MIDI 1.0 - Message Format

Message Format

Every MIDI connection is a one-way connection from the MIDI Out connector of the sending device to the MIDI In connector of the receiving device. Each such connection can carry a stream of MIDI messages, with most messages representing 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. All of those messages include channel number. There are 16 possible channels in the protocol. The channels are used to separate "voices" or "instruments", somewhat like tracks in a multi-track mixer.

The ability to multiplex 16 "channels" onto a single wire makes it possible to control several instruments at once using a single MIDI connection. When a MIDI instrument is capable of producing several independent sounds or "voices" simultaneously (a multitimbral instrument), MIDI channels are used to address these sections independently. (This should not be confused with "polyphonic"; the ability to play several notes simultaneously in the same "voice".)

In more detail, MIDI 1.0 defines several basic message types of channel messages:

• Note messages can represent any note from '''C (i.e. five octaves below middle C or 8.176 Hz in common Western musical tuning; designated as MIDI note 0) to g (i.e. five octaves above the G above middle C or 12,544 Hz; designated as MIDI note 127) with precision down to the semitone. A note-on message starts a note, and a separate note-off message is needed to end it. If running status is 'active', a note-on message with the velocity byte set to zero is used in place of a note-off message.

• Pitch-bend messages range in ±2 semitones (sometimes adjustable with Registered Parameter Numbers), with precision of 1/8192 semitone (The human ear cannot hear the difference between adjacent pure tones that differ by less than 1/20 semitone). Most synthesizers allow you to adjust the pitch bend range over several octaves.

• "Control Change" messages (frequently wrongly called Continuous Controller) are quite versatile; they are usually generated by a musician using knobs, sliders, footswitches, or pressure on a physical MIDI controller (or MIDI-equipped instrument). While the response to these messages is generally totally up to the receiving device, they are typically used to change the tone, timbre, or volume of an instrument's sound. In non-musical applications of MIDI, Control Change messages can be used to move motorized faders, to dim lights, or even to move a motorized joint in an animatronic figure.

• Program change messages are sent to an instrument on a particular channel to instruct it to recall another patch, or program. The MIDI protocol uses 7 bits for this message, supporting only 128 programs to change to. Many devices which are more modern than the MIDI specification store far more than 128 programs. To overcome the limitation, a bank-switching method has been added to the spec (Each bank of 127 programs can be selected using a controller message, enabling access to 127² = 16129 programs).

• Aftertouch messages (also known as Poly Pressure messages) are sent in some instruments to indicate pressure changes on the note while it is being played. Similarly, channel pressure changes the pressure for the entire instrument, not just one note. The channel pressure messages are more commonly implemented in most synthesizers, while the individual pressure sensors that aftertouch messages require are reserved mainly for expensive, high-end synthesizers.

In addition to the channel-based messages, there are system-related messages not addressed to any particular channel. These include:

• 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.

• Real Time System Exclusive messages include the significant MIDI Show Control extension which enables all types of entertainment equipment to easily communicate with each other through the process of show control.

• 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 can be used to provide facilities for playing in musical tunings different from the 12 tone per octave, equal-tempered tuning used in most western musical traditions. However, apart from using pitch-bend to control each note, these features have not been implemented by all instrument manufacturers.