From Linux Man Pages
jackd, jackstart - JACK Audio Connection Kit sound server
SYNOPSYS
jackd [options] -d backend [backend-parameters]
jackstart [options] -d backend [backend-parameters]
jackd --help
DESCRIPTION
jackd invokes the JACK audio server daemon. The jackstart command provides JACK's built-in support for Linux
2.4.x kernels with the realtime capabilities patch. Use jackd for Linux 2.6.x and all other systems. All param-
eters are the same.
JACK is a low-latency audio server. Originally written for the GNU/Linux operating system, it also supports Mac
OS X and various Unix platforms. JACK can connect a number of different client applications to an audio device
and also to each other. Most clients are external, running in their own processes as normal applications. JACK
also supports internal clients, which run within the jackd process using a loadable "plugin" interface.
JACK differs from other audio servers in being designed from the ground up for professional audio work. It
focuses on two key areas: synchronous execution of all clients, and low latency operation.
For the latest JACK information, please consult the web site, <http://www.jackaudio.org>.
OPTIONS
-d, --driver backend [backend-parameters ]
Select the audio interface backend. The current list of supported backends is: alsa, coreaudio, dummy,
freebob, oss and portaudio. They are not all available on all platforms. All backend-parameters are
optional.
-h, --help
Print a brief usage message describing the main jackd options. These do not include backend-parameters,
which are listed using the --help option for each specific backend. Examples below show how to list them.
-m, --no-mlock
Do not attempt to lock memory, even if --realtime.
-n, --name server-name
Name this jackd instance server-name. If unspecified, this name comes from the $JACK_DEFAULT_SERVER envi-
ronment variable. It will be "default" if that is not defined.
-p, --port-max n
Set the maximum number of ports the JACK server can manage. The default value is 256.
-R, --realtime
Use realtime scheduling. This is needed for reliable low-latency performance. On most systems, it
requires jackd to run with special scheduler and memory allocation privileges, which may be obtained in
several ways. The simplest, and least-secure method is to run jackd with root privileges. This means
that all JACK clients must also run as root. With a Linux 2.6 kernel, ordinary users can run jackd and
its clients using options of the realtime LSM. Linux 2.4 kernels need "POSIX draft capabilities" enabled
(see the <linux/capability.h> include file). Using that method, ordinary users who invoke the daemon
using jackstart, can later launch JACK clients without running them as root. See http://jackit.source-
forge.net/docs/faq.php#a52 for more information.
-P, --realtime-priority int
When running --realtime, set the scheduler priority to int.
--silent
Silence any output during operation.
-T, --temporary
Exit once all clients have closed their connections.
-t, --timeout int
Set client timeout limit in milliseconds. The default is 500 msec. In realtime mode the client timeout
must be smaller than the watchdog timeout (5000 msec).
-u, --unlock
Unlock libraries GTK+, QT, FLTK, Wine.
-v, --verbose
Give verbose output.
-c, --clocksource ( c(ycle) | h(pet) | s(ystem) )
Select a specific wall clock (Cycle Counter, HPET timer, System timer).
-V, --version
Print the current JACK version number and exit.
ALSA BACKEND OPTIONS
-C, --capture [ name ]
Provide only capture ports, unless combined with -D or -P. Parameterally set capture device name.
-d, --device name
The ALSA pcm device name to use. If none is specified, JACK will use "hw:0", the first hardware card
defined in /etc/modules.conf.
-z, --dither [rectangular,triangular,shaped,none]
Set dithering mode. If none or unspecified, dithering is off. Only the first letter of the mode name is
required.
-D, --duplex
Provide both capture and playback ports. Defaults to on unless only one of -P or -C is specified.
-h, --help Print a brief usage message describing only the
alsa backend parameters.
-M, --hwmeter
Enable hardware metering for devices that support it. Otherwise, use software metering.
-H, --hwmon
Enable hardware monitoring of capture ports. This is a method for obtaining "zero latency" monitoring of
audio input. It requires support in hardware and from the underlying ALSA device driver.
When enabled, requests to monitor capture ports will be satisfied by creating a direct signal path between
audio interface input and output connectors, with no processing by the host computer at all. This offers
the lowest possible latency for the monitored signal.
Presently (March 2003), only the RME Hammerfall series and cards based on the ICE1712 chipset (M-Audio
Delta series, Terratec, and others) support --hwmon. In the future, some consumer cards may also be sup-
ported by modifying their mixer settings.
Without --hwmon, port monitoring requires JACK to read audio into system memory, then copy it back out to
the hardware again, imposing the basic JACK system latency determined by the --period and --nperiods
parameters.
-i, --inchannels int
Number of capture channels. Default is maximum supported by hardware.
-n, --nperiods int
Specify the number of periods of playback latency. In seconds, this corresponds to --nperiods times
--period divided by --rate. The default is 2, the minimum allowable. For most devices, there is no need
for any other value with the --realtime option. Without realtime privileges or with boards providing
unreliable interrupts (like ymfpci), a larger value may yield fewer xruns. This can also help if the sys-
tem is not tuned for reliable realtime scheduling.
For most ALSA devices, the hardware buffer has exactly --period times --nperiods frames. Some devices
demand a larger buffer. If so, JACK will use the smallest possible buffer containing at least --nperiods,
but the playback latency does not increase.
-o, --outchannels int
Number of playback channels. Default is maximum supported by hardware.
-P, --playback [ name ]
Provide only playback ports, unless combined with -D or -C. Optionally set playback device name.
-p, --period int
Specify the number of frames between JACK process() calls. The default is 1024. If you need low latency,
set -p as low as you can go without seeing xruns. A larger period size yields higher latency, but makes
xruns less likely. The JACK capture latency in seconds is --period divided by --rate.
-r, --rate int
Specify the sample rate. The default is 48000.
-S, --shorts
Try to configure card for 16-bit samples first, only trying 32-bits if unsuccessful. Default is to prefer
32-bit samples.
-s, --softmode
Ignore xruns reported by the ALSA driver. This makes JACK less likely to disconnect unresponsive ports
when running without --realtime.
COREAUDIO BACKEND PARAMETERS
-c --channel
Maximum number of channels (default: 2)
-i --channelin
Maximum number of input channels (default: 2)
-o --channelout
Maximum number of output channels (default: 2)
-C --capture
Whether or not to capture (default: true)
-P --playback
Whether or not to playback (default: true)
-D --duplex
Capture and playback (default: true)
-r --rate
Sample rate (default: 44100)
-p --period
Frames per period (default: 128)
-n --name
Driver name (default: none)
-I --id
Audio Device ID (default: 0)
DUMMY BACKEND PARAMETERS
-C, --capture int
Specify number of capture ports. The default value is 2.
-P, --playback int
Specify number of playback ports. The default value is 2.
-r, --rate int
Specify sample rate. The default value is 48000.
-p, --period int
Specify number of frames per period. The default value is 1024. If you need low latency, set -p as low as
you can go without seeing xruns. A larger period size yields higher latency, but makes xruns less likely.
The JACK capture latency in seconds is --period divided by --rate.
-w, --wait int
Specify number of usecs to wait between engine processes. The default value is 21333.
OSS BACKEND PARAMETERS
-r, --rate int
Specify the sample rate. The default is 48000.
-p, --period int
Specify number of frames between JACK process() calls. The default is 1024. If you need low latency, set
-p as low as you can go without seeing xruns. A larger period size yields higher latency, but makes xruns
less likely. The JACK capture latency in seconds is --period divided by --rate.
-n, --nperiods int
Specify the number of periods in the hardware buffer. The default is 2. The period size (-p) times
--nperiods times four is the JACK buffer size in bytes. The JACK output latency in seconds is --nperiods
times --period divided by --rate.
-w, --wordlength int
Specify the sample size in bits. The default is 16.
-i, --inchannels int
Specify how many channels to capture (default: 2)
-o, --outchannels int
Specify number of playback channels (default: 2)
-C, --capture device_file
Specify input device for capture (default: /dev/dsp)
-P, --playback device_file
Specify output device for playback (default: /dev/dsp)
-b, --ignorehwbuf boolean
Specify, whether to ignore hardware period size (default: false)
PORTAUDIO BACKEND PARAMETERS
-c --channel
Maximum number of channels (default: all available hardware channels)
-i --channelin
Maximum number of input channels (default: all available hardware channels)
-o --channelout
Maximum number of output channels (default: all available hardware channels)
-C --capture
Whether or not to capture (default: true)
-P --playback
Whether or not to playback (default: true)
-D --duplex
Capture and playback (default: true)
-r --rate
Sample rate (default: 48000)
-p --period
Frames per period (default: 1024)
-n --name
Driver name (default: none)
-z --dither
Dithering mode (default: none)
EXAMPLES
Print usage message for the parameters specific to each backend.
jackd -d alsa --help
jackd -d coreaudio --help
jackd -d dummy --help
jackd -d freebob --help
jackd -d oss --help
jackd -d portaudio --help
Run the JACK daemon with realtime priority using the first ALSA hardware card defined in /etc/modules.conf.
jackstart --realtime --driver=alsa
Run the JACK daemon with low latency giving verbose output, which can be helpful for trouble-shooting system
latency problems. A reasonably well-tuned system with a good sound card and a low-latency kernel can handle
these values reliably. Some can do better. If you get xrun messages, try a larger buffer. Tuning a system for
low latency can be challenging. The JACK FAQ, http://jackit.sourceforge.net/docs/faq.php has some useful sugges-
tions.
jackstart -Rv -d alsa -p 128 -n 2 -r 44100
Run jackd with realtime priority using the "sblive" ALSA device defined in ~/.asoundrc. Apply shaped dithering
to playback audio.
jackd -R -d alsa -d sblive --dither=shaped
Run jackd with no special privileges using the second ALSA hardware card defined in /etc/modules.conf. Any xruns
reported by the ALSA backend will be ignored. The larger buffer helps reduce data loss. Rectangular dithering
will be used for playback.
jackd -d alsa -d hw:1 -p2048 -n3 --softmode -zr
Run jackd in full-duplex mode using the ALSA hw:0,0 device for playback and the hw:0,2 device for capture.
jackd -d alsa -P hw:0,0 -C hw:0,2
Run jackd in playback-only mode using the ALSA hw:0,0 device.
jackd -d alsa -P hw:0,0
ENVIRONMENT
JACK is evolving a mechanism for automatically starting the server when needed. Any client started without a
running JACK server will attempt to start one itself using the command line found in the first line of
$HOME/.jackdrc if it exists, or /etc/jackdrc if it does not. If neither file exists, a built-in default command
will be used, including the -T flag, which causes the server to shut down when all clients have exited.
As a transition, this only happens when $JACK_START_SERVER is defined in the environment of the calling process.
In the future this will become normal behavior. In either case, defining $JACK_NO_START_SERVER disables this
feature.
To change where JACK looks for the backend drivers, set $JACK_DRIVER_DIR.
$JACK_DEFAULT_SERVER specifies the default server name. If not defined, the string "default" is used. If set in
their respective environments, this affects jackd unless its --name parameter is set, and all JACK clients unless
they pass an explicit name to jack_client_open().
SEE ALSO:
http://www.jackaudio.org
The official JACK website with news, docs and a list of JACK clients.
http://lists.sourceforge.net/mailman/listinfo/jackit-devel
The JACK developers' mailing list. Subscribe, to take part in development of JACK or JACK clients. User ques-
tions are also welcome, there is no user-specific mailing list.
http://www.jackosx.com/
Tools specific to the Mac OS X version of JACK.
http://www.alsa-project.org
The Advanced Linux Sound Architecture.
BUGS
Please report bugs to
http://jackit.sourceforge.net/mantis/main_page.php
CATEGORY