JACK

JACK, JACK Audio Connection Kit, is a sound server for professional audio production that provides low-latency communication for applications that implement the JACK API, such as Ardour and mpv.

Background

The following is a general introduction to JACK; some parts of it might be outdated.

JACK adds a layer to ALSA that guarantees constant sound latency and synchronous operation to all its clients. JACK can only use one sound card at a time, and will provide separated outputs and inputs for each audio channel of the sound card. JACK further provides the ability to view, manipulate or pipe audio streams, both hardware and software streams, in a similar manner to using cables to interface different audio equipment. For example, JAMin can intercept an audio stream before exporting the stream to another audio application and/or through analog or S/PDIF outputs. The user-owned JACK daemon is usually started using media-sound/qjackctl , which provides many other functions, including audio stream connections.

JACK does not handle A/52 encoded (AC-3 or Dolby/DTS) material on input. It's just a sound server that deals with separated channels of audio streams, with the capability to connect - simultaneously and synchronously - any output stream on any input, and any input to any output stream, on any hardware and software audio component, with a constant latency. This makes JACK an outstanding tool for audio production and creation. The ac3jack tool can be used for encoding multiple separated audio channels to AC-3 streams.

When playing A/52 encoded media, the player will request as many outputs from JACK as audio channels in the media; this will fail if the sound card being used doesn't have the requested amount of audio outputs. For example, when using MPlayer, it will need to be configured to output a 5.1 stream on a 7.1 output configuration, or a 7.1 stream on a 2 channel stereo output.

One solution is to configure the kernel and ALSA according to the JACK installation instructions for providing low-latency audio. Since ALSA is said to already be very efficient and low latency, providing very good quality playback with no additional mixing when appropriately configured via an ~/.asoundrc and/or /etc/asoundrc, JACK probably isn't needed for most situations. The main audience of JACK is audio producers and musicians (in studio, live performances, DJing, etc). The proaudio-gentoo overlay is available via eselect-repository.

For an older solution using the zita-ajbridge, refer to "Using an ALSA Loopback device and zita-ajbridge" on the ProAudio Gentoo overlay wiki.

Using zita-ajbridge, it is also possible to add additional real sound cards into JACK.

Those wanting to try JACK should know that a realtime (RT) kernel and RT operations are only needed when using the computer as a Digital Audio Workstation (DAW). A DAW necessitates something PulseAudio is not able to achieve: synchronous operations, and constant sound latency as low or lower than 20 msec. (PulseAudio is not able to provide constant sound latency at all.) JACK can be an alternative to PulseAudio for users that want a professional solution.

Installation

JACK uses ALSA for accessing the sound card hardware, be sure ALSA is working.

There are two implementations of the JACK API, both implementations are considered equivalent. JACK2 is usually the one to go for, JACK 1 is no longer under active development.

JACK is the original implementation, it uses a C API and has built-in Linux MIDI integration. JACK2 is a re-implementation in C++ that has support for multiprocessing and D-Bus, while MIDI support is handled by ALSA. Visit what are the differences between JACK 1 and JACK2 for an in-depth comparison.

USE flags

Emerge

Emerge the sound server:

The JACK 1 package is media-sound/jack-audio-connection-kit (link provides a list of JACK 1 USE flags).

Global JACK USE flag

The global jack USE flag enables support for JACK in other packages, so they can submit sound to a JACK server:

The euse command is part of app-portage/gentoolkit .

After setting this, be sure to update the system so the changes take effect:

Additional software

There is a JACK configuration and control software package media-sound/cadence :

Another alternative is media-sound/qjackctl :

See the article on music production for software to use with JACK.

The JACK website has a page on sofware that uses JACK.

Configuration

Sound server

All users that need JACK should be in the audio group:

JACK can be configured using the jack_control utility. A basic configuration script for JACK could look like:

jack_control start
jack_control ds alsa
jack_control dps device hw:2,0
jack_control dps rate 48000
jack_control dps nperiods 2
jack_control dps period 64

To determine the appropriate playback device (instead of hw:2,0):

Bridging

JACK can route sound which was sent to other sound systems like PulseAudio ^[2] and ALSA through itself.

ALSA

To use the ALSA only applications with JACK, you will need 2 things: the snd_aloop module and the jack ALSA plugin.

The snd-aloop is the Loopback virtual sound card. This will be used as the default ALSA sound card. That allow the ALSA applications to use it without the need to change their default configuration, the jack ALSA plugin will then route these applications to the JACK graph.

ALSA application <=> Loopback <=> ALSA jack plugin <=> JACK

Loopback device

Kernel configuration

Activate the following kernel options:

Device Drivers --->
 <M> Sound card support
 <M> Advanced Linux Sound Architecture --->
 <*> Generic Sound Devices --->
 <M> Generic loopback driver (PCM)

To configure the Loopback as the default ALSA sound card, we need to create the file /etc/modprobe.d/alsa.conf. Example with several sound cards, it's probably an overkill due to the evolution of the default ALSA configuration, but it has been working from years on several computers with very minor changes. It can also be used in other cases when the computer have several sound cards and a constancy order between reboot is a must.

Sound card 0 is the Loopback virtual device, sound card 1 and 2 are the built-in audio card which have 2 devices, one being the HDMI, sound card 3 is for an USB card.

# Alsa kernel modules' configuration file.
options snd slots=snd-aloop,snd-hda-intel,snd-hda-intel,snd-usb-audio
options snd-hda-intel index=1,2 model=1002:1637,1022:15e3

The cards index start from 0. For more details about that file, see Alsa Opensrc Org support page for multiple sound cards configuration.

To get the model strings, use lspci (or lsusb for USB cards):

04:00.1 Audio device: Advanced Micro Devices, Inc. [AMD/ATI] Renoir Radeon High Definition Audio Controller
04:00.6 Audio device: Advanced Micro Devices, Inc. [AMD] Family 17h/19h HD Audio Controller

04:00.1 0403: 1002:1637

04:00.6 0403: 1022:15e3

We instruct the kernel to load the virtual sound card:

snd-aloop

Jack ALSA plugin

The jack ALSA plugin provide a better and more elegant solution than the previous ones like the zita-ajbridge (see Using an ALSA Loopback device and zita-ajbridge on the ProAudio Gentoo overlay wiki - the author of zita-ajbridge will maybe disagree on that...).

Installation:

We need at least the jack USE flag.

Configuration:

pcm.!default {
 type plug
 slave { pcm "jack" }
}
 
pcm.jack {
 type jack
 playback_ports {
 0 system:playback_1
 1 system:playback_2
 }
 capture_ports {
 0 system:capture_1
 1 system:capture_2
 }
}
 
ctl.mixer0 {
 type hw
 card 0
}

That file is necessary when using qjackctl or the command line to manage the JACK server. Cadence will manage it for You: On its System tab, select Bridge Type: ALSA -> Jack (Plugin).

Now, You can configure JACK to use the wanted and real sound card as usual, and all the ALSA applications will be available as per magic into the JACK graph.

PulseAudio

To use PulseAudio's JACK module media-sound/pulseaudio needs to have the jack USE flag enabled.

In order to route all audio from PulseAudio to JACK, the JACK sink needs to be configured:

PulseAudio will recognize that JACK started and will bridge its audio to JACK.

Troubleshooting

"Cannot use real-time scheduling" error

Some applications may show a permission error when trying to connect to Jack, for example:

ERROR: JACK: Cannot create thread res = 1
ERROR: JACK: JackClient::AcquireSelfRealTime error
ERROR: JACK: Cannot use real-time scheduling (RR/5) (1: Operation not permitted)

Adding the current user to the realtime group should solve this:

Log out and log back in to apply group change.

See also

• ALSA — the Linux kernel's API for sound cards, together with an associated software framework
• PipeWire — low-latency, graph-based, processing engine and server, for interfacing with audio and video devices.
• PulseAudio — a multi-platform, open source, sound server that provides a number of features on top of the low-level audio interface ALSA

References

• Stale
• Audio
• Sound devices
• Audio production tools