by Stéphane Rollandin

What is µO ?

µO (or muO, or MuO, just as you like) stands for "Musical Objects". It is a quite large ecosystem of tools and representations for musical composition.

Unlike most programs, it does not provide a rigid framework for editing a document; instead it is an open playfield were musical ideas can be tested, visualized and manipulated in a very rich way.

The kind of data produced by µO includes MIDI files, MIDI and OSC output streams, Csound scores and orchestras, graphical representations and sound files. ABC format for tunes and Scala format for scales are also supported.
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 does not make sound by itself; it is not designed for real-time performance; it does not compose music automatically; it is neither simple nor easy to use; it is probably not what you want.

µO includes a port of the older GeoMaestro system for KeyKit; it also includes the Csound Blocks application.

µO is based on 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 does not have a specific sound. It is an environment for composition. It does not favor a specific composition style either.
Here are a few pieces composed in µO: Boeuf de Wei, Ressac, Bulle qui rulle, Striures, Ishtar Biting.

See this page for more...

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 covering all the topics that should be covered. 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 in the making; here are the current draft documents:
Other links:

Testing µO: the fast & easy way

Download the ready-to-go 302 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 May 21rd, 2015

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 4.1 or newer Squeak image (up to 4.5; versions above 304 should also work on the newest 4.6 but minor glitches are to be expected). 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) the Squeak virtual machine (for Windows), a Squeak 4.5 image and its .source file
(2) a bundle of all third-party packages required by muO
(3) the current µO code: muO.304.sar

- uncompress (1) and (2) in an empty folder; keep this folder untouched for later installations.
- copy the folder, open the copy and drag the Squeak4.5.image file onto squeak.exe: you will see the Squeak window.
- now drop (3) in the window: a menu appears, select item "install SAR".
.. that´s it: just wait for the installation to proceed. It may take a while.

- to upgrade, only download the newer .sar file, and start again from a new copy of the base folder.
(older and possibly newer versions of the .sar package are available from this page:

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