An audio unit is a plug- in Core Audio technology of Apple's Mac OS X. Its task is the processing of audio material ( as an effect ) or the conversion of MIDI data into audio signals ( as a software instrument). This process can be done in real time ( depending on the computational effort delayed) or in so-called off-line computation even faster.
The principle of Audio Units is the collection of all plugins in one place. Support any program, called the host software must CoreAudio, so as to obtain access to all plugins. The Audio Units therefore solve Mac OS X, and in particular the sequencing program Logic Steinberg VST interface that is supported since version 6 is no longer of Logic. Unlike VSTs but AUs are available as part of Core Audio platform tied to Mac OS X and do not run on other operating systems like Windows or Linux, which also limits their distribution. The file name extension of the Audio Unit plugins component is usually.
Audio Units can be developed by other manufacturers and can be installed additionally, some are already integrated into the operating system. Most manufacturers of software instruments and effects ported their VSTs after the announcement of the new interface and set AU and VST versions available. The plugin " VST to AudioUnit Adapter" the company FXPansion makes it possible to integrate VST plug-ins in an AU interface by it in turn functions of the AU host as a plugin and for the VST plugin host.
Another common audio plug-in interface is Real Time Audio Suite, developed by Digidesign.