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What is JACK?

Have you ever wanted to take the audio output of one piece of software and send it to another? How about taking the output of that same program and send it to two others, then record the result in the first program? Or maybe you're a programmer who writes real-time audio and music applications and who is looking for a cross-platform API that enables not only device sharing but also inter-application audio routing, and is incredibly easy to learn and use? If so, JACK may be what you've been looking for.

JACK is system for handling real-time, low latency audio (and MIDI). It runs on GNU/Linux, Solaris, FreeBSD, OS X and Windows (and can be ported to other POSIX-conformant platforms). It can connect a number of different applications to an audio device, as well as allowing them to share audio between themselves. Its clients can run in their own processes (ie. as normal applications), or can they can run within the JACK server (ie. as a "plugin"). JACK also has support for distributing audio processing across a network, both fast & reliable LANs as well as slower, less reliable WANs.

JACK was designed from the ground up for professional audio work, and its design focuses on two key areas: synchronous execution of all clients, and low latency operation. More background information is available.

Understanding JACK in different ways

The term "JACK" doesn't really identify anything very precisely. What "JACK" means depends a little bit on your perspective. There are several ways of understanding the term, all equally valid:

The JACK API
The definition of the data structures, protocols and functions used by programs that use JACK
An implementation of the API
Actual software, including a server application, often called "jack", and a library used by clients. There are already two implementations of the API, known as "jack1" and "jack2" (formerly "jackmp"). Different implementations are (in almost every way) drop-in replacements for each other.
A GUI control application
The existing implementations do not come with any GUI at all. To make life easier for most users, GUI control apps exist that allow easy modification of JACK startup parameters, and ways to monitor the state of a running JACK system. qjackctl is the most widely used GUI control application.
A running instance of JACK with a variety of JACK-aware applications
This will likely include the server application, the control GUI, and zero or more actual applications that use JACK, such as Ardour (a DAW), Hydrogen (a drum machine) or many, many more.