Outline of the Standard MIDI File Structure Go Back

A standard MIDI file is composed of "chunks". It starts with a header chunk and is followed by one or more track chunks. The header chunk contains data that pertains to the overall file. Each track chunk defines a logical track.

 
   SMF = <header_chunk> + <track_chunk> [+ <track_chunk> ...]

A chunk always has three components, similar to Microsoft RIFF files (the only difference is that SMF files are big-endian, while RIFF files are usually little-endian). The three parts to each chunk are:

1. The track ID string which is four charcters long. For example, header chunk IDs are "MThd", and Track chunk IDs are "MTrk".
2. next is a four-byte unsigned value that specifies the number of bytes in the data section of the track (part 3).
3. finally comes the data section of the chunk. The size of the data is specified in the length field which follows the chunk ID (part 2).

The header chunk consists of a literal string denoting the header, a length indicator, the format of the MIDI file, the number of tracks in the file, and a timing value specifying delta time units. Numbers larger than one byte are placed most significant byte first.

 
   header_chunk = "MThd" + <header_length> + <format> + <n> + <division>
 

"MThd" 4 bytes

the literal string MThd, or in hexadecimal notation: 0x4d546864. These four characters at the start of the MIDI file indicate that this is a MIDI file.

<header_length> 4 bytes

length of the header chunk (always 6 bytes long--the size of the next three fields which are considered the header chunk).

<format> 2 bytes

0 = single track file format
1 = multiple track file format
2 = multiple song file format (i.e., a series of type 0 files)

<n> 2 bytes

number of track chunks that follow the header chunk

<division> 2 bytes

unit of time for delta timing. If the value is positive, then it represents the units per beat. For example, +96 would mean 96 ticks per beat. If the value is negative, delta times are in SMPTE compatible units.

A track chunk consists of a literal identifier string, a length indicator specifying the size of the track, and actual event data making up the track.

 
   track_chunk = "MTrk" + <length> + <track_event> [+ <track_event> ...]
 

"MTrk" 4 bytes

the literal string MTrk. This marks the beginning of a track.

<length> 4 bytes

the number of bytes in the track chunk following this number.

<track_event>

a sequenced track event.

Track Event

A track event consists of a delta time since the last event, and one of three types of events.

 
   track_event = <v_time> + <midi_event> | <meta_event> | <sysex_event>
 

<v_time>

a variable length value specifying the elapsed time (delta time) from the previous event to this event.

<midi_event>

any MIDI channel message such as note-on or note-off. Running status is used in the same manner as it is used between MIDI devices.

<meta_event>

an SMF meta event.

<sysex_event>

an SMF system exclusive event.

Meta Event

Meta events are non-MIDI data of various sorts consisting of a fixed prefix, type indicator, a length field, and actual event data..

 
   meta_event = 0xFF + <meta_type> + <v_length> + <event_data_bytes>
 

<meta_type> 1 byte

meta event types:

Type	Event	Type	Event
0x00	Sequence number	0x20	MIDI channel prefix assignment
0x01	Text event	0x2F	End of track
0x02	Copyright notice	0x51	Tempo setting
0x03	Sequence or track name	0x54	SMPTE offset
0x04	Instrument name	0x58	Time signature
0x05	Lyric text	0x59	Key signature
0x06	Marker text	0x7F	Sequencer specific event
0x07	Cue point

<v_length>

length of meta event data expressed as a variable length value.

<event_data_bytes>

the actual event data.

System Exclusive Event

A system exclusive event can take one of two forms:

sysex_event = 0xF0 + <data_bytes> 0xF7 or sysex_event = 0xF7 + <data_bytes> 0xF7

In the first case, the resultant MIDI data stream would include the 0xF0. In the second case the 0xF0 is omitted.

A variable length value uses the low order 7 bits of a byte to represent the value or part of the value. The high order bit is an "escape" or "continuation" bit. All but the last byte of a variable length value have the high order bit set. The last byte has the high order bit cleared. The bytes always appear most significant byte first.

Here are some examples:

   Variable length              Real value
   0x7F                         127 (0x7F)
   0x81 0x7F                    255 (0xFF)
   0x82 0x80 0x00               32768 (0x8000)