Using JACK with PulseAudio

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The Jack Audio Connection Kit is used by most of the serious audio applications on Linux. It provides real-time, low latency connections for both audio and MIDI data between applications that implement its API. NixOS uses the dbus version of JACK2 (jackdbus). This can be used together with pulseaudio with a little configuration. The result is that you don't have to manually hunt down applications which are using the sound device and kill them before starting JACK. You can also continue to use non-JACK aware applications (e.g. flash) at the same time as using JACK applications (e.g. Ardour).

1. Load the sequencer and midi kernel modules

   boot.kernelModules = [ "snd-seq" "snd-rawmidi" ];

2. Enable JACK support

   In your configuration file:

   hardware.pulseaudio.package = pkgs.pulseaudio.override { jackaudioSupport = true; };
   

3. Ensure that the JACK enabled pulseaudio is being used

   ~/.config/pulse/client.conf

   daemon-binary=/var/run/current-system/sw/bin/pulseaudio

4. Configure QjackCtl
   1. Enable jackdbus

      Setup -> Settings -> Server Path: jackdbus

      

      Setup -> Misc -> Enable D-Bus interface: check

      

   2. Load the jack modules for pulseaudio after starting jackdbus

      Setup -> Settings -> Options -> Execute script after Startup: check

      pactl load-module module-jack-sink channels=2; pactl load-module module-jack-source channels=2; pacmd set-default-sink jack_out

      Setup -> Settings -> Options -> Execute script on Shutdown: check

      pactl unload-module `pactl list|grep -A 3 jack-source|tail -1|awk '{ print $NF }'`;pactl unload-module `pactl list|grep -A 3 jack-sink|tail -1|awk '{ print $NF }'`

      

You should now be able to start JACK with QjackCtl, you will notice a new playback and capture device in your sound mixer along with your normal devices.

Troubleshooting JACK and PulseAudio

$ pactl load-module module-jack-sink channels=2
Failure: Module initalization failed

Check if you have previous settings in ~/.config/jack/conf.xml. Try renaming this file and running the pactl command again.

Otherwise, you may get further info by disabling PulseAudio respawning (see above) and starting it in verbose mode:

$ pulseaudio -vvv

System optimizations for low latency audio with JACK

Some of the following settings, documented in http://wiki.linuxmusicians.com/doku.php?id=system_configuration and https://wiki.archlinux.org/index.php/Pro_Audio can be very helpful to reduce xruns and improve responsiveness and are required for certain programs to run at all e.g. Ardour. The kernelPackages section was taken from https://github.com/rockfabrik/deployment/blob/master/modules/profiles/dj.nix#L32

An easy way of setting all of these, plus a true realtime kernel, is here: https://github.com/musnix/musnix

boot = {
  kernelModules = [ "snd-seq" "snd-rawmidi" ];
  kernel.sysctl = { "vm.swappiness" = 10; "fs.inotify.max_user_watches" = 524288; };
  kernelParams = [ "threadirq" ];
  kernelPackages = let 
    rtKernel = pkgs.linuxPackagesFor (pkgs.linux.override {
      extraConfig = ''
        PREEMPT_RT_FULL? y
        PREEMPT y
        IOSCHED_DEADLINE y
        DEFAULT_DEADLINE y
        DEFAULT_IOSCHED "deadline"
        HPET_TIMER y
        CPU_FREQ n
        TREE_RCU_TRACE n
      '';
    }) pkgs.linuxPackages;
  in rtKernel;

  postBootCommands = ''
    echo 2048 > /sys/class/rtc/rtc0/max_user_freq
    echo 2048 > /proc/sys/dev/hpet/max-user-freq
    setpci -v -d *:* latency_timer=b0
    setpci -v -s $00:1b.0 latency_timer=ff
  '';
  # The SOUND_CARD_PCI_ID can be obtained like so:
  # $ lspci ¦ grep -i audio
};

powerManagement.cpuFreqGovernor = "performance";

fileSystems."/" = { options = "noatime errors=remount-ro"; };

security.pam.loginLimits = [
  { domain = "@audio"; item = "memlock"; type = "-"; value = "unlimited"; }
  { domain = "@audio"; item = "rtprio"; type = "-"; value = "99"; }
  { domain = "@audio"; item = "nofile"; type = "soft"; value = "99999"; }
  { domain = "@audio"; item = "nofile"; type = "hard"; value = "99999"; }
];

services = {
  udev = {
    packages = [ pkgs.ffado ]; # If you have a FireWire audio interface
    extraRules = ''
      KERNEL=="rtc0", GROUP="audio"
      KERNEL=="hpet", GROUP="audio"
    '';
  };
  cron.enable =false;
};

shellInit = ''
  export VST_PATH=/nix/var/nix/profiles/default/lib/vst:/var/run/current-system/sw/lib/vst:~/.vst
  export LXVST_PATH=/nix/var/nix/profiles/default/lib/lxvst:/var/run/current-system/sw/lib/lxvst:~/.lxvst
  export LADSPA_PATH=/nix/var/nix/profiles/default/lib/ladspa:/var/run/current-system/sw/lib/ladspa:~/.ladspa
  export LV2_PATH=/nix/var/nix/profiles/default/lib/lv2:/var/run/current-system/sw/lib/lv2:~/.lv2
  export DSSI_PATH=/nix/var/nix/profiles/default/lib/dssi:/var/run/current-system/sw/lib/dssi:~/.dssi
'';

users = {
  extraUsers.yourname= {
   extraGroups = [ "wheel" "audio" ];
  };
};