Supercollider [McCartney, 2002,Pope, 2004] is a language for sound and image processing developed by James McCartney. It can be considered as a Music-N style language in its use of unit generators and other concepts such as instrument, orchestra or events. Nevertheless it presents important differences in respect to traditional Music-N languages such as CSound (see 2.6.1). The main differences are: (1) most Supercollider programs can run in real-time and process live sound and MIDI inputs/outputs; (2) Supercollider is a comprehensive general-purpose programming language with facilities for file input/output, list processing, and OO programming; and (3) Supercollider is an integrated development environment.

Supercollider has been implemented in Apple Macintosh and Be computers though more ports are planned. It is a high-level programming language with a syntax derived from C++ and Smalltalk. Its development environment includes a program text editor, rapid compiler, run-time system and a GUI builder. Supercollider Instruments can take their inputs from real-time MIDI controllers and can process audio files and live sound input.

Motivations for the design of Supercollider were the ability to realize sound processes that were different every time they played, write pieces describing ranges of possibilities rather than fixed entities and to facilitate live improvisation by a composer/performer.

Supercollider computes control functions and other values at a lower rate than the sampling rate called the ``sub-frame'' size. Its default value is 64 though it can be set to any value between 4 and 256. In Supercollider, data is played as it is generated.

SC's syntax is an OO programming language, with a syntax mixture of C++ and Smalltalk. In SC one can program in two styles: function-oriented or message-passing. As in most programming languages, there are different kinds of statements: comments, declarations, assignments and control structures. The language includes everything you would expect to find in a general programming language but also includes specific functions for music and signal processing.

In a typical SC program there are several parts: (1) Header: title, comment, date, version...; (2) Declarations: declare output buffers, sound files, function tables...(required); (3) Init function: run at compile time if present (optional); (4) Start function: called at run-time if present, runs the instruments (normally present though optional); (5) Instrument functions: can be called from the Start function.

Unit generators are regular OO objects with constructors, and evaluation methods. SC also has support for OO classes and inheritance. The UGen class provides the abstraction of a unit generator, and the Synth class represents a group of UGens operating as a group to generate an output. The unit generator API is a simple C interface.

An Instrument is constructed functionally. When writing a sound-processing function one is actually writing a function that creates and connects unit generators. This is different from a static object specification of a network of unit generators. Instruments in Supercollider can ``generate'' a network of unit generators.

A composition can be considered as a sequence of events and this abstraction is accomplished via the concept of a stream. A stream is an object to which the next message can be sent to get the next element. A stream ends when it returns nil. By default, all objects in Supercollider respond to next by returning themselves so any object can be used as an infinite stream of itself. An event stream returns dictionaries that map symbols to values, the composition code does not need to know anything about an instrument argument list and may contain any set of parameters.

Supercollider was originally designed to combine a high-level language and a synthesis engine. But later some reasons were found to separate the composition language from a synthesis engine [McCartney, 2002]. The main reason is that some synthesis processing time must be consumed generating events, if the composition language is separated it can run in the background generating events.

That is why in Supercollider Server the synthesis engine and the language were separated and are now two applications that communicate via a slightly modified version of Open Sound Control (OSC) [Wright, 1998a]. This allows to run several instances of the synthesis engine either in different processors or machines. Controlling the synthesis engine is as simple as opening a socket and sending commands, so any program (Max, a C++ program...) could control it.

In Supercollider 3 Synth Server, while synthesis is playing new modules can be created, destroyed or repatched and sample buffers can be created and reallocated. All commands are received via TCP or UDP using the simplified version of OSC. If MIDI is desired, it is up to the client to convert it to OSC commands for the synthesis engine.

There are two versions of the Supercollider Server synthesis engine. One uses a block computation model and unit generator plug-ins. Instruments are loaded as files that describe patches of these unit generators. This version has a control rate and audio rate. The other version implements single-sample computation with the instruments loaded as compiled plug-ins. The synthesis class library can generate C++ code to be loaded by the synthesis engine. Instead of a single control rate any unit generator may run at any power of two division of the audio clock rate. Only source unit generators need to specify the computation rate.

All running modules are ordered in a tree of nodes that define an order of execution. There are two types of nodes: Synths and Groups. A Synth is just a collection of unit generators that can be addressed together. A Group is a collection of Nodes.

Synths send audio and control signals to each other via a a pair of global arrays of audio and control buses. Using buses allows to connect Synths without a priori knowledge about them. The lowest-numbered audio busses get written to the audio hardware outputs, then there are the audio input buses.

The Supercollider user interface has a program text editor, a message output view and an instrument user interface view. But a GUI can also be created with Supercollider.