ALSA, Jack, Pulseaudio and Systemd User Sessions

This is the audio setup for my hardware. Goals are:

• Use systemd user sessions, so the sound settings are persistent across logins.
• Use Pulseaudio for non-realtime audio but bridge it to Jack.
• Get my Focusrite Scarlett 8i6 interface to work as good as possible, because it suffers from minor bugs in the kernel driver. Of the 6 mixes only 4 are working, looks like on offset by 2 error. After initialization some channels need to be toggled on/off, or switched to another input and back until they route audio.

ALSA Setup

These are the cards for my hardware:

cat /sys/class/sound/card*/id

Nvidia

NVIDIA HDMI, Card 0, 4 Devices

Generic

Motherboard HD Audio (ALC 1220), Card 1, 2 Devices

Adapter

USB Rocksmith Guitar Adapter, Card 2, 1 Device

USB

Focusrite Scarlet 8i6 in, Card 3, 1 Device

and here is the configuration for kernel modules and alsa. The kernel module options make sure that the modules are loaded in the same order every boot.

# Alsa kernel modules' configuration file.

# ALSA portion
alias char-major-116 snd
options snd major=116 cards_limit=5 slots=snd_aloop,snd_hda_intel,snd_hda_intel,snd_usb_audio,snd_usb_audio

# Loopback
alias snd-card-0 snd_aloop
options snd_aloop index=0

# Internal Audio and NVidia HDMI Audio
alias snd-card-1 snd_hda_intel
alias snd-card-2 snd_hda_intel
options snd_hda_intel index=1,2

# USB: index 3 = Scarlett 8i6, 4 = Rocksmith Adapter
alias snd-card-3 snd_usb_audio
alias snd-card-4 snd_usb_audio
options snd_usb_audio index=3,4 vid=0x8002,0x00ff pid=0x1235,0x12ba 

The alsa config snippet gives some nicer names to the hardware cards. We use these later.

pcm.hdmi0 {
    type hw
    card NVidia
    device 0
    hint.description "HDMI Audio Output"
}
 
ctl.hdmi0 {
    type hw
    card NVidia
    device 0
}
 
pcm.hdmi1 {
    type hw
    card NVidia
    device 1
    hint.description "HDMI Audio Output"
}
 
ctl.hdmi1 {
    type hw
    card NVidia
    device 1
}
 
pcm.hdmi2 {
    type hw
    card NVidia
    device 2
    hint.description "HDMI Audio Output"
}
 
ctl.hdmi2 {
    type hw
    card NVidia
    device 2
}
 
pcm.hdmi3 {
    type hw
    card NVidia
    device 3
    hint.description "HDMI Audio Output"
}
 
ctl.hdmi3 {
    type hw
    card NVidia
    device 3
}
 
pcm.analog {
    type hw
    card Generic
    device 0
    hint.description "Internal HD audio, analog"
}
 
ctl.analog {
    type hw
    card Generic
    device 0
}
 
pcm.digital {
    type hw
    card Generic
    device 1
    hint.description "Internal HD audio, optical digital"
}
 
ctl.digital {
    type hw
    card Generic
    device 1
}
 
pcm.scarlett {
    type hw
    card USB
    device 0
    hint.description "Focusrite Scarlett 8i6 USB"
}
 
ctl.scarlett {
    type hw
    card USB
    device 0
}
 
pcm.rocksmith {
    type hw
    card Adapter
    device 0
    hint.description "Rocksmith USB Adapter"
}
 
ctl.rocksmith {
    type hw
    card Adapter
    device 0
}
 
pcm.default pulse
ctl.default pulse

The alsa-setup.service restores the alsa cards settings from a file in ~/.config/alsa/<CARD_ID>.state. Unfortunately this is not enough for my Focusrite Scarlett 8i6, the Channels are muted unless they are toggled on/off one time. So the alsa-setup script also calls ~/.config/alsa/<CARD_ID>.setup with the CARD_ID as parameter to do any further necessary setup.

Jack

Jack is started after pulseaudio, to give pulseaudio the chance to load its Jack bridging modules. It might be wise to disable the ALSA cards Jack should use in pulseaudio via pavucontrol.

In case jack can not switch to realtime mode when started from a user session systemd service, but does it just fine from the X11 session dbus, check if /etc/pam.d/systemd-user contains the line

session required pam_limits.so

and, if not, add it.

jack.service starts the DBus variant of Jack (Jack2). It stores its configuration in ~/config/jack/conf.xml. For ease of importing these settings into dependent service unit files, the settings are exported into a shell variable like environment file in ~/.config/jack/conf.env.

Additional services for a ALSA-MIDI/Jack-MIDI bridge and the import of the ‘digital’ ALSA card into Jack exists, which read this environment file.

Pulseaudio

I want Pulseaudio for the ease of integrating non Jack-aware desktop applications. It is bridged to Jack via module-jack-source and module-jack-sink. Pulseaudio writes an error about not beeing able to set realtime scheduling for the jack modules into the journal, despite the correct parameters in the service files. There is an open issue for this in the PulseAudio bugtracker. I think this is only on issue if you run Jack in sync-mode.

Systemd and DBus

Systemd and DBus need to support User-Session semantics for this setup to work. In Gentoo they need to be emerged with the user-session USE flag.

The package uses environment generators to make plugin paths and to put jack parameters into the environment for usage in dependent services. I also makes a file ~/.profile.env, with these variables and the ones from ~/.config/environment.d/*.conf, which can be sourced from a shell startup script.

Installation

Copy or link user-environment-generators/audio-plugin-path-generator into /etc/systemd/user-environment-generators and enable pulseaudio.service, jack.service and any other services you like.