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study/sabre/os/files/Sound/MIDI/StandardMIDIFileFormat.txt
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study/sabre/os/files/Sound/MIDI/StandardMIDIFileFormat.txt
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Standard MIDI File Format
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Dustin Caldwell
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The standard MIDI file format is a very strange beast. When viewed as a
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whole, it can be quite overwhelming. Of course, no matter how you look at it,
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describing a piece of music in enough detail to be able to reproduce it
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accurately is no small task. So, while complicated, the structure of the midi
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file format is fairly intuitive when understood.
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I must insert a disclaimer here that I am by no means an expert with
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midi nor midi files. I recently obtained a Gravis UltraSound board for my PC,
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and upon hearing a few midi files (.MID) thought, "Gee, I'd like to be able to
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make my own .MID files." Well, many aggravating hours later, I discovered that
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this was no trivial task. But, I couldn't let a stupid file format stop me.
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(besides, I once told my wife that computers aren't really that hard to use,
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and I'd hate to be a hypocrite) So if any errors are found in this
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information, please let me know and I will fix it. Also, this document's scope
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does not extend to EVERY type of midi command and EVERY possible file
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configuration. It is a basic guide that should enable the reader (with a
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moderate investment in time) to generate a quality midi file.
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1. Overview
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A midi (.MID) file contains basically 2 things, Header chunks and Track
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chunks. Section 2 explains the header chunks, and Section 3 explains the track
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chunks. A midi file contains ONE header chunk describing the file format,
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etc., and any number of track chunks. A track may be thought of in the same
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way as a track on a multi-track tape deck. You may assign one track to each
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voice, each staff, each instrument or whatever you want.
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2. Header Chunk
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The header chunk appears at the beginning of the file, and describes the
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file in three ways. The header chunk always looks like:
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4D 54 68 64 00 00 00 06 ff ff nn nn dd dd
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The ascii equivalent of the first 4 bytes is MThd. After MThd comes the 4-byte
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size of the header. This will always be 00 00 00 06, because the actual header
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information will always be 6 bytes.
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ff ff is the file format. There are 3 formats:
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0 - single-track
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1 - multiple tracks, synchronous
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2 - multiple tracks, asynchronous
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Single track is fairly self-explanatory - one track only. Synchronous multiple
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tracks means that the tracks will all be vertically synchronous, or in other
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words, they all start at the same time, and so can represent different parts
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in one song. Asynchronous multiple tracks do not necessarily start at the same
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time, and can be completely asynchronous.
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nn nn is the number of tracks in the midi file.
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dd dd is the number of delta-time ticks per quarter note. (More about this
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later)
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3. Track Chunks
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The remainder of the file after the header chunk consists of track chunks.
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Each track has one header and may contain as many midi commands as you like.
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The header for a track is very similar to the one for the file:
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4D 54 72 6B xx xx xx xx
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As with the header, the first 4 bytes has an ascii equivalent. This one is
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MTrk. The 4 bytes after MTrk give the length of the track (not including the
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track header) in bytes.
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Following the header are midi events. These events are identical to the
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actual data sent and received by MIDI ports on a synth with one addition. A
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midi event is preceded by a delta-time. A delta time is the number of ticks
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after which the midi event is to be executed. The number of ticks per quarter
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note was defined previously in the file header chunk. This delta-time is a
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variable-length encoded value. This format, while confusing, allows large
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numbers to use as many bytes as they need, without requiring small numbers to
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waste bytes by filling with zeros. The number is converted into 7-bit bytes,
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and the most-significant bit of each byte is 1 except for the last byte of the
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number, which has a msb of 0. This allows the number to be read one byte at a
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time, and when you see a msb of 0, you know that it was the last (least
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significant) byte of the number. According to the MIDI spec, the entire delta-
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time should be at most 4 bytes long.
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Following the delta-time is a midi event. Each midi event (except a
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running midi event) has a command byte which will always have a msb of 1 (the
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value will be >= 128). A list of most of these commands is in appendix A. Each
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command has different parameters and lengths, but the data that follows the
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command will have a msb of 0 (less than 128). The exception to this is a meta-
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event, which may contain data with a msb of 1. However, meta-events require a
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length parameter which alleviates confusion.
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One subtlety which can cause confusion is running mode. This is where
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the actual midi command is omitted, and the last midi command issued is
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assumed. This means that the midi event will consist of a delta-time and the
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parameters that would go to the command if it were included.
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4. Conclusion
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If this explanation has only served to confuse the issue more, the
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appendices contain examples which may help clarify the issue. Also, 2
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utilities and a graphic file should have been included with this document:
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DEC.EXE - This utility converts a binary file (like .MID) to a tab-delimited
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text file containing the decimal equivalents of each byte.
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REC.EXE - This utility converts a tab-delimited text file of decimal values
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into a binary file in which each byte corresponds to one of the decimal
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values.
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MIDINOTE.PS - This is the postscript form of a page showing note numbers with
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a keyboard and with the standard grand staff.
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Appendix A
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1. MIDI Event Commands
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Each command byte has 2 parts. The left nybble (4 bits) contains the actual
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command, and the right nybble contains the midi channel number on which the
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command will be executed. There are 16 midi channels, and 8 midi commands (the
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command nybble must have a msb of 1).
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In the following table, x indicates the midi channel number. Note that all
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data bytes will be <128 (msb set to 0).
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Hex Binary Data Description
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8x 1000xxxx nn vv Note off (key is released)
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nn=note number
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vv=velocity
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9x 1001xxxx nn vv Note on (key is pressed)
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nn=note number
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vv=velocity
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Ax 1010xxxx nn vv Key after-touch
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nn=note number
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vv=velocity
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Bx 1011xxxx cc vv Control Change
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cc=controller number
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vv=new value
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Cx 1100xxxx pp Program (patch) change
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pp=new program number
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Dx 1101xxxx cc Channel after-touch
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cc=channel number
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Ex 1110xxxx bb tt Pitch wheel change (2000H is normal or no
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change)
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bb=bottom (least sig) 7 bits of value
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tt=top (most sig) 7 bits of value
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The following table lists meta-events which have no midi channel number. They
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are of the format:
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FF xx nn dd
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All meta-events start with FF followed by the command (xx), the length, or
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number of bytes that will contain data (nn), and the actual data (dd).
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Hex Binary Data Description
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00 00000000 nn ssss Sets the track's sequence number.
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nn=02 (length of 2-byte sequence number)
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ssss=sequence number
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01 00000001 nn tt .. Text event- any text you want.
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nn=length in bytes of text
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tt=text characters
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02 00000010 nn tt .. Same as text event, but used for
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copyright info.
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nn tt=same as text event
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03 00000011 nn tt .. Sequence or Track name
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nn tt=same as text event
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04 00000100 nn tt .. Track instrument name
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nn tt=same as text event
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05 00000101 nn tt .. Lyric
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nn tt=same as text event
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06 00000110 nn tt .. Marker
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nn tt=same as text event
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07 00000111 nn tt .. Cue point
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nn tt=same as text event
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2F 00101111 00 This event must come at the end of each
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track
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51 01010001 03 tttttt Set tempo
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tttttt=microseconds/quarter note
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58 01011000 04 nn dd ccbb Time Signature
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nn=numerator of time sig.
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dd=denominator of time sig. 2=quarter
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3=eighth, etc.
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cc=number of ticks in metronome click
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bb=number of 32nd notes to the quarter
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note
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59 01011001 02 sf mi Key signature
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sf=sharps/flats (-7=7 flats, 0=key of C,
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7=7 sharps)
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mi=major/minor (0=major, 1=minor)
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7F 01111111 xx dd .. Sequencer specific information
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xx=number of bytes to be sent
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dd=data
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The following table lists system messages which control the entire system.
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These have no midi channel number. (these will generally only apply to
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controlling a midi keyboard, etc.)
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Hex Binary Data Description
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F8 11111000 Timing clock used when synchronization is
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required.
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FA 11111010 Start current sequence
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FB 11111011 Continue a stopped sequence where left
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off
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FC 11111100 Stop a sequence
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The following table lists the numbers corresponding to notes for use in note
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on and note off commands.
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Octave|| Note Numbers
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# ||
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|| C | C# | D | D# | E | F | F# | G | G# | A | A# | B
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-----------------------------------------------------------------------------
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0 || 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11
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1 || 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20 | 21 | 22 | 23
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2 || 24 | 25 | 26 | 27 | 28 | 29 | 30 | 31 | 32 | 33 | 34 | 35
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3 || 36 | 37 | 38 | 39 | 40 | 41 | 42 | 43 | 44 | 45 | 46 | 47
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4 || 48 | 49 | 50 | 51 | 52 | 53 | 54 | 55 | 56 | 57 | 58 | 59
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5 || 60 | 61 | 62 | 63 | 64 | 65 | 66 | 67 | 68 | 69 | 70 | 71
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6 || 72 | 73 | 74 | 75 | 76 | 77 | 78 | 79 | 80 | 81 | 82 | 83
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7 || 84 | 85 | 86 | 87 | 88 | 89 | 90 | 91 | 92 | 93 | 94 | 95
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8 || 96 | 97 | 98 | 99 | 100 | 101 | 102 | 103 | 104 | 105 | 106 | 107
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9 || 108 | 109 | 110 | 111 | 112 | 113 | 114 | 115 | 116 | 117 | 118 | 119
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10 || 120 | 121 | 122 | 123 | 124 | 125 | 126 | 127 |
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BIBLIOGRAPHY
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"MIDI Systems and Control" Francis Rumsey 1990 Focal Press
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"MIDI and Sound Book for the Atari ST" Bernd Enders and Wolfgang Klemme
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1989 M&T Publishing, Inc.
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MIDI file specs and general MIDI specs were also obtained by sending e-mail
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to LISTSERV@AUVM.AMERICAN.EDU with the phrase GET MIDISPEC PACKAGE
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in the message.
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------------------------------- DEC.CPP ------------------------------------
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/* file dec.cpp
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by Dustin Caldwell (dustin@gse.utah.edu)
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*/
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#include <dos.h>
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#include <stdio.h>
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#include <stdlib.h>
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void helpdoc();
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main()
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{
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FILE *fp;
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unsigned char ch, c;
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if((fp=fopen(_argv[1], "rb"))==NULL) /* open file to read */
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{
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printf("cannot open file %s\n",_argv[1]);
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helpdoc();
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exit(-1);
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}
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c=0;
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ch=fgetc(fp);
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while(!feof(fp)) /* loop for whole file */
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{
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printf("%u\t", ch); /* print every byte's decimal equiv. */
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c++;
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if(c>8) /* print 8 numbers to a line */
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{
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c=0;
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printf("\n");
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}
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ch=fgetc(fp);
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}
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fclose(fp); /* close up */
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}
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void helpdoc() /* print help message */
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{
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printf("\n Binary File Decoder\n\n");
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printf("\n Syntax: dec binary_file_name\n\n");
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printf("by Dustin Caldwell (dustin@gse.utah.edu)\n\n");
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printf("This is a filter program that reads a binary file\n");
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printf("and prints the decimal equivalent of each byte\n");
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printf("tab-separated. This is mostly useful when piped \n");
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printf("into another file to be edited manually. eg:\n\n");
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printf("c:\>dec sonata3.mid > son3.txt\n\n");
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printf("This will create a file called son3.txt which can\n");
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printf("be edited with any ascii editor. \n\n");
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printf("(rec.exe may also be useful, as it reencodes the \n");
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printf("ascii text file).\n\n");
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printf("Have Fun!!\n");
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}
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---------------------------- REC.CPP ----------------------------------
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/* File rec.cpp
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by Dustin Caldwell (dustin@gse.utah.edu)
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*/
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#include <dos.h>
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#include <stdio.h>
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#include <ctype.h>
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#include <stdlib.h>
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void helpdoc();
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main()
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{
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FILE *rfp, *wfp;
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unsigned char ch, c;
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char s[20];
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if((rfp=fopen(_argv[1], "r"))==NULL) /* open the read file */
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{
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printf("cannot open file %s \n",_argv[1]);
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helpdoc();
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exit(-1);
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}
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if((wfp=fopen(_argv[2], "wb"))==NULL) /* open the write file */
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{
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printf("cannot open file %s \n",_argv[1]);
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helpdoc();
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exit(-1);
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}
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c=0;
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ch=fgetc(rfp);
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while(!feof(rfp)) /* loop for whole file */
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{
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if(isalnum(ch)) /* only 'see' valid ascii chars */
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{
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c=0;
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while(isdigit(ch)) /* only use decimal digits (0-9) */
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{
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s[c]=ch; /* build a string containing the number */
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c++;
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ch=fgetc(rfp);
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}
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s[c]=NULL; /* must have NULL terminator */
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fputc(atoi(s), wfp);/* write the binary equivalent to file */
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}
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ch=fgetc(rfp); /* loop until next number starts */
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}
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fclose(rfp); /* close up */
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fclose(wfp);
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}
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void helpdoc() /* print help message */
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{
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printf("\n Text File Encoder\n\n");
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printf("\n Syntax: rec text_file_name binary_file_name\n\n");
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printf("by Dustin Caldwell (dustin@gse.utah.edu)\n\n");
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printf("This is a program that reads an ascii tab-\n");
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printf("delimited file and builds a binary file where\n");
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printf("each byte of the binary file is one of the decimal\n");
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printf("digits in the text file.\n");
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printf(" eg:\n\n");
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printf("c:\>rec son3.txt son3.mid\n\n");
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printf("(This will create a file called son3.mid which is\n");
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printf("a valid binary file)\n\n");
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printf("(dec.exe may also be useful, as it decodes binary files)\n\n");
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printf("Have Fun!!\n");
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}
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Reference in New Issue
Block a user