This is a very basic tutorial that should give you an idea of how to use the QjackCtl Connections window.
You do not need to use JACK if you only want to make MIDI connections. If, however, you would also like to route audio connections, make sure JACK is configured as described at HowToJACKConfiguration. Users that only want to make MIDI connections can also follow this tutorial, skipping the parts about audio connections.
The Connections window in QjackCtl provides a quick and easy way for you to make your audio and MIDI connections between applications supporting ALSA or JACK MIDI. It isn't as advanced as the Patchbay, in that it can't save and load your settings to files, but it is easier to get going quickly, and is good if you don't care about saving your configuration to reload at a later time.
For this simple tutorial, we will assume that you have QjackCtl installed. We will show you how to connect the Virtual MIDI Keyboard application to ZynAddSubFX, and get some nice sounds on your speakers. At the end of this tutorial, you will have a basic understanding of MIDI and audio routing with QjackCtl, and you should be able to apply these concepts to meet your needs.
Firstly, install the applications we will be using:
sudo apt-get install vkeybd zynaddsubfx
Now go to Applications > Sound & Video and launch the following applications: JACK Control, VKeyBd, and ZynAddSubFX. Be sure to load JACK Control first, the launch order of other applications doesn't matter.
You should now have all three applications loaded. Click on the Connect button in the QjackCtl main window.
This will open up a new window, which is the Connections window, on the Audio tab. If you are not using JACK, you can skip to the MIDI tab now. In the left pane, you have the Readable Client list (audio outputs), and in the right pane you have the Writable Client list (audio inputs). We want to have the output of ZynAddSubFX connected to the sound card's ALSA input so that we can hear ZynAddSubFX's sounds.
Click on the ZynAddSubFX entry in the left pane, and click on the alsa_pcm entry in the right pane, and then click on the Connect button.
You will now see a line drawn between the left and right panes. This represents an audio connection, and is like a virtual audio patch cable. Think of it this way: you just plugged your synthesizer into a set of speakers.
Now click on the MIDI tab.
In the left pane, you have the Readable Client list (MIDI outputs), and in the right pane you have the Writable Client list (MIDI inputs). We want to have the MIDI output of VKeyBd connected to ZynAddSubFX's MIDI input so that we can play notes on ZynAddSubFX using the VKeyBd application.
Click on the Virtual Keyboard entry in the left pane, and click on the ZynAddSubFX entry in the right pane, and then click on the Connect button.
Again, you see the line drawn between the left and right panes. This represents a MIDI connection, and is like a virtual MIDI patch cable. Think of it this way: you just plugged your MIDI keyboard controller into your synthesizer.
Click the close button to hide the Connections window, as we're done making connections.
Now, switch to the VKeyBd window, and press some keys on your computer keyboard, or use your mouse to click on the graphical keyboard. Any notes that you press will highlight in blue, as shown in the image below.
If you look at the ZynAddSubFX window, you should see the volume meters (at the very bottom of the window) showing activity, and you should hear some sounds coming out your speakers.
Congratulations, you have just made your first JACK audio and MIDI connections! You should have a pretty basic understanding of how connections work.
MIDI and JACK Connections are very flexible, as you can plug any audio output into any audio input, and any MIDI output into any MIDI input. For example, you could plug the MIDI output of your USB MIDI keyboard into the MIDI input of seq24, and the MIDI output of seq24 into the MIDI input of ZynAddSubFX.
If you are using JACK, you can also plug the audio output of ZynAddSubFX to the input of JACK-Rack; the audio output of JACK-Rack into Ardour, and then record it to disk.
Have fun and make music!