Ubuntu – How to use Jack for all the system sounds

Jackd does not normally replace ALSA. Jackd usually uses ALSA to provide it's output. What Jackd provides is a low-latency audio patching/routing network that other software can use. Only software designed to use Jackd will use JackD. Jackd only needs to be run when you are running software that uses it (e.g. Ardour, Rosegarden, Jack-rack). When you are not running a program that uses jackd, you don't have to run jackd.

The best program I have found for controlling jackd is qjackctl, which can be installed through the Ubuntu Software Center as Jack Control. It allows starting/stopping jackd, configuring how it is run, and setting up connections.

In order to get the lowest latency possible, jackd really would like to use real-time priority threads. Enabling real-time processing for jackd is good thing to do and should not hurt your system. (The machine I am typing on right now has had it enabled for 4 years under 2 different Linux distros and 2 different hardware incarnations without any lock-ups.)

BTW, once you start using some of the features of jackd, it gets pretty addictive. By starting up different effects racks (jack-rack instances), and setting up the connections, I can take take a stereo recording and create a real-time 4.2 output where I have different delays reverb/delay for the rear speakers, a custom center channel mix, and a sub-woofer output. (I am still trying to find a good octave doubler for that.)

To let Jack and PulseAudio work together better, you should use pasuspender.

With the options -m alsa -a alsa, amsynth should not use Jack. (You can the use aconnect to connect the keyboard to the synth.)