midilib is a pure Ruby MIDI library useful for reading and writing standard MIDI files and manipulating MIDI event data.

The GitHub project page and Web site of midilib is github.com/jimm/midilib and the RubyGems.org page is rubygems.org/gems/midilib, where you can also find all the RDoc documentation.

midilib is compatible with Ruby 2.6 and higher.


midilib does not require any other packages. The test suite in the tests directory requires the testing framework TestUnit, which comes with Ruby 1.8 and later and can also be found in the Ruby Application Archive (raa.ruby-lang.org).

To rebuild the gem or RDocs or run the tests easily, you can use the Rakefile which requires Rake (rake.rubyforge.org).


RubyGems Installation

To install midilib as a gem, type

% gem install midilib

or, if you already have a previous version, use

% gem update midilib

You may need root privileges to install or update the gem.

Manual Installation

After downloading and expanding the archive, you can install midilib with the command

% ruby install.rb


% ruby install.rb --install-dir=my_directory

You may need root privileges to install.


% rake test

runs all of the tests in the test directory.


midilib can read and write MIDI file formats 0 (one single track) or 1 multiple tracks. By default, it writes format 1 which is the most common format. MIDI file format 2 is not yet supported.


A sequence contains a collection of tracks and global information like the sequence's pulses per quarter note (ppqn) and time signature.

The first track in a sequence is special; it holds meta-events like tempo and sequence name. Don't put any notes in this track.

MIDI::Sequence also contains some convenience methods that let you set and retrieve the sequence's name, the time signature, and to retrieve the first tempo event's beats-per-minute value.

Normally instances of MIDI::IO::SeqReader and MIDI::IO::SeqWriter are used when a sequence reads itself from or writes itself to a MIDI file. You can change that by setting a sequence's reader_class or writer_class attributes. Instances of the classes contained in those attributes are created and used whenever the sequence reads or writes itself.


A track contains an array of events.

When you modify the events array, make sure to call recalc_times so each event gets its time_from_start recalculated. You don't have to do that after every event you add; just remember to do so before using the track in a way that expects the list of events to be ordered correctly.

A Track also holds a bit mask that specifies the channels used by the track. This bit mask is set when the track is read from the MIDI file by a SeqReader but is not kept up to date by any other methods. Specifically, if you add events to a track at any other time, the bit mask will not be updated.


This class contains information about a measure from the sequence. Measure data is based on the time signature information from the sequence and is not stored in the sequence itself.


The class MIDI::Sequence method get_measures returns a MIDI::Measures object. MIDI::Measures is a subclass of Array. It is a specialized container for MIDI::Measure objects, which can be use to map event times to measure numbers. Please note that this object has to be remade when events are deleted/added in the sequence.

MIDI::Measure and MIDI::Measures are brought to us by Jari Williamsson <[email protected]>, who also contributed some improvements to the MIDI::Event and MIDI::Track classes.


Each event holds not only its delta time but also its time from the start of the track. The track is responsible for recalculating its events' start times. You can call MIDI::Track#recalc_times to do so.

Subclasses of MIDI::Event implement the various MIDI messages such as note on and off, controller values, system exclusive data, and realtime bytes.

MIDI::Realtime events have delta values and start times, just like all the other midilib event types do. (MIDI real time status bytes don't have delta times, but this way we can record when in a track the realtime byte was received and should be sent. This is useful for start/continue/stop events that control other devices, for example.) Note that when a MIDI::Realtime event is written out to a MIDI file, the delta time is not written.

MIDI::MetaEvent events hold an array of bytes named 'data'. Many meta events are string holders (text, lyric, marker, etc.) Though the 'data' value is always an array of bytes, MIDI::MetaEvent helps with saving and accessing string. The MIDI::MetaEvent#data_as_str method returns the data bytes as a string. When assigning to a meta event's data, if you pass in a string it will get converted to an array of bytes.

How To Use

The following examples show you how to use midilib to read, write, and manipulate MIDI files and modify track events. See also the files in the examples directory, which are described below.

Reading a MIDI File

To read a MIDI file, create a MIDI::Sequence object and call its #read method, passing in an IO object.

The #read method takes an optional block. If present, the block is called once after each track has finished being read. Each time, it is passed the track object, the total number of tracks and the number of the current track that has just been read. This is useful for notifying the user of progress, for example by updating a GUI progress bar.

require 'midilib/io/seqreader'

# Create a new, empty sequence.
seq = MIDI::Sequence.new()

# Read the contents of a MIDI file into the sequence.
File.open('my_midi_file.mid', 'rb') { | file |
    seq.read(file) { | track, num_tracks, i |
        # Print something when each track is read.
        puts "read track #{i} of #{num_tracks}"

Writing a MIDI File

To write a MIDI file, call the write method, passing in an IO object.

require 'midilib/io/seqwriter'

# Start with a sequence that has something worth saving.
seq = read_or_create_seq_we_care_not_how()

# Write the sequence to a MIDI file.
File.open('my_output_file.mid', 'wb') { | file | seq.write(file) }

Editing a MIDI File

Combining the last two examples, here is a script that reads a MIDI file, transposes some events, and writes the sequence out to a different file. This is a useful template for programatically manipulating MIDI data.

This code transposes all of the note events (note on, note off, and poly pressure) on channel 5 down one octave.

Transposing One Channel

require 'midilib/io/seqreader'
require 'midilib/io/seqwriter'

# Create a new, empty sequence.
seq = MIDI::Sequence.new()

# Read the contents of a MIDI file into the sequence.
File.open('my_input_file.mid', 'rb') { | file |
    seq.read(file) { | track, num_tracks, i |
        # Print something when each track is read.
        puts "read track #{i} of #{num_tracks}"

# Iterate over every event in every track.
seq.each { | track |
    track.each { | event |
        # If the event is a note event (note on, note off, or poly
        # pressure) and it is on MIDI channel 5 (channels start at
        # 0, so we use 4), then transpose the event down one octave.
        if MIDI::NoteEvent === event && event.channel == 4
            event.note -= 12

# Write the sequence to a MIDI file.
File.open('my_output_file.mid', 'wb') { | file | seq.write(file) }

Manipulating tracks

If you modify a track's list of events directly, don't forget to call MIDI::Track#recalc_times when you are done.

track.events[42, 1] = array_of_events
track.events << an_event

Calculating delta times

A few methods in MIDI::Sequence make it easier to calculate the delta times that represent note lengths. MIDI::Sequence#length_to_delta takes a note length (a multiple of a quarter note) and returns the delta time given the sequence's current ppqn (pulses per quarter note) setting. 1 is a quarter note, 1.0/32.0 is a 32nd note (use floating-point numbers to avoid integer rounding), 1.5 is a dotted quarter, etc. See the documentation for that method for more information.

MIDI::Sequence#note_to_length takes a note name and returns a length value (again, as a multiple of a quarter note). Legal note names are those found in MIDI::Sequence::NOTE_TO_LENGTH, and may begin with “dotted” and/or end with “triplet”. For example, “whole”, “sixteenth”, “32nd”, “quarter triplet”, “dotted 16th”, and “dotted 8th triplet” are all legal note names.

Finally, MIDI::Sequence#note_to_delta takes a note name and returns a delta time. It does this by calling note_to_length, then passing the result to length_to_delta.

Example Scripts

Here are short descriptions of each of the examples found in the examples directory.

  • examples/from_scratch.rb shows you how to create a new sequence from scratch and save it to a MIDI file. It creates a file called 'from_scratch.mid'.

  • examples/seq2text.rb dumps a MIDI file as text. It reads in a sequence and uses the to_s method of each event.

  • examples/reader2text.rb dumps a MIDI file as text. It subclasses MIDI::SeqReader instead of creating a sequence containing tracks and events.

  • examples/transpose.rb transposes all note events (note on, note off, poly pressure) on a specified channel by a specified amount.

  • There is also one MIDI file, examples/NoFences.mid. It is a little pop ditty I wrote. The instruments in this file use General MIDI patch numbers and drum note assignments. Since I don't normally use GM patches, the sounds used here are at best approximations of the sounds I use.


The Ruby Web site (www.ruby-lang.org/en/index.html) contains an introduction to Ruby, the Ruby Application Archive (RAA) at raa.ruby-lang.org, and pointers to more information.

<cite>Programming Ruby, The Pragmatic Programmer's Guide</cite>, by David Thomas and Andrew Hunt, is a well-written and practical introduction to Ruby. Its Web page at www.rubycentral.com/book also contains a wealth of Ruby information. Though the first edition book is available online, I encourage you to purchase a copy of the latest edition.

A description of the MIDI file format can be found in a few places such as www.csie.ntu.edu.tw/~r92092/ref/midi/.

The MIDI message reference at www.jimmenard.com/midi_ref.html describes the format of MIDI commands.

To Do

:include: TODO.rdoc




Jim Menard ([email protected])


Copyright © 2003-2013 Jim Menard


Distributed under the same license as Ruby.


midilib is copyrighted free software by Jim Menard and is released under the same license as Ruby. See the Ruby license at www.ruby-lang.org/en/LICENSE.txt.

midilib may be freely copied in its entirety providing this notice, all source code, all documentation, and all other files are included.

midilib is Copyright © 2003-2013 by Jim Menard.

The song “No Fences” contained in the MIDI file examples/NoFences.mid is Copyright © 1992 by Jim Menard ([email protected]). It may be freely used for non-commercial purposes as long as the author is given credit.

Recent Changes

Changes for 2.0.5:

Updated install.rb to work with newer versions of Ruby by using fileutils instead of ftools.

Changes for 2.0.3:

New MIDI::Sequence.pulses_to_seconds method.

Changes for 2.0.2:

Stop monkeypatching Array in MIDI::Track.

Changes for 2.0.0:

MIDI::NoteOnEvent and MIDI::NoteOffEvent renamed to MIDI::NoteOn and MIDI::NoteOff. The old names will still work for a while.

The MIDI::Event boolean methods like meta? and note? have been removed. Use the event classes themselves (for example, MIDI::MetaEvent === my_event or my_event.kind_of?(MIDI::MetaEvent)). Case statements that use classes work, too:

case my_event
when MIDI::NoteEvent # superclass of note on, note off, poly press
when MIDI::Controller

Introduced Adam Murray's stable sorting code for MIDI::Track#recalc_delta_from_times. See wiki.github.com/adamjmurray/cosy/midilib-notes and github.com/adamjmurray/cosy/blob/master/lib/cosy/helper/midi_file_renderer_helper.rb for details.

Aliased MIDI::Track#sort to MIDI::Track#recalc_delta_from_times, since all sort did was sort the events then call recalc_delta_from_times, and recalc_delta_from_times sorts the events before doing anything else.

MIDI::Tempo#mpq_to_bpm now returns a float.

Changes for 1.2.0:

Use byte arrays instead of strings for passing around data. All tests now pass for both Ruby 1.8.X and 1.9.X.

New code repository

The midilib code is now hosted at Github (github.com/jimm/midilib).

Changes for 1.1.4:

  • Fixed a bug in KeySig.data_as_bytes. Thanks to Noah Thorp for finding this and the bug fixed in 1.1.3.

Changes for 1.1.3:

  • Fixed the way midilib detects the behavior of IO.getc.

Changes for 1.1.2:

  • Define MIDI::IO::MIDIFile.getc differently for different Ruby versions, instead of checking for String.bytes every time we read a byte.

Changes for 1.1.1:

  • Make MIDI::IO::MIDIFile.getc do the right thing for both Ruby 1.8 and 1.9.

Changes for 1.1.0:

  • Added test/test.mid to list of files to be included when packaging midifile for distribution.

Changes for 1.0.0:

  • Fixed the bug in Track#recalc_delta_from_times found by Christopher Rose.

Changes for 0.8.7:

  • Fixed the misspelled POLY_PRESSURE constant, thanks to Mario Pehle.

Changes for 0.8.6:

  • Added missing test/test.mid.

Changes for 0.8.5:

  • Fixed bugs in MIDI::PitchBend reading and writing, thanks to Emanuel Borsboom.

  • Fixed a bug in MIDI::Track#quantize.

  • The argument to MIDI::Track#quantize has changed: it is now either a note name (“sixteenth”, “32nd”, “8th triplet”) or a length (1 = quarter, 0.25 = sixteenth). This is a drastic change that will break all previous calls to quantize. However, since that method was broken already, I don't feel it's a burden to anybody to change the arguments.

Changes for 0.8.4:

  • Realtime status bytes now set @is_realtime to true and return true when realtime? is called.

  • All system common events now set @is_system to true and return true when system? is called, not just system exclusive events.

  • Added examples/from_scratch.rb, which shows how to create a sequence manually.

  • New MIDI::Sequence methods that turn note length names like “32nd”, “dotted quarter”, and “16th triplet” into delta times. See the docs below and MIDI::Sequence::length_to_delta, MIDI::Sequence::note_to_length, and MIDI::Sequence::note_to_delta.

Changes for 0.8.3:

  • Added MIDI::NoteEvent.note_to_s, which returns note name as a string like “C4” or “F#6”.

  • Added new boolean attributes to MIDI::Event: @print_decimal_numbers and @print_note_names. These are used by all Event to_s methods. See examples/seq2text.rb for an example.

Changes for 0.8.2:

  • Changed MIDI::MetaEvent.type to MIDI::MetaEvent.event_type to avoid runtime complaints about Object#type calls.

  • Added 'b' binary flag to file open modes for Windows.

  • Fixed $LOAD_PATH in example files.

  • Fixed read and write block arguments.

  • Fixed other example script bugs.

Changes for 0.8.1:

  • Fixed track sorting.

  • Fixed track's recalc_delta_from_times method.

  • Fixed event quantization.

  • More tests and documentation.


This software is provided “as is” and without any express or implied warranties, including, without limitation, the implied warranties of merchantability and fitness for a particular purpose.