by Stéphane Rollandin

What is µO ?

µO (or muO, or MuO) stands for "musical objects".
It is a quite large ecosystem of tools and representations for musical composition, in the form of an open playfield were musical ideas can be tested, visualized and manipulated in a very rich and personal way, up to the assembling of fully fledged pieces of music.

Data produced by µO includes MIDI files, MIDI and OSC output streams, Csound scores and orchestras, graphical representations and sound files. µO can also read scores in MusicXML and ABC formats, and read scales in Scala format.
Since µO is also a programming language, it can potentially import and export about anything.

What µO is not: it is not a digital audio workstation; it is not too good at making sound by itself; it does not compose nice music automatically; it is neither simple nor easy to use; it is probably not what you want.

µO features the evolution of the older GeoMaestro system for KeyKit.

µO includes smaller standalone applications such as the Boadebui MIDI sequencer.

µO is built upon the Squeak open source multimedia Smalltalk implementation; it is a full-fledged programming language.

µO and Squeak form the right hemisphere of Surmulot's brain, whose left hemisphere is Csound-x for Emacs.

µO is forever in beta stage.

What does it sound like ?

µO is an environment for computer-assisted composition. It does not have a specific sound.
It does not favor a specific composition style either.

Here are a few pieces of music entirely composed in µO.

The following are direct recordings of VST instruments rendering a real-time MIDI output stream from µO:

Tout va bien (merci)
Bulle qui rulle
Blue glue
I'll get there, again
Day one
Busy Claude is late again
Lava me, baby
Nouménal blues

Ressac is a Csound piece; its score has been graphically composed in µO:


All compositions above are Creative Commons. More music here.

Where is the documentation ?

Documentation, examples and tutorials are included within µO itself.

Most of it is interactive: it is much more effective than static external documentation. Note that µO being under active development, the current documentation is far from comprehensive. If you feel that an aspect of µO really needs better documentation, please tell me. Feedback is always welcome !

Reference papers about specific topics are (slowly) in the making; here are the current draft documents:
Other links:

Getting µO: the fast & easy way

Download the ready-to-go 360 image archive, unzip it anywhere and start Squeak.exe if you are on Windows. For other platforms, you will need to install Squeak and use it to open the .image file.

That's it; you don't need the following instructions.
Updated on January 27th, 2023
(more recent versions may exist; see the links below)

Building µO (for experienced squeakers)

The current code for µO is available on SqueakMap: check up the entry "muO".
Before installing, download the archive in SqueakMap package "muO dependencies" and uncompress it in the same directory as your Squeak image; this should give you a "muo-cache" folder providing all required third-party code.

Latest releases require a 5.3 or newer Squeak image.
Get it at or just below.

Building µO on Windows (for novices)

To install the current development snapshot of µO on a Windows system, follow these steps:

- Download the following files:
(1) a Squeak virtual machine (for Windows), a Squeak 5.3 image and its .source file
(2) a bundle of all third-party packages required by µO
(3) the current µO code: muO.360.sar

- Uncompress (1) and (2) in an empty folder; keep this folder untouched for later upgrades (see below).
- Copy the folder, open the copy and launch squeak.exe: you will see the Squeak window.
- Drop (3) in the window: a menu appears, select item "install SAR".
- Wait for the installation to proceed.
- Save the image (you can do that from the World menu in Squeak which you get by clicking in the background)

- To upgrade, just download the newer muO.***.sar file, and start again from a new copy of the base folder.
If the upgrade fails check for new versions of (1) and (2) since they are being updated from time to time.

Older versions of the muO.***.sar package are available on SqueakMap.


(click images to enlarge, click text to listen)

Part of the score for Missed angry cows.

A few of the projectors used for Modalités et juridictions.

Circular rhythm editors and piano-roll display for Mesures dragoniennes.

The full score of Mister Brown, with the exposed contents of one of the boxes.

A study of interlocking melodic lines for Suspension of Belief.

The full microtonal score of Small ambitions.

The full score of Tu quoque.

Melodic sculpting and harmonic exploration for Etude de masse.

Elaboration of the pitch bending and amplitude envelopes used in Ressac.

Editors on the dissonant chords at the beginning of Nom Gustav Zarkaoi.

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