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The Jack Audio Connection Kit (JACK) is a suite of professional-grade, low-latency tools for routing digital audio.

Configure Rivendell 3 For Use With JACK

Overview

You have a several choices for getting audio out of a Rivendell system. Notably Rivendell supports ALSA, the native Linux audio subsystem, and JACK, a layer above ALSA.

JACK enables multiple audio applications to simultaneously access audio devices on Linux systems.

In 2021, Rivendell processes and JACK processes must be run as the same user ID. And since Rivendell processes (in the packaged version available from Paravel Systems) always run as root, this means jackd(8) (the JACK daemon) must also run as root.

Configuring Rivendell to use JACK

So, you’ve decided to use JACK for your Rivendell audio. First, make sure you tell Rivendell to not use any ALSA devices on your computer using the RDAlsaConfig Rivendell app. Run this app from the Rivendell menu and simply unhighlight all devices in the list, as shown here:

RDAlsaConfig Ready for JACK
(As an aside, when you use JACK for Rivendell’s audio routing, you can also use any other JACK-enabled audio app alongside Rivendell.)
(Another aside: know that Rivendell works fine using both JACK clients and Audio Science adapters.)

Set up JACK on a per-host basis by modifying the configuration in RDAdmin->Manage Hosts->hostname. In the host configuration window, touch the JACK Settings button to manage the Jack settings.

Ensure that Rivendell starts the JACK app by checking the Start JACK Server checkbox. Unless you have a really good reason to change it (beyond the scope of this article), leave the JACK Server Name set to (default) as shown here:

Configured JACK Clients in Reivendell 3.4.1

It is easiest to set the JACK Command Line to the full path of a shell script so you can make changes to the way in which the JACK app is started.

Note that regardless of how you configure the JACK Command Line if you make any changes to the JACK Settings you must restart the Rivendell service, interrupting audio playout, in order for the changes to take effect. Plan your changes accordingly.

Here is an example JACK startup script:

#!/bin/sh

#######################################################################
##
## This script is started by Rivendell caed when the Rivendell daemons
## are launched. The script assumes a "default" Rivendell installation
## with the username 'rd' automatically logged in and running the
## windowing system.
##
## Change the JACK sample rate to match your Rivendell sample rate
## set in RDAdmin->System Settings.
##
#######################################################################

export LANG=en_US.UTF-8
export TERM=xterm
export QTDIR=/usr/lib64/qt4
export PATH=/sbin:/bin:/usr/bin:/usr/sbin:/usr/local/sbin:/usr/local/bin:/home/rd/bin
export XDG_DATA_DIRS=/usr/local/share/:/usr/share/
export XDG_RUNTIME_DIR=/run/user/1000
export QT_GRAPHICSSYSTEM_CHECKED=1

exec /usr/bin/jackd --name default -d dummy -r 48000 2>&1 > /var/tmp/jackd.log

Copy the above text and save it in a file named /usr/local/bin/jackd-start. Make the file executable with sudo chmod +x /usr/local/bin/jackd-start. Enter the full path of the shell script into the JACK Command Line text box.

When using the above script, notice that we initialize JACK with a “dummy“ device. We do this because the default device name is always “system”. Not only is this a useless name, it is also something one cannot change without editing the JACK source code. In the above script, we assign the “dummy” device to the “system” JACK client in order to “get it out of the way”. We then use jack-plumbing or some other means to connect JACK client inputs and outputs.

Adding JACK Clients

After configuring Rivendell to start the JACK subsystem, you probably need to also start some JACK clients. Do this by adding each one in the JACK Clients to Start section (see the figure above for examples).

You might consider using the pmw-jack-setup script (or something like it) to configure your ALSA sound devices to work with JACK. This script accepts two parameters: the name of the ALSA device to configure, and the “human-readable” name you want to give the JACK client. You can learn your ALSA device names by running the command sudo aplay -l (or su - root -c 'aplay -l') in a terminal window. Running this command results in output like:

**** List of PLAYBACK Hardware Devices ****
card 0: Intel [HDA Intel], device 0: CX20561 Analog [CX20561 Analog]
  Subdevices: 0/1
  Subdevice #0: subdevice #0
card 0: Intel [HDA Intel], device 1: CX20561 Digital [CX20561 Digital]
  Subdevices: 1/1
  Subdevice #0: subdevice #0

In the above output, the device name for card 0 is Intel. Many people prefer more appropriate names for their JACK devices. One common name for the built-in sound device (the “sound card” on the PC‘s motherboard) is “Onboard”. Set up your first JACK Client in RDAdmin as follows:

  • touch Add
  • enter a meaningful description in the Description text box; experience (and observing the source code) has shown that the JACK Clients are started in the alpha-numeric order of the Description label. Use this knowledge to guide your choices of descriptions
  • enter the path and parameters to the setup script: /usr/local/bin/pmw-jack-setup Intel Onboard in the Command Line text box
  • touch OK to save your settings

Repeat the above steps for as many ALSA-compatible hardware audio devices as you want to use with Rivendell. Note that pmw-jack-setup merely configures your hardware audio devices for use with JACK. It does not connect them to Rivendell. Also note that using pmw-jack-setup requires that you also download and install the “helper script”, zsh-functions in /usr/local/bin. zsh-functions in turn may also require the installation of additional packages.

The version of pmw-jack-setup on github attempt to discern the number of input and output channels for the ALSA device. You can override this with the --ci NUM-CHANNELS (capture channels) and --co NUM-CHANNELS (playback channels) options, where NUM-CHANNELS is the actual number of input and output channels you want to use.

As with /usr/local/bin/jackd-start, be sure to set the permissions on pmw-jack-setup with sudo chmod +x /usr/local/bin/pmw-jack-setup.

Connecting JACK Inputs and Outputs

Once you have successfully configured Rivendell to launch JACK and configure your audio devices, you need to “connect” the devices to Rivendell. There are several ways to do this, including:

  • run one or more Rivendell Macros to make the connections (see JC)
  • run a JACK plumbing service daemon such as jack-plumbing to automate the connections
  • run the command line jack_connect application to make the connections
  • use a graphical tool such as qjackctl or patchage to make the connections

The most reliable methods for making JACK connections are those that perform the work automatically and in the background. Unfortunately, CentOS does not package the jack-plumbing command, so one must either download the source code from the Debian website: here or here and compile it (the original source code for jack-plumbing seems to have been lost in the bit bucket), or choose a different “plumbing” tool to manage the JACK connections.

Here is an example of a configuration file for jack-plumbing. Note that the “comment character” is a semicolon (“;”) rather than the more common hash symbol (“#”). In general, the jack-plumbing connect statement format is (connect "source" "destination"). If you place this in /etc/jack-plumbing you can start the app with simply /bin/jack-plumbing (see the figure above):

; JACK port connection rules.
; See man jack-plumbing for details.
; dklann@broadcasttool.com, Fri Aug 14 09:20:55 PDT 2020

; Connect the first Rivendell output port to the Onboard card.
; See also pmw-jack-setup and the JACK client name for the
; built-in audio device. Substitute your audio device for
; "Onboard" as necessary...
(connect "rivendell_0:playout_0L" "Onboard:playback_1")
(connect "rivendell_0:playout_0R" "Onboard:playback_2")

; Connect the first Rivendell input port to the Onboard card.
(connect "Onboard:capture_1" "rivendell_0:record_0L")
(connect "Onboard:capture_2" "rivendell_0:record_0R")

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
; Audio processing with Calf plugins
; Use this along with calfjackhost to connect some Calf audio
; plugins for real-time audio processing.
;
; Rivendell Primary output to BassEnhancer
;(connect "rivendell_0:playout_0L" "calf:bassenhancer_in_l")
;(connect "rivendell_0:playout_0R" "calf:bassenhancer_in_r")
; -or- Rivendell Primary output to MultibandCompressor input
;(connect "rivendell_0:playout_0L" "calf:multibandcompressor_in_l")
;(connect "rivendell_0:playout_0R" "calf:multibandcompressor_in_r")
; MultibandCompressor output to Limiter input
;(connect "calf:multibandcompressor_out_l" "calf:multibandlimiter_in_l")
;(connect "calf:multibandcompressor_out_r" "calf:multibandlimiter_in_r")

; BassEnhancer output to MultibandCompressor input
;(connect "calf:bassenhancer_out_l" "calf:multibandcompressor_in_l")
;(connect "calf:bassenhancer_out_r" "calf:multibandcompressor_in_r")

; MultibandCompressor output to MultibandLimiter input
;(connect "calf:multibandcompressor_out_l" "calf:multibandlimiter_in_l")
;(connect "calf:multibandcompressor_out_r" "calf:multibandlimiter_in_r")

; MultibandLimiter output to Onboard input
;(connect "calf:multibandlimiter_out_l" "Onboard:playback_1")
;(connect "calf:multibandlimiter_out_r" "Onboard:playback_2")

; Limiter output to glasscoder input
; This assumes you are running GlassCoder with the JACK
; client name "glass".
;(connect "calf:multibandlimiter_out_l" "glass:input_1")
;(connect "calf:multibandlimiter_out_r" "glass:input_2")

Restart Rivendell Service

After making changes to your JACK configuration in Rivendell you must restart the Rivendell service. Do this by either rebooting your Rivendell computer, or running the following command in a terminal window:

sudo systemctl restart rivendell.service

Future Work

To do: make JACK work with PulseAudio.

This is complete as of April, 2021: To do: in pmw-jack-setup: use data in /proc/asound to automatically discern the number of input and output channels for ALSA devices.

In 2021 the pipewire audio subsystem is maturing and aims to be compatible with and to eventually replace both the PulseAudio and the JACK audio systems. Pipewire may include more “plumbing” tools over the long run.