Midi (Musical Instrument Digital Interface) is a digital interface allowing the user to control virtual music instruments on his computer or on other hardware components. In general, the Midi protocol is used to exchange commands like "play note X with velocity Y on instrument Z" or similar.
Those commands are usually stored and exchanged using so-called Midi events. They can contain information about the notes which are currently played, the sounds which belong to each (virtual) instrument, or general information about the song, e.g. about the key, the meter, or the song's name. While the above mentioned commands are definitely the most often used, there are many other commands, which can e.g. control the sustain pedal of a piano or define an instrument's overall volume. The events are presented below in more detail.
The different virtual instruments are organized in tracks, where each track refers to one instrument or a group of instruments. Hence, each Midi event is assigned to a track. Each note will be assigned to the track of the instrument which plays the note, while events, which do not belong to a specific instrument (key, meter, etc.), are usually assigned to a specific track ("Tempo Track"). Moreover, each note and other instrument-specific events are assigned to a channel. There are 16 channels where each channel has a defined sound (e.g. "Piano" or "String Ensemble"). While it may sound redundant to have each note assigned to a track and a channel, the concept makes sense. This is, because the information about the different tracks is only known to the device requesting another device to play the note. The other device will only receive information about the channel on which to play the note, which reduces the traffic needed to exchange the information. Hence, the tracks are (mostly) for a user's orientation when editing a file. Usually, all notes within a single track are also assigned to one specific channel. However, there are exceptions, e.g. when a group of strings is divided into two parts where one part plays pizzicato and the other part plays legato; in this case both groups can be noted in the same track but they have to use different channels, as each group has another sound.
The following list shows the different Midi events and provides an overview of their functionality.
Each note is represented by two events, where one event indicates the start of a note and the other event is used to stop the note. The note height is represented by a number from 0 (C0) to 127 (G10) and the note's velocity (loudness) has a range from 0 to 127. The note is assigned to a channel, where the channel's configuration specifies the voice of the note.
A Tempo Change Events sets the current tempo. The tempo is set in beats per minute (BPM). Higher values indicate a faster tempo.
Any meter change is represented by a Time Signature Event in Midi. The event holds two parameters, the numerator and the denominator of the meter. E.g., in order to represent an alla breve meter, the numerator would be set to 2 and the denominator would be set to half notes.
A Key Signature Event defines the song's key (C major, G minor, ...).
In order to define the voice (or instrument) assigned to each channel, a Program Change Event can be used. E.g., after the program Church Organ has been set for channel 5, all notes played on channel 5 will be played with an organ's voice.
Besides the voice assigned to each channel, a lot of other properties regarding a channel's sound can be controlled using Control Change Events. Up to 128 different controls exist for each channel where a value between 0 and 127 can be assigned to each control. The available controls include the sustain pedal of a piano, the channel's overall volume, and the modulation wheel.
These events control the pressure for each note (Key Pressure Events) or for all notes played on a given channel (Channel Pressure Events).
The Pitch Bend Events are used to modify the pitch of a given channel.
Lyrics, instrument/tracknames or general information (Copyright etc.) can be stored in TextEvents.
System Exclusive Events are used by manufacturers to provide a way to send device-specific messages.
While synthesizers can easily be installed and run on a computer, a lot of hardware exists which can be used to play or record Midi data (e.g. digital pianos or hardware synthesizers). Such additional hardware is usually connected to the computer using a USB port. Whether or not a synthesizer or a keyboard is running on a computer or an additional hardware which is connected to a computer does not affect the way how the device is used within a Midi software (e.g. MidiEditor). In both cases, a Midi port will be existing for each device to allow the software to receive data from or to send data to the device.