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USING ALSA DIRECTLY
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Switch to root account.
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Install the required packages:
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dnf install alsa-plugins-jack
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Add yourself to the jackuser group.
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usermod -a -G audio,jackuser "<your username>"
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Now you will need to log out and log back in, or simply restart the system.
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Start the jack daemon (the best parameters for your sound card may be different):
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jackd -R -d alsa -d hw:0
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This will start jack on the first sound card, at 48KHz sampling rate (can be
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changed with the "-r" command line parameter), with a period of 1024 frames
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(change with "-p", determines the latency) and with 2 interrupts per period
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(change with -n). USB sound cards can benefit from running with "-n 3" or
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"-n 4". All of these optional parameters should be specified after "-d alsa".
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It is best to replace the card number with the card name as the numbering can
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change from boot to boot if there is more than one sound card. The name of a
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particular card can be found in the output of "cat /proc/asound/cards", for each
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card it is the string between square brackets without the trailing blanks.
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Note: Jack is a sound server and not a daemon - while it does work "in the
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background" it is not something similar to unix daemons which are started at
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boot time.
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QJACKCTL
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This application can be used to control the jack sound server with a graphical
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interface. It can be started via
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qjackctl -s
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"-s" parameter makes the jack sound server start immediately.
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When starting jack through qjackctl it is important to note that you should (at
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least) change the "Interface" field in the "Setup" dialog to _not_ be "(default)"
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which would point Jack to use Pulse Audio. The proper "Interface" is the direct
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hw ALSA device (hw:0 for the first card, etc). Even best to use the name of the
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card as stated above.
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USING THE JACK DBUS INTERFACE
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Control applications such as qjackctl can be configured to interact with the jack
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sound server using D-Bus. This has the advantage of allowing jack to request the
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release of the sound device from Pulseaudio (see below).
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jackdbus is shipped in a separate package. To install this (as root) do:
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dnf install jack-audio-connection-kit-dbus
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and check "Enable D-Bus interface" in Setup -> Misc within Qjackctl
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INTEGRATE JACK WITH PULSEAUDIO
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Jack will ask Pulse Audio through D-Bus for ownership of the sound card. Pulse
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Audio will grant it and Jack will have complete control of it. No conflicts and
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no configuration necessary. In this case Pulse Audio will stop using the card
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and if it is the only one, you will not have sound for system sounds, browser
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media playback, etc (which is usually what you want). Otherwise it is possible
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to load a Pulse Audio module that redirects PA to use Jack.
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RUNNING JACK SERVER IN REALTIME MODE
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NOTE: This chapter explains why you have done the above changes in more detail.
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It is kept for informational purposes. The changes suggested below is done
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automatically by your Jack installation and no manual modification of the
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pam configuration should be necessary.
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The JACK server jackd has the capability to run in a real-time mode
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which greatly decreases the chance of audio glitches. The real-time mode
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is enabled by passing the -R or --realtime option to jackd when starting
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the server. It is only possible to run jackd in real-time mode as a
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non-root user by modifying your PAM configuration, PAM stands for
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Pluggable Authentication Modules and is the primary authentification
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mechanism used on Fedora. The primary source of PAM documentation can be
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found at the following at http://www.kernel.org/pub/linux/libs/pam/
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The specific PAM configuration file that needs to be modified is
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/etc/security/limits.conf and it controls the system resource limits. It
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is important to understand that modifying the resource limits
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configuration files can decrease the security of your system.
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Documentation specific to the resource limits PAM module can be found at
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http://www.kernel.org/pub/linux/libs/pam/Linux-PAM-html/pam-6.html#ss6.12
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Alternatively, one can create a new file
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/etc/security/limits.d/<priority>-<name>.conf
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that contains the resource limits for individual users and/or groups. The
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jack-audio-connection-kit package that comes with Fedora contains the file
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/etc/security/limits.d/99-jack.conf
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that does this job for you.
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The resource limits that need to be changed to allow jackd to run in
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realtime mode are named rtprio and memlock. To increase the limits for a
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specific user named fred you would add the following to
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/etc/security/limits.conf
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fred - rtprio 20
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fred - memlock 50000
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The value of rtprio can be set in the range 0 - 99 where any value
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greater that 0 will allow the user to change the scheduling policy to
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"real-time". By default the JACK server requires a minimum rtprio
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setting of 20 but jackd will accept a command line parameter -P or
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--realtime-priority which will change the minimum required value of
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rtprio that is needed, but the default of 20 is nearly always
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sufficient.
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The appropriate value for memlock is dependent on the amount of memory
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present in the system but a minimum value of 50000(50MB) and a maximum
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value of half the available memory can be used as a rough guideline.
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To verify that the resource limits have been modified you can use the
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bash built-in ulimit command, for example:
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$ulimit -a
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core file size (blocks, -c) 0
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data seg size (kbytes, -d) unlimited
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max nice (-e) 0
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file size (blocks, -f) unlimited
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pending signals (-i) 8191
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max locked memory (kbytes, -l) 50000
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max memory size (kbytes, -m) unlimited
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open files (-n) 1024
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pipe size (512 bytes, -p) 8
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POSIX message queues (bytes, -q) 819200
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max rt priority (-r) 20
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stack size (kbytes, -s) 10240
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cpu time (seconds, -t) unlimited
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max user processes (-u) 8191
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virtual memory (kbytes, -v) unlimited
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file locks (-x) unlimited
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file locks (-x) unlimited
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Keep in mind that you may have to re-login before changes to limits.conf,
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or additions to the limits.d/ directory to take effect.
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Arkady L. Shane