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JACK Audio Connection Kit¶

JACK Audio Connection Kit (or JACK; a recursive acronym) is a professional sound server daemon that provides real-time, low-latency connections for both audio and MIDI data between applications that use its API.

There are two JACK implementations (JACK1 and JACK2), see this comparison for the difference between the two. In short, JACK1 and JACK 2 are equivalent implementations of the same protocol. JACK2 uses dbus to talk with other applications such as PulseAudio, X. Using JACK2 without X requires some additional hacks. So we use JACK1 instead of JACK2 here.

Install¶

sudo apt install jackd1

Use JACK as an ALSA PCM¶

Edit /etc/asound.conf (system wide settings) to have these lines:

pcm.rawjack {
    type jack
    playback_ports {
        0 system:playback_1
    }
}

pcm.jack {
    type plug
    slave { pcm "rawjack" }
    hint {
        description "JACK Audio Connection Kit"
    }
}

Run¶

jackd -v -d alsa -d hw:Codec -P

Play audio¶

aplay -v -D jack audio.wav

Configure¶

$ cat /etc/security/limits.d/audio.conf 
# Provided by the jackd package.
#
# Changes to this file will be preserved.
#
# If you want to enable/disable realtime permissions, run
#
#    dpkg-reconfigure -p high jackd

@audio   -  rtprio     95
@audio   -  memlock    unlimited
#@audio   -  nice      -19

Multiple users¶

To setup JACK for multiple users, we can run JACK in promiscuous mode.

start JACK with a systemd service

echo '
[Unit]
Description=Start the JACK server in promiscuous mode
After=sound.target

[Service]
Type=simple
Environment="JACK_PROMISCUOUS_SERVER="
UMask=0
ExecStart=/usr/bin/jackd -d alsa -d hw:Codec -P -o 1

[Install]
WantedBy=multi-user.target

' | sudo tee /etc/systemd/system/jackd.service
sudo systemctl start jackd
sudo systemctl enable jackd

export the environment variable JACK_PROMISCUOUS_SERVER to let a JACK client use JACK
```
JACK_PROMISCUOUS_SERVER=
export JACK_PROMISCUOUS_SERVER
aplay -v audio.wav
```