Standard MIDI File Format
Dustin Caldwell
The standard MIDI file format is a very strange beast. When viewed as a
whole, it can be quite overwhelming. Of course, no matter how you look at it,
describing a piece of music in enough detail to be able to reproduce it
accurately is no small task. So, while complicated, the structure of the midi
file format is fairly intuitive when understood.
I must insert a disclaimer here that I am by no means an expert with
midi nor midi files. I recently obtained a Gravis UltraSound board for my PC,
and upon hearing a few midi files (.MID) thought, "Gee, I'd like to be able to
make my own .MID files." Well, many aggravating hours later, I discovered that
this was no trivial task. But, I couldn't let a stupid file format stop me.
(besides, I once told my wife that computers aren't really that hard to use,
and I'd hate to be a hypocrite) So if any errors are found in this
information, please let me know and I will fix it. Also, this document's scope
does not extend to EVERY type of midi command and EVERY possible file
configuration. It is a basic guide that should enable the reader (with a
moderate investment in time) to generate a quality midi file.
1. Overview
A midi (.MID) file contains basically 2 things, Header chunks and Track
chunks. Section 2 explains the header chunks, and Section 3 explains the track
chunks. A midi file contains ONE header chunk describing the file format,
etc., and any number of track chunks. A track may be thought of in the same
way as a track on a multi-track tape deck. You may assign one track to each
voice, each staff, each instrument or whatever you want.
2. Header Chunk
The header chunk appears at the beginning of the file, and describes the
file in three ways. The header chunk always looks like:
4D 54 68 64 00 00 00 06 ff ff nn nn dd dd
The ascii equivalent of the first 4 bytes is MThd. After MThd comes the 4-byte
size of the header. This will always be 00 00 00 06, because the actual header
information will always be 6 bytes.
ff ff is the file format. There are 3 formats:
0 - single-track
1 - multiple tracks, synchronous
2 - multiple tracks, asynchronous
Single track is fairly self-explanatory - one track only. Synchronous multiple
tracks means that the tracks will all be vertically synchronous, or in other
words, they all start at the same time, and so can represent different parts
in one song. Asynchronous multiple tracks do not necessarily start at the same
time, and can be completely asynchronous.
nn nn is the number of tracks in the midi file.
dd dd is the number of delta-time ticks per quarter note. (More about this
later)
3. Track Chunks
The remainder of the file after the header chunk consists of track chunks.
Each track has one header and may contain as many midi commands as you like.
The header for a track is very similar to the one for the file:
4D 54 72 6B xx xx xx xx
As with the header, the first 4 bytes has an ascii equivalent. This one is
MTrk. The 4 bytes after MTrk give the length of the track (not including the
track header) in bytes.
Following the header are midi events. These events are identical to the
actual data sent and received by MIDI ports on a synth with one addition. A
midi event is preceded by a delta-time. A delta time is the number of ticks
after which the midi event is to be executed. The number of ticks per quarter
note was defined previously in the file header chunk. This delta-time is a
variable-length encoded value. This format, while confusing, allows large
numbers to use as many bytes as they need, without requiring small numbers to
waste bytes by filling with zeros. The number is converted into 7-bit bytes,
and the most-significant bit of each byte is 1 except for the last byte of the
number, which has a msb of 0. This allows the number to be read one byte at a
time, and when you see a msb of 0, you know that it was the last (least
significant) byte of the number. According to the MIDI spec, the entire delta-
time should be at most 4 bytes long.
Following the delta-time is a midi event. Each midi event (except a
running midi event) has a command byte which will always have a msb of 1 (the
value will be >= 128). A list of most of these commands is in appendix A. Each
command has different parameters and lengths, but the data that follows the
command will have a msb of 0 (less than 128). The exception to this is a meta-
event, which may contain data with a msb of 1. However, meta-events require a
length parameter which alleviates confusion.
One subtlety which can cause confusion is running mode. This is where
the actual midi command is omitted, and the last midi command issued is
assumed. This means that the midi event will consist of a delta-time and the
parameters that would go to the command if it were included.
4. Conclusion
If this explanation has only served to confuse the issue more, the
appendices contain examples which may help clarify the issue. Also, 2
utilities and a graphic file should have been included with this document:
DEC.EXE - This utility converts a binary file (like .MID) to a tab-delimited
text file containing the decimal equivalents of each byte.
REC.EXE - This utility converts a tab-delimited text file of decimal values
into a binary file in which each byte corresponds to one of the decimal
values.
. Appendix A
1. MIDI Event Commands
Each command byte has 2 parts. The left nybble (4 bits) contains the actual
command, and the right nybble contains the midi channel number on which the
command will be executed. There are 16 midi channels, and 8 midi commands (the
command nybble must have a msb of 1).
In the following table, x indicates the midi channel number. Note that all
data bytes will be <128 (msb set to 0).
Hex Binary Data Description
8x 1000xxxx nn vv Note off (key is released)
nn=note number
vv=velocity
9x 1001xxxx nn vv Note on (key is pressed)
nn=note number
vv=velocity
Ax 1010xxxx nn vv Key after-touch
nn=note number
vv=velocity
Bx 1011xxxx cc vv Control Change
cc=controller number
vv=new value
Cx 1100xxxx pp Program (patch) change
pp=new program number
Dx 1101xxxx cc Channel after-touch
cc=channel number
Ex 1110xxxx bb tt Pitch wheel change (2000H is normal or no
change)
bb=bottom (least sig) 7 bits of value
tt=top (most sig) 7 bits of value
.The following table lists meta-events which have no midi channel number. They
are of the format:
FF xx nn dd
All meta-events start with FF followed by the command (xx), the length, or
number of bytes that will contain data (nn), and the actual data (dd).
Hex Binary Data Description
00 00000000 nn ssss Sets the track's sequence number.
nn=02 (length of 2-byte sequence number)
