1. Messages

Each MIDI connection carries a stream of MIDI messages made up of 8-bit bytes. The messages are extremely compact, due to the low bandwidth of the connection and the need for real-time accuracy. Most messages consist of a status byte (channel number in the low 4 bits, for a total of 16 channels, and an opcode in the high 4 bits), followed by one or two data bytes.

The channels are used to control several instruments at once using a single MIDI connection. A multitimbral instrument is capable of producing several independent sounds or "voices" simultaneously on separate MIDI channels.

To further optimize the data stream, "Running status" convention allows the status byte to be omitted if it would be the same as that of the previous message. This is possible since status byte always starts with high bit 1, while all data bytes are encoded as 7-bit values and always start with high bit 0.

MIDI messages can be channel messages sent on only one of the 16 channels and monitored only by devices on that channel, or system messages that all devices can receive.

1.1. Channel Voice Messages

Channel Voice messages transmit real-time performance data over a single channel. Most messages represent a common musical performance event or gesture such as note-on, note-off, controller value change (including volume, pedal, modulation signals, etc.), pitch bend, program change, aftertouch, channel pressure.

• Note messages represent notes numbered from 0 to 127) with precision down to the semitone. A note-on message starts a note, and a separate note-off message (or note-on message with the velocity set to zero) is needed to end it.
• Pitch-bend Wheel messages range in ±2 semitones (adjustable with Registered Parameter Number 0x0000), with precision of 1/8192 semitone.
• Control Change messages are usually generated using knobs, sliders, footswitches, or pressure on a physical MIDI controller. They are typically used to change the tone, timbre, or volume of an instrument's sound, and change parameters like pitchbend range and musical tuning.
• Program Change messages recall another patch/program on a particular channel, with additional Bank Select controllers that enable access to 16,384 banks of 128 programs.
• Aftertouch messages include Polyphonic Pressure to indicate pressure changes on each note while it is being played, or Channel Pressure messages that indicate pressure changes on the entire keyboard instead of individual keys.

^O Optional (not widely implemented).

1.2. Registered Parameter Numbers

Registered Parameters provide additional control over instrument setup. Control Changes #101 and #100 select the parameter number, then Control Changes #6 and #38 set the actual data, while #96 and #97 set relative changes. The same approach is used for Non-Registered Parameter Numbers .

1.3. Channel Mode Messages

Channel Mode messages are controller changes 120-127 which affect the entire instrument; they include the Omni/mono/poly mode on and off messages, as well as messages to reset all controllers to their default state or to send "note-off" messages for all notes.

1.4. System Common Messages

System messages contain meta-data about other MIDI messages. A sequencer, for example, often sends MIDI clock messages during playback that correspond to the MIDI timecode, so the device receiving the messages (usually a synthesizer) will be able to keep time. Also, some devices will send Active Sense messages, used only to keep the connection between the sender and the receiver alive after all MIDI communication has ceased. MIDI time code is an example of a System Common message. System messages do not include channel numbers, and are received by every device in the MIDI chain.

1.5. System Exclusive Messages

System Exclusive (SysEx) messages are proprietary manufacturer's defined control data. SysEx messages are addressed to a specific device in a system. Each manufacturer has a unique identifier and instruments also include a SysEx ID setting, to address two devices of the same model independently.

Manufacturer's System Exclusive messages (also known as Manufacturer SysEx, Manuf Sysx, etc.) are defined by the manufacturer of the sequencer/synthesizer and can be any length. These messages are commonly used to send non-MIDI data over a MIDI connection, such as patch settings, a sound sample, or a sequencer's memory dump. Because they are defined by the device's manufacturer, they are mainly used for backup purposes and rarely (if ever) useful in another MIDI device.

Start of SysEx is followed by either a Manufacturer ID byte, or three Manufacturer ID bytes when the first byte is zero:

F0 <ID number> <data Bytes>... F7

F0 00 <ID number> <ID number> <data Bytes>... F7

ID number and data bytes use 7-bit values and their high bit is always set to 0.

Universal System Exclusive messages

Universal System Exclusive messages are formed from Manufacturer ID number 0x7E for non-realtime and 0x7F for realtime messages, a SysEx 'Device ID' (SysEx 'channel' set in each instrument's settings) or 0x7F to broadcast to all devices, then one or two Sub-ID bytes to indicate function then data bytes:

F0 <7E or 7F> <device ID> <sub ID#1> ... <data Bytes> ... F7

They include the significant MIDI Machine Control and MIDI Show Control extensions which enable all types of equipment to easily communicate with each other, as well as MIDI Time Code and MIDI tuning standard.

System Real-Time messages

System Real-Time messages provide means for synchronization, and include MIDI clock and Active Sensing.

1.7. Implementation Chart

Devices typically do not respond to every type of message defined by the MIDI specification. A device can be configured to only listen to specific channels and to ignore the messages sent on other channels ("Omni Off" mode), or it can listen to all channels, effectively ignoring the channel address ("Omni On").

An individual device may be monophonic (the start of a new "note-on" MIDI command implies the termination of the previous note), or polyphonic (multiple notes may be sounding at once, until the polyphony limit of the instrument is reached, or the notes reach the end of their decay envelope, or explicit "note-off" MIDI commands are received). Receiving devices can typically be set to all four combinations of "omni off/on" versus "mono/poly" modes.

The MIDI implementation chart was standardized by the MMA as a way for users to see what specific capabilities an instrument has, and how it responds to messages. A specific MIDI Implementation Chart is usually published for each MIDI device within the device documentation.

1.8. Bandwidth

The serial nature of MIDI messages means that long strings of MIDI messages take an appreciable time to send, at times even causing audible delays, especially when dealing with dense musical information or when many channels are particularly active.

2. Hardware Transport

2.1. Connectors

MIDI connectors are standard 5-pin 180° DIN connectors. Only two of the five pins (pins 4 and 5) are used for MIDI signal transmission.

Most MIDI capable instruments feature a MIDI IN, MIDI OUT, and occasionally a MIDI THRU connection in the form of five-pin DIN connectors. In order to build a two-way physical connection between two devices, a pair of cables must be used. The MIDI THRU jack simply echoes the signal entering the device at MIDI-IN. This makes it possible to control several devices from a single source.

Computer sound cards included a game ports with 15-pin D-subminiature connectors that provides MIDI IN/MIDI OUT- a short adapter cable can converts the D-subminiature pinout into standard DIN connectors.^[3]

2.2. Electrical Interface

An electrical schematic of the MIDI electrical/optical interconnection. https://handwiki.org/wiki/index.php?curid=2065266

The MIDI specification for the electrical interface is based on a fully isolated current loop. It provides a one-way (simplex) digital current loop electrical connection sending asynchronous serial communication data at 31,250 bits per second. The serial transmission comes in 8-N-1 format, i.e. one start bit (must be 0), eight data bits, no parity bit and one stop bit (must be 1), so up to 3,125 bytes per second can be sent

The current loop on the transmitter side drives the LED of an opto-isolator on the receiver side. The MIDI Out port nominally provides a +5 volt source through a 220 ohm resistor out through pin 4 on the DIN connector, in on pin 4 of the receiving device's MIDI In connector, through a 220 ohm protection resistor and the LED of an opto-isolator. The current then returns via pin 5 on the MIDI In port to the originating device's MIDI Out port pin 5, again with a 220 ohm resistor in the path, giving a nominal current of about 5 milliamperes.

Only one end of the loop is referenced to ground, with the other end "floating", to prevent ground loops which may otherwise cause interference and hum in analog audio signals. The MIDI specification provides for a ground "wire" and a braid or foil shield, connected on pin 2, protecting the two signal-carrying conductors on pins 4 and 5. However to improve EMI/EMC performance and filtering of RF noise, shield can be connected to ground directly or through a small ceramic capacitors and ferrite beads can be installed in the signal path.^[4] The maximum distance is specified at 15 meters (50 feet), although it can normally go much farther.^[5]

2.3. Signal Flow

The opto-isolator must be a high-speed type, with less than 2 μs risetime. As most opto-isolators have asymmetrical positive-going and negative-going slew rates, they slightly alter the signal's duty cycle. If several MIDI devices are connected in series by daisy-chaining the MIDI THRU to the next device's MIDI-IN, the signal gets more and more distorted, until receive errors occur due to pulse narrowing.

At the physical layer (MIDI cable), a pair of wires carry the MIDI signal. The voltage difference is normally 0 volts (both at positive potential referenced to ground) in the idle state, which is seen as a '1' at the MIDI receiver due to logic inversion by the Opto-isolator. A MIDI message start bit (0) causes a voltage differential on the wire pair (current loop) which is seen at the MIDI receiver as a '0'. The 8 data bits can be either '0' (low) or '1' (high) with the stop bit (1) seen at the MIDI receiver as a '1'. To summarize:

• Logic 1 → High → no current flow → Opto-isolator LED off → MIDI receiver sees High, logic '1' (data bits, stop bit or idle)
• Logic 0 → Low → current loop flow → Opto-isolator LED on → MIDI receiver sees Low, logic '0' (data bits, start bit)