Audio Units (AU) are a system-level plug-in architecture provided by

Core Audio Core Audio is a low-level API for dealing with sound in Apple's macOS and iOS operating systems. It includes an implementation of the cross-platform OpenAL. Apple's Core Audio documentation states that "in creating this new architecture on Ma ...

in Apple's macOS and

iOS iOS (formerly iPhone OS) is a mobile operating system created and developed by Apple Inc. exclusively for its hardware. It is the operating system that powers many of the company's mobile devices, including the iPhone; the term also includes ...

operating systems. Audio Units are a set of

application programming interface An application programming interface (API) is a way for two or more computer programs to communicate with each other. It is a type of software interface, offering a service to other pieces of software. A document or standard that describes how t ...

(API) services provided by the operating system to generate, process, receive, or otherwise manipulate streams of audio in near-real-time with minimal latency. It may be thought of as Apple's architectural equivalent to another popular plug-in format,

Steinberg Steinberg Media Technologies GmbH (trading as Steinberg) is a German musical software and hardware company based in Hamburg. It develops music writing, recording, arranging, and editing software, most notably Cubase, Nuendo, and Dorico. It also ...

's Virtual Studio Technology (VST). Because of the many similarities between Audio Units and VST, several commercial and free wrapping technologies are available (e.g
Symbiosis
and

FXpansion FXpansion is a Ltd company that produces music software including Geist2, Strobe2, Tremor, Etch, Bloom, Maul and DCAM Dynamics. History FXpansion is a company that develops music software, and was founded in 1999 in London, United Kingd ...

br>VST-AU Adapter
.

Celemony Software Celemony Software GmbH is a German musical software company that specializes in digital audio pitch correction software. It produces Melodyne, a popular audio pitch modification tool similar to Auto-Tune, although the program itself is a manual ...

and PreSonus have also developed the

Audio Random Access Audio Random Access (commonly abbreviated to ARA) is an extension for audio plug-in interfaces, such as AU, VST and RTAS, allowing them to exchange a greater amount of audio information with digital audio workstation (DAW) software. It was dev ...

(ARA) extension, which works for both AU and VST, allowing greater integration between the plug-ins and DAW software.

Use

Audio Units allows sound file

audio time stretching and pitch scaling Time stretching is the process of changing the speed or duration of an audio signal without affecting its pitch. Pitch scaling is the opposite: the process of changing the pitch without affecting the speed. Pitch shift is pitch scaling implement ...

(e.g., timestretch), sample rate conversion, and streaming over a Local Area Network. It also comes with a set of AU plug-ins such as EQ filters, dynamic processors, delay, reverb, and a Soundbank Synthesizer Instrument. AU are used by Apple applications such as GarageBand, Soundtrack Pro,

Logic Express Logic Express was a "light" version of Logic Pro, a MIDI sequencer and digital audio workstation software application maintained by Apple that runs on the Mac OS X platform. It was announced on January 15, 2004, for release in March 2004. Logi ...

, Logic Pro,

Logic Pro X Logic Pro is a digital audio workstation (DAW) and MIDI sequencer software application for the macOS platform. It was originally created in the early 1990s as Notator Logic, or Logic, by German software developer C-Lab which later went by Em ...

, Final Cut Pro,

MainStage MainStage is a music application developed by Apple Inc. designed for use in live performance. Features MainStage might be thought of as a companion app to Logic Pro. It works in much the same way and bears a similar user interface. The focu ...

and most 3rd party audio software developed for macOS such as

Ableton Live Ableton Live is a digital audio workstation for macOS and Windows developed by the German company Ableton. In contrast to many other software sequencers, Ableton Live is designed to be an instrument for live performances as well as a tool ...

, Amadeus Pro, Ardour,

Audio Hijack Rogue Amoeba Software, Inc. is a software company that produces audio software, specializing in applications for audio capture for the Macintosh platform. Rogue Amoeba's best-known products are Audio Hijack and Audio Hijack Pro (both now known ...

DaVinci Resolve DaVinci Resolve (originally known as da Vinci Resolve) is a color grading, color correction, visual effects, and audio post-production video editing application for macOS, Windows, and Linux, originally developed by da Vinci Systems, and now dev ...

, Digital Performer,

Gig Performer Gig Performer is a cross-platform audio plug-in host software package developed by Deskew Technologies. It is designed to provide a solution for playing an instrument and effect plug-ins live, without using a DAW. It was originally released i ...

REAPER A reaper is a farm implement or person that reaps (cuts and often also gathers) crops at harvest when they are ripe. Usually the crop involved is a cereal grass. The first documented reaping machines were Gallic reapers that were used in Roma ...

, and Studio One.

Competing technologies

* Avid's Avid Audio eXtension (a successor to the RTAS format) *

Digidesign Avid Audio (formerly Digidesign) is an American digital audio technology company. It was founded in 1984 by Peter Gotcher and Evan Brooks. The company began as a project to raise money for the founders' band, selling EPROM chips for drum mac ...

's Real Time AudioSuite (now obsolete) *

LADSPA LADSPA is an acronym for Linux Audio Developer's Simple Plugin API. It is an application programming interface (API) standard for handling audio filters and audio signal processing effects, licensed under LGPL-2.1-or-later. It was originally des ...

and

Disposable Soft Synth Interface Disposable Soft Synth Interface (DSSI) is a virtual instrument (software synthesizer) plugin architecture for use by music sequencer applications. It was designed for applications running under Linux, although there is nothing specific to Linux ...

(DSSI) for

Linux Linux ( or ) is a family of open-source Unix-like operating systems based on the Linux kernel, an operating system kernel first released on September 17, 1991, by Linus Torvalds. Linux is typically packaged as a Linux distribution, whi ...

* LV2 (crossplatform successor to LADSPA) *

Microsoft Microsoft Corporation is an American multinational technology corporation producing computer software, consumer electronics, personal computers, and related services headquartered at the Microsoft Redmond campus located in Redmond, Washi ...

DirectX Microsoft DirectX is a collection of application programming interfaces (APIs) for handling tasks related to multimedia, especially game programming and video, on Microsoft platforms. Originally, the names of these APIs all began with "Direct" ...

's Virtual Studio Technology

References

Music software plugin architectures macOS APIs {{mac-stub