Allow Computer Outputs while Jack is Running?

Question

I've played around with jack in the past, but I have never really been successful with it because it feels overwhelming and it doesn't allow my computer audio output to continue functioning while jack is running. Trying to play songs in my media player has them hang at 0 seconds attempting to play them; presumably they are writing PCM or something to pulseaudio or alsa and that write is never completing.

Is there a way to have jack running most or all of the time in the background while still being able to hear computer audio output?

In my case, I have a USB audio interface (a Focusrite Scarlett) which is connected to a pair of speakers which I use as my default audio output, but I'm not sure whether the output type (i.e. USB or an internal sound card) matters here.

I'm on Ubuntu 20.04 LTS running the latest HWE kernel (actually elementary OS Odin, but based on Ubuntu 20.04).

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EDIT: I was able to completely bypass all of this by using PipeWire.

PipeWire is a low-latency audio system which can speak PulseAudio, ALSA, and Jack protocols, and can easily replace both PulseAudio and Jack. I replaced my desktop's PulseAudio server with PipeWire, it isn't too hard, and enabled PipeWire for Jack as well, so now everything goes to one unified place and I don't need to do silly workarounds to make things work, they just do, and everything is none the wiser.

I've written up how to do this for my distribution, elementary OS 6, in this Gist.

Answer 1

The USB audio interface will probably accept only a single audio datastream at any time, and so you would need the audio from jack and the "computer audio output" to be mixed together in software.

On systems without jack, PulseAudio will usually be the component doing that, but since jack wants to provide absolutely minimum latency and sample synchronization, it needs exclusive access to the sound device.

You might want to read this webpage: https://jackaudio.org/faq/pulseaudio_and_jack.html

In particular, I'd consider option 3 on that page, which refers to another page: https://github.com/jackaudio/jackaudio.github.com/wiki/WalkThrough_User_PulseOnJack

In a nutshell: Ubuntu probably already has ALSA defaulting to PulseAudio, so you'll only need to ensure the pulseaudio-module-jack package is installed, and place this configuration into your ~/.config/pulse/default.pa if you intend to run jack all the time:

load-module module-native-protocol-unix
load-module module-jack-sink channels=2
load-module module-jack-source channels=2
load-module module-null-sink
load-module module-stream-restore
load-module module-rescue-streams
load-module module-always-sink
load-module module-suspend-on-idle
set-default-sink jack_out
set-default-source jack_in

Depending on the capabilities of your USB audio interface, you might need some or all of these settings in your ~/.config/pulse/daemon.conf:

default-sample-format = float32le
default-sample-rate = 48000
realtime-scheduling = yes
exit-idle-time = -1

(Most USB audio interfaces accept input at a fixed sample rate only - typically 48000 - so this makes sure Pulseaudio knows about this restriction and converts everything incoming from the desktop audio side to that format, to make jack's job easier.)

If you are using qjackctl to launch jackd, you'll need to configure a post-startup action like this to switch PulseAudio to jack:

pactl load-module module-jack-sink channels=2; pactl load-module module-jack-source channels=2; pacmd set-default-sink jack_out

If you only run jack some of the time, then this post-startup action will essentially do the job of the ~/.config/pulse/default.pa configuration above, telling PulseAudio to divert its output to jack when it has been started up.