FAQ026

From the ALSA wiki

How can I change the default ALSA device?

Ok, if you want to change the default device on your soundcard, you need to edit your ~/.asoundrc file and put something like this into it:

pcm.!default {
    type hw
    card 0
    device 2
}

This selects hw:0,2 as the default device. If you want to do sample-rate conversion, etc, using the plug plugin, you can try this:

pcm.!default {
    type plug {
        slave.pcm "hw,0,2"
    }
}

If you need to change the default soundcard you need to edit one of the ALSA configuration files:

• /etc/asound.conf for system-wide options that affect all users
• ~/.asoundrc for options that affect only your own user account

To one of these files (probably /etc/asound.conf) add something similar to the following:

pcm.!default {
    type hw
    card 2
}
ctl.!default {
    type hw           
    card 2
}

You can find out which number you need to put in the card line by looking at /proc/asound/cards. If this file doesn't exist or the contents are empty, it means ALSA is not working properly on your computer. Let's assume ALSA is working ok. For example:

$ cat /proc/asound/cards

0 [Dummy      ]: Dummy - Dummy
                 Dummy 1

1 [VirMIDI    ]: VirMIDI - VirMIDI
                 Virtual MIDI Card 1

2 [AudioPCI   ]: ENS1371 - Ensoniq AudioPCI
                 Ensoniq AudioPCI ENS1371 at 0xe400, irq 11

In this example we can see that soundcard number 0 "Dummy" is a dummy soundcard (useful for certain types of testing), soundcard number 1 "VirMIDI" is a virtual soundcard (useful for testing MIDI programs), and last but most importantly, soundcard number 2 "AudioPCI" is the real soundcard. Question: How can you tell which soundcard is a real soundcard? Answer: real soundcards always have an irq. In this example, only soundcard 2 AudioPCI has an irq, so it must be the real soundcard.

So the general case is:

pcm.!default {
    type hw
    card X
    device Y
}

Also if you ever want to change your default soundcard, you should change the ctl.!default device to that card:

ctl.!default {
    type hw
    card X
}

Note by mschiff: IMO its better to use the card name instead of the number because as you can see in FAQ042 the order of the cards can change if you reboot. I set my default soundcard like that:

Find out the name of the card:

$ cat /proc/asound/cards
 0 [Live           ]: EMU10K1 - SBLive! Value [CT4832]
                      SBLive! Value [CT4832] (rev.8, serial:0x80271102) at 0xb400, irq 201
 1 [Bt878          ]: Bt87x - Brooktree Bt878
                      Brooktree Bt878 at 0xf9000000, irq 177
 2 [V8237          ]: VIA8237 - VIA 8237
                      VIA 8237 with AD1980 at 0x1000, irq 209

(Last time my computer booted the "Live" card was card "2")

Set it as default in the ALSA configuration file:

$ cat /etc/asound.conf
# use Soundblaster Live as default device
# (from /proc/asound/cards)
#
#

pcm.!default {
    type hw
    card Live
}
ctl.!default {
    type hw
    card Live
}

You can also make the default device use any other defined pcm device:

pcm.foo {
    type dmix
    slave.pcm "hw:0,0"
}
pcm.!default {
    type plug
    slave.pcm "foo"
}

In this example the slave definition references the "foo" pcm device defined above.

"Real soundcards always have an IRQ" - this is not true for USB soundcards:

$ cat /proc/asound/cards
0 [External   ]: USB-Audio - SB Live! 24-bit External
                 Creative Technology SB Live! 24-bit External at usb-0000:00:10.1-2, full speed

Look at /usr/share/alsa/alsa.conf and find parameters...

# defaults

defaults.ctl.card 0
defaults.pcm.card 0

Change it to the card of your choice.

Is it possible to use/set environment variables to change the default ALSA device?

you can use ALSA_PCM_CARD or ALSA_CARD to set the default card, either by name or by number. If both are set, ALSA_PCM_CARD takes precedence. The device on the card can be set in the variable ALSA_PCM_DEVICE.

You can check aplay -L for this information. This is part of the output I see.

       @args.CARD {
               type string
               default {
                       @func getenv
                       vars {
                               0 ALSA_PCM_CARD
                               1 ALSA_CARD
                       }
                       default {
                               @func refer
                               name 'defaults.pcm.card'
                       }
               }
       }

Now, under default, you see vars and the two variables (ALSA_PCM_CARD and ALSA_CARD) that are queried.