What exactly is the default pcm ALSA device?

Question

I'm on Lubuntu 20.04, with no PulseAudio installed. I'm having some trouble editing my ALSA setting, as any change I make interferes with my microphone.

In particular, if I use the following basic configuration file:

pcm.!default {
    type hw
    card 2
}

ctl.!default {
    type hw
    card 2
}

Then I am unable to run OBS and Discord in parallel, as the first tries to open the microphone in stereo mode, while the latter in mono. The last to try always fails to open the device.

However, with just the lines

defaults.pcm.card 2
defaults.ctl.card 2

Everything works correctly. This hints to me that the default device that ALSA provides is more flexible than a simple type hw plugged to the correct device. I tried to look into somehow making ALSA print its defaults, but could not find anything about it.

How can I replicate the default ALSA device in my configuration file, so that I can make and test my changes as diffs to what ALSA already does for me?

Answer 1

The default definition of the default device can be found in /usr/share/alsa/pcm/default.conf. If it does not redirect to a driver-specific default, it is defined like this:

pcm.!default {
    type plug
    slave.pcm {
        type hw
        card 2
    }
}

The plug plugin implements automatic sample rate/format conversion.

Most drivers do have their own default definition. In particular, most motherboard devices are handled by /usr/share/alsa/cards/HDA-Intel.conf, which defines something like this to allow multiple clients:

pcm.!default {
    type asym
    playback.pcm {
        type plug
        slave.pcm "dmix:2"
    }
    capture.pcm {
        type plug
        slave.pcm "dsnoop:2"
    }
}