jackd

jackd [options]
jackd -d [driver_options]

-d
    Specifies the audio driver to use (e.g., alsa, firewire, portaudio, dummy). This is typically the first option after the command name.

-r
    Sets the sample rate in Hz (e.g., 44100, 48000, 96000). All JACK clients will operate at this rate.

-p
    Sets the number of frames per period (buffer size). Smaller values yield lower latency but require more CPU power.

-n
    Sets the number of periods/buffers per process cycle. Often set to 2 or 3 for stable operation.

-S
    Starts the JACK server in SUSPENDED mode. It will not start processing audio until a client explicitly activates it.

-P
    Sets the real-time priority for the JACK server thread. Higher numbers (e.g., 85) indicate higher priority.

-t
    Sets the server timeout in milliseconds. If no clients connect within this time, the server exits.

-X
    Provides a unique name for this JACK server instance. Useful when running multiple JACK servers.

-m
    Enables memory locking (mlockall), preventing JACK's memory from being swapped out, which is crucial for real-time performance.

jackd is the daemon for the JACK Audio Connection Kit, a professional sound server API and daemon that provides low-latency audio and MIDI connectivity between applications. It runs in the background, managing audio streams, synchronizing applications, and routing connections between them. Designed primarily for professional audio and music production on Linux, jackd offers features like sample-accurate synchronization, flexible routing capabilities, and support for various audio drivers such as ALSA, FFADO (FireWire), and PortAudio.

It enables a wide range of audio software, including DAWs, synthesizers, and effects processors, to work together seamlessly with minimal latency, making it an essential component for serious audio work on Linux.

Optimal performance of jackd, particularly very low latency, typically requires a real-time kernel and appropriate user permissions (e.g., being part of the 'audio' group). Without these, you may experience 'xruns' (buffer underruns/overruns) or higher latency.

Configuration can be complex; understanding the interplay of sample rate, period size, and number of periods is key to stable, low-latency operation. Interoperability with other sound systems like PulseAudio can also require careful configuration to ensure devices are released for JACK's exclusive use.

For critical audio applications, it's highly recommended to run jackd on a Linux system configured with a real-time kernel. Additionally, the user running jackd and its clients should be a member of the 'audio' group, which typically grants permissions for real-time scheduling and memory locking. This setup minimizes latency and prevents audio dropouts (xruns).

While jackd can be launched from the command line, most users find it much easier to manage and configure using a graphical front-end like QjackCtl (qjackctl(1)). QjackCtl provides an intuitive interface for starting/stopping the server, setting parameters, managing connections between applications, and monitoring performance.

The JACK Audio Connection Kit was primarily developed by Paul Davis, with its initial public release around 2002. It was created to address the need for a robust, low-latency audio interconnection system for Linux, similar to proprietary solutions on other operating systems. JACK quickly became the de-facto standard for professional audio and music production on the Linux platform, fostering a vibrant ecosystem of audio applications. Its design emphasizes stability, low latency, and flexible routing, which were significant advancements for open-source audio development at the time.

qjackctl(1), jack_lsp(1), jack_connect(1), jack_disconnect(1), alsa(7), pulseaudio(1)