SYSJACK is an aid to simplify installation and maintenance of a JACK daemon for devices to which it is assumed that haven't access to terminal or GUI.
I am carrying out a project for a headless synth with raspbery pi 3 and 4, and I'm using a modular approach to better manage and share mine work. SYSJACK is one of these modules.
SYSJACK locates the sound cards available with ALSA, allows you to choose and set your configuration, and then save it to a .json file, so you can reuse JACK's settings with other applications.
As a bonus, SYSJACK can create other systemd services, which can be really useful if you are planning to do a simple synth, i.e. a single process running on JACK.
Benefits of this approach are:
- all systemd benefits including restart on crash;
- hierarchy: run other stuff only after JACKD is started;
- configurable: sometimes you have to change parameters over and over to get the right configuration. Some other times you need to know certain parameters. This helped me a lot in checking out the right configuration for my headless pi.
You need a Linux OS with a low latency kernel. To find out if yours kernels are realtime, write:
uname -a
if PREEMPT is present in the name of your operating system, you have low latency kernels. You can try to build one instead.
If you are on Raspberry pi3/pi4, give a chance to the 64 bit edition of Raspberry OS, as of 5.10 Raspberry Pi OS is pre-built for low latency.
Install jack daemon first:
sudo apt-get install --no-recommends jackd2
sudo /sbin/setcap cap_ipc_lock,cap_sys_nice=ep /usr/bin/jackd
The latter I have found on this thread and seems to be necessary for jackd to be able to obtain realtime privileges. May be needed for any process you want to run realtime.
jackd2 should be preferred to jackd1 as it should have a fix with the infamous d-bus error. SYSJACK does take care of that too.
An external sound card is recommended since pi's internal audio card is hardly capable of handling realtime audio, as it works with an intermediate buffer. I never managed to run jackd succesfully with the builtin audio card. I guess it would be possible with the highest possible latency...
sudo apt-get install git perl libjson-perl libasound2-dev
This will help you install PERL and some C libraries for ALSAcap, a small utility listed below. It will also help you install git, useful for downloading sysjack!
ALSACAP is a small piece of code by Volker Schatz. Source code is included here as it is not easy to find. It will enumerate any audio device capability, including sample rate and sample format. You may need tools such as make and autotools to compile it. It is not necessary but useful. It is embedded with sysjack.
- First, clone this repository.
git clone https://github.com/StefanoMarina/sysjack.git sysjack; - Enable execution of perl files
sudo chmod +x *.pl; - Let's configure SYSJACK! do
./configure.plto start the process and follow the instructions. - Now that a config.json file was made, run
./install.pl jackdto create a jackd.service. Press 'i' when asked to install or local, to install it on system directory. - Done!
Ensure that everything is ok: first do pgrep jackd and check out the process number.
If there is no number, Jackd failed! do systemctl status jackd or cat /var/log/syslog | grep jackd and start bugging from there.
If jackd is running, do chrt --pid and the number which comes out with pgrep. The output should look like this:
current scheduling policy: SCHED_RR
current scheduling priority: 80 <- or wathever you put when you configured it
configure.pl will accept the following syntax: .configure.pl (config=configfile) (key=jsonkey)
- config: you can specify the json file to write.
- key: this setting tells configure that the json file is not all about sysjack. configure will search the matching key and write there all his setting, preserving other stuff. This is useful if you are using a single, large json file for configuration.
configure.pl will ask you if you want to create an /etc/asound.conf file. This file will set your selected audio card as ALSA default audio card, and will create an alias for SYSJACK alsa device called SYSJACK, so you can use i.e. aplay -D SYSJACK if you don't want to change your default audio card.
Any major file change is always backupped in home directory/.sysjack as a .backup file.
install.pl accepts this sintax: ./install.pl unit-name (config=configfile) (key=jsonkey) (-y).
coonfig and key are the same value as configure.pl.
The -y(es) flag will skip the install/local question and just install the required service. The -s(tring) flag will simply output the command line string. This is useful if you want to install your services on bash profiles instead of systemd.
SYSJACK works on a simple string replacement method. the json file has 3 categories:
- card containing all card info;
- jack containing jackd params;
- user contaning misc stuff
here is a simple .json
{
"card" : {
"card_id" : "1",
"samplerate" : "48000",
"card_longname" : "USB Audio CODEC",
"alsa_id" : "hw:1,0",
"device_name" : "USB Audio ",
"device_id" : "0",
"card_shortname" : "CODEC"
},
"units" : {
"jackd" : "/usr/bin/jackd -R -p{jack/ports} -t{jack/timeout} -d alsa -d{card/alsa_id} -{jack/alsa_mode} -p {jack/buffersize} -n {jack/alsa_periods} -r {card/samplerate} -s"
},
"user" : {
"sub_priority" : "80"
},
"jack" : {
"alsa_periods" : "2",
"buffersize" : 512,
"timeout" : "2000",
"alsa_mode" : "P",
"ports" : 16,
"priority" : "80"
}
}The units property contains key/value pairs for processes. Values are strings containing {stuff inside curly brackets}.
When installing, SYSJACK will simply replace the content from the brackets with the value inside the parameter.
So, if you want to pass the alsa HW id as a parameter, you just write {card/alsa_id}. SYSJACK will look for the alsa_id value inside the card property.
You may add everything everywhere, but keep in mind that launching ./configure.pl again will cleanse all your customization. If the key parameter is specified, everything outside the key will be preserverd. Backup! always backup!
To add a service, add a new entry in units, use the name you want for a service, then pass the parameter string as value: remeber that systemd requires absolute paths only!
...
"units": {
"mynewunit": "/usr/bin/myunitexe unit parameters"
}
...
Let's say you want zynaddsubfx, the famous open source synth, to automatically start after JACKD. You also want to make it OSC responsive, so you need to have the port as a property that can be read elsewhere.
Let's open config.json and add this bit under the 'user' property:
"user" : {
"zyn_OSC": "7777"
"sub_priority": "80",
}now let's write the unit, add this bit under "units", just before "jackd":
"units" : {
"zynasfx": "/usr/local/bin/zynaddsubfx -U -A=0 -I JACK -O JACK -o 256 -b {jack/buffersize} -r {card/samplerate} -P {user/zyn_OSC}",
"jackd" : "/usr/bin/jackd -R -p{jack/ports} -t{jack/timeout} -d alsa -d{card/alsa_id} -{jack/alsa_mode} -p {jack/buffersize} -n {jack/alsa_periods} -r {card/samplerate} -s"
}run ./install.pl zynasfx and press (l) when asked to install. this will create a .service file inside SYSJACK directory. check it out - if the file is correct, run install.pl with the -y flag to install the service.
Let's say you need to customize you service but you don't want to change it everytime.
cp src/default.service.in src/myservice.service.in where myservice is your unit name (i.e. zynasfx.service.in). This will tell install.pl to use a customized template when recustomizing services.