I have installed MIXXX on my Ubuntu laptop and now I’m researching on supported Deck Controller with integrated Soundcard. Can someone advice me the one I can use please? The other thing is not clear to me is: I connect a controller via USB, but how do I direct a digital output to it? I don’t see USB is listed in the “Preferences / Sound Hardware / Output” section.
Which controller do you currently have attached?
Regarding a controller recommendation, I sugggest you take a look at https://github.com/mixxxdj/mixxx/wiki/Hardware%20compatibility and check each controller if it suits your needs
There are many 2 deck controllers supported by Mixxx already. As ronso suggested, please check for a suitable one at the wiki.
To answer your other question. There is no USB output as such but instead it shows sound devices. Once you attach an external audio interface (like with your controller) to USB then the system will find the audio device and you can select it. So instead of looking for a free USB port you need to attach the audio device (make sure it is switched on) which then will be detected and shown in the drop menu