM2, GS, XG) add more instruments and controllers.

4.2 MIDI Show Control (MSC)

MSC is a SysEx‑based extension for controlling theatrical and multimedia equipment. Commands include “Go,” “Stop,” “Load,” and “Reset,” sent to device IDs. MSC is widely used in theme parks, concerts, and Broadway shows.

4.3 Standard MIDI File (SMF) variations (Type 0/1/2)

SMF Type 0 stores all data in a single track (monophonic track chunk). Type 1 uses multiple tracks, each containing a separate part (e.g., piano track, bass track). Type 2 is rarely used; it allows independent pattern‑oriented sequences.

4.4 Alternative protocols (OSC, hui, Mackie Control)

Open Sound Control (OSC) is a newer, high‑resolution protocol often used for networked music performance. HUI and Mackie Control are proprietary protocols designed for DAW control surfaces; they emulate MIDI but carry extended data for faders, automation, and transport.

5 Cultural and industrial impact

5.1 Democratization of music production

MIDI lowered the barrier to entry for music creation. By the late 1980s, home computers with MIDI interfaces allowed hobbyists to compose entire arrangements without expensive multitrack tape. The standard enabled affordable sequencers, sound modules, and eventually software instruments, leading to a boom in bedroom production and independent music.

5.2 Role in video game music and chiptune

In the 1980s and 1990s, game consoles and PC sound cards (e.g., AdLib, Sound Blaster) used MIDI‑like protocols to play back music with minimal data. Many classic game soundtracks, from *Doom* to *Final Fantasy*, rely on General MIDI or custom sound banks. Chiptune culture later embraced the raw sound of early MIDI synthesizers and drum machines, often using MIDI sequencers to compose 8‑bit style tracks.

5.3 Legacy and continued relevance

Despite being over 40 years old, MIDI remains ubiquitous. Its efficiency, simplicity, and open licensing have never been surpassed for performance control. The introduction of MIDI 2.0 ensures its relevance for decades to come. From symphonic orchestras to electronic dance music, MIDI is the silent backbone that connects artists, instruments, and technology.