According to the MIDI protocol certain MIDI events are never sent over MIDI ports, but exist in MIDI files. These events are called meta events and the messages that they carry are called meta messages.
MIDI meta messages are messages that contains information about the MIDI sequence and that are not to be sent over MIDI ports.
Meta messages have the following standard structure.
|Field||Length||Starts at byte||Value|
|Status byte||One byte||0x00||0xFF always|
|Meta type||One byte||0x01||Variable, see below|
Status byte – the first byte of the MIDI meta message is the "status byte" as with any other MIDI message. The status byte of a MIDI meta message is always 0xFF.
Meta type – the "meta type" byte defines how the rest of the message should be interpreted (see below for a table of defined meta types).
Length – the "length" contains the length in bytes of the data to follow so that MIDI devices know how much data to expect (not including the status byte, meta type, and the length itself). The length of certain meta messages (see below) is predefined, whereas other meta messages have variable length.
Data – the "data" portion of the message defines, obviously, something that the MIDI device must interpret.
The following is a list of defined meta messages.
|Message ||Meta type ||Data length ||Contains ||Occurs at |
|Sequence number ||0x00 ||2 bytes ||The number of a sequence ||At delta time 0 |
|Text ||0x01 ||variable ||Some text ||Anywhere |
|Copyright notice ||0x02 ||variable ||A copyright notice ||At delta time 0 in the first track |
|Track name ||0x03 ||variable ||A track name ||At delta time 0 |
|Instrument name ||0x04 ||variable ||The name of an instrument in the current track ||Anywhere |
|Lyrics ||0x05 ||variable ||Lyrics, usually a syllable per quarter note ||Anywhere |
|Marker ||0x06 ||variable ||The text of a marker ||Anywhere |
|Cue point ||0x07 ||variable ||The text of a cue, usually to prompt for some action from the user ||Anywhere |
|Channel prefix ||0x20 ||1 byte ||A channel number (following meta events will apply to this channel) ||Anywhere |
|End of track ||0x2F ||0 || ||At the end of each track |
|Set tempo ||0x51 ||3 ||The number of microseconds per beat ||Anywhere, but usually in the first track |
|SMPTE offset ||0x54 ||5 ||SMPTE time to denote playback offset from the beginning ||Anywhere |
|Time signature ||0x58 ||4 ||Time signature, metronome clicks, and size of a beat in 32nd notes ||Anywhere |
|Key signature ||0x59 ||2 ||A key signature ||Anywhere |
|Sequencer specific ||0x7F ||variable ||Something specific to the MIDI device manufacturer ||Anywhere |
Note on status byte 0xFF
The reset message (a system realtime message) carries the same status byte as a meta message: 0xFF. By convention, the MIDI reset message is never placed in MIDI files but is only sent over MIDI ports upon a request by the user. Thus, if a MIDI device receives the status byte 0xFF over a MIDI port, the device will interpret it as a "reset" message. If the MIDI device reads the status byte 0xFF from within some MIDI file, the device will interpret the message as a meta message.
Note on meta types
Since all meta message have the same status byte (0xFF), the MIDI meta type ensures that MIDI devices can distinguish between different meta messages. For example, if the meta type in the second byte of a meta message is equal to 0x02 then the message contains a copyright notice (see the table above).
Musical Instrument Digital Interface (MIDI)