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How do I refer to a soundcard so that the name always works?

The precise order in which a Linux system discovers any soundcards can vary each time the machine is rebooted. This is particularly true of USB devices. The device that used to be "hw:0" can become "hw:1" after a reboot, or vice versa. Fortunately, you don't need to use names like this, and can instead use a form that isn't dependent on the order that the Linux discovers your soundcards.

First, in a terminal window, run this command: cat /proc/asound/cards. You'll see output a bit like this:

 0 [SB             ]: HDA-Intel - HDA ATI SB
                      HDA ATI SB at 0xfcef8000 irq 16
 1 [DSP            ]: H-DSP - Hammerfall DSP
                      RME Hammerfall DSP + Digiface at 0xfcff0000, irq 20
 2 [NVidia         ]: HDA-Intel - HDA NVidia
                      HDA NVidia at 0xfe57c000 irq 32

The "name" of each soundcard is in square brackets.

With this information, you can now refer to a particular device as, for example hw:DSP (for the RME Hammerfall DSP in the above example. Even if the devices end up re-ordered for any reason, hw:DSP will still refer to the same device.

The one case where this fails is if you have multiple instances of the same type of soundcard. This is a much trickier situation to deal with and is covered in detail here.

Where do I use this name?

You can type it into the device name selector in the upper left of qjacktl's setup dialog. You can use it as the argument to the -d flag of JACK's ALSA backend (e.g. jackd -d alsa -d hw:DSP.