MIDIfile documentation

MIDIfiles are more or less MIDI data plus timing stored in a file.

Each file contains one or more parallel streams of events.

All files contain a header block followed by one or more track blocks.

Byte ordering

MIDIfile data is stored in 'BigEndian' format with multibyte numbers coming high-byte first in the file - hence, multibyte data has to be byte-swapped for PC's.

Variable Length Numbers

Many values in MIDI files are stored in 'variable length' multi-byte format. The numbers are stored at 7 bits per byte with the most-significant end coming first. All bytes except the last have the most significant bit set. Hence, values between 0 and 127 fit simply into a single byte. Numbers up to 14 bits in length need two bytes and so on.

eg:

Number	MIDIfile representation	0x00000000	00	0x00000040	40	0x0000007F	7F	0x00000080	81 00	0x00002000	C0 00	0x00003FFF	FF 7F	0x001FFFFF	FF FF 7F	0x08000000	C0 80 80 00	0x0FFFFFFF	FF FF FF 7F

Although the format theoretically could store numbers of any length, a limit of 32 bits is imposed - requiring 5 bytes.

Header Block

Offset	#Bytes	Data-type	Description	Usual content
0x0000	4	char	Magic	'MThd'
0x0004	4	int	Length of rest of header	6
0x0008	2	short	Format	See Note #1
0x000A	2	short	Num track blocks	-
0x000C	2	short/bytes	Tempo	See Note #2

NOTE #1: Format

• 0 - one, single multi-channel track
• 1 - one or more simultaneous tracks
• 2 - one or more sequentially independent single-track patterns

NOTE #2: Tempo

• If tempo is negative :
  • The absolute value of the high byte = Number of frames per second.
  • Low byte : Resolution within one frame.
• If tempo is positive:
  • Tempo is the division of a quarter-note.

Track Blocks

Track blocks are essentially a stream of MIDI data preceded by delta times.

Each event comprises a delta-time and an event number. The time is a variable-length quantity and stores the time that has to elapse until the NEXT event is due. A time of 0 means that there is no delay before the next event.

At the start of each track block there is a track block header:

Offset	#Bytes	Data-type	Description	Usual content
0x0000	4	char	ID	'MTrk'
0x0004	4	int	Length of rest of header	0

There are three kinds of event:

1. MIDI event
2. System exclusive event (SysEx)
3. Meta event (non-MIDI information).

Meta Event

Offset	#Bytes	Data-type	Description	Value
0x0000	1	byte	ID	0xFF
0x0001	1	byte	Type	<= 0x7F
0x0002	?	variable-length number	Length of data in bytes
?	?	bytes

Predefined Meta events:

ID	Type	Length	Data	Name	Description
FF	00	02	ssss	Sequence Number	Note #1.
FF	01	len	text	Text Event	ASCII text describing anything. Generally found at the start with the track's name, etc.
FF	02	len	text	Copyright Notice
FF	03	len	text	Sequence/Track Name
FF	04	len	text	Instrument Name
FF	05	len	text	Lyric	Each syllable should be a lyric event with the appropriate timing.
FF	06	len	text	Section Marker	A rehearsal note or section name (eg "Second chorus")
FF	07	len	text	Cue Point	Something happening in a related film/stage performance.
FF	2F	00	(nothing)	End of Track	Provides timing for final delay at the end of the track.
FF	51	03	3-byte int	Tempo change	Note #2
FF	54	05	hour min sec frame ff	SMPTE Offset	Note #3
FF	58	04	nn dd cc bb	Time Signature	Note #4
FF	59	02	sf mi	Key Signature	Note #5
FF	7F	len	data	Sequencer-Specific Meta-Event	Note #6

Note #1: Sequence number:

• Optional event must occur at the beginning of a track, before any time has elapsed and before any MIDI events have to be sent.

Note #2: Tempo change:

• Can be envisaged as usec per MIDI quarter-note.
• Can be envisaged as 24ths of a microsecond per MIDI clock.

Note #3: SMPTE Offset:

• Designates the SMPTE time at which the track block is supposed to start.
• The hour is encoded in SMPTE format, same as MIDI Time Code.
• The ff field contains fractional frames, in 100ths of a frame.

Note #4: Time Signature:

• The time signature is four 1 byte numbers.
• nn & dd are the numerator and denominator of the time signature as it would be notated.
• dd is a negative power of two:
  • 2 = quarter-note,
  • 3 = eighth-note, etc.
• cc : number of MIDI clocks in a metronome click.
• bb : number of notated 32nd-notes in a MIDI quarter-note (24 MIDI Clocks).

Note #5: Key Signature

• st field:
  • sf = -7: 7 flats
  • sf = -1: 1 flat
  • sf = 0: key of C
  • sf = 1: 1 sharp
  • sf = 7: 7 sharps
• mi field:
  • mi = 0: major key
  • mi = 1: minor key

Note #6: Sequencer-Specific Meta-Event

• First byte: is a manufacturer ID.
• After that - all bets are off!

System Exclusive Events

SysEx is used as an escape sequence that specifies completely arbitary data to be sent to a sequencer or synthesizer.

There are two versions:

• The 0xF7 event is used if a 0xF0 needs to be sent inside the data...otherwise use 0xF0
• SysEx events must end with an 0xF7 event.

Offset	#Bytes	Data-type	Description	Value
0x0000	1	byte	ID	0xF0 or 0xF7
0x0001	?	variable-length number	Length
?	?	byte	data