pcm.headset {
    # The "plug" plugin is misleadingly named; it has nothing to do with being
    # a plugin.  What it should really be named is the "auto-re-sample" plugin.
    # It allows this device to be used at any sample rate.
    type plug

    # A "plug" plugin needs a "slave" PCM, which is a definition of another
    # "pcm" configuration that corresponds to the device which will be
    # re-sampled to.
    slave.pcm {
        # The "asym" plugin is a virtual "asymmetric" device; where capture and
        # playback on the same virtual device may be going to two different
        # underlying devices.  In fact, we are sending both playback and
        # capture to the same device in this case, but we need to use two
        # different plugins ("dmix" and "dsnoop") to do it.  It requires a
        # "playback.pcm" and "capture.pcm" argument to describe the two halves
        # of the device.
        type asym

        playback.pcm {
            # The "dmix" plugin is our automatic playback-mixing device.
            type dmix

            # Now, need to specify a number to identify the shared memory area
            # where this dmix plugin will do its mixing.  This is meaningless,
            # it just needs to be unique.
            ipc_key 4321

            # When naming that shared-memory area, don't add the current user's
            # ID in there, so that other users can find it.
            ipc_key_add_uid false

            # Allow other users in my group to access that area so they can
            # also open this device.  This is especially important if you want
            # to put this device into a global configuration file.
            ipc_perm 0660

            # Similar to "plug", "dmix" needs a description of the actual audio
            # device it's doing mixing to.  The usual ALSA idiom is to use
            # 'hw:2' or somesuch, but that's not very stable in the face of
            # unplugging USB devices, rebooting, or moving the device to a
            # different machine.  This identifier appears to be unique, and you
            # can get it from 'aplay -l'.
            slave.pcm "hw:U0x47f0xad01"
        }
        capture.pcm {
            # This is the recording half of the device.  The "dsnoop" plugin is
            # just like "dmix" except it does capture rather than playback.
            type dsnoop
            ipc_key 5432
            ipc_key_add_uid false
            ipc_perm 0660
            slave.pcm "hw:U0x47f0xad01"
        }
    }
}

# Finally, we need to describe a 'control' interface (for volume knobs and
# such) whose name corresponds exactly.  This is important because VoIP
# programs and the like will assume they can tweak the volume on their input or
# output devices by looking for a similarly-named ctl device once they have a
# pcm.

# Unlike with playback and capture, ALSA *does* allow multiple processes to
# access the same control device simultaneously, so we don't need to do
# anything special here.
ctl.headset {
     type hw
     card "U0x47f0xad01"
}

# A list of 'hooks' is effectively a list of additional files to load.  I don't
# really understand the nuances of 'func' vs. '@func', but this seems to work.
@hooks [
    {
        func load
        files [
            {
                # I'm loading exactly one file, based on an environment
                # variable.  If ALSA_BONUS_CONFIG is defined, then its value
                # goes into this list.
                @func getenv
                vars [ ALSA_BONUS_CONFIG ]
                default {
                    # Otherwise, I look for a file named ".asoundrc.empty" in
                    # the user's home directory (or /tmp, if the user has no
                    # home directory, although that isn't likely).  This file
                    # should be present but empty, but if it doesn't exist the
                    # only consequence will be some ugly warning messages from
                    # programs that use ALSA.
                    @func concat
                        strings [
                            {
                                @func getenv
                                vars [ HOME ]
                                default "/tmp"
                            }
                            "/.asoundrc.empty"
                        ]
                }
            }
        ]
    }
]