pcm.!default plug:both

ctl.!default {
  type hw
  card "Intel"
}

pcm.both {

  type route	# The "route" plugin routes channels from its input card to its output.
  
  slave {	# Slave is the input card.

    # The virtual input card has 4 channels. Yes, this is circumstantial and
    # also must be specified explicitly.
    channels 4

    pcm {	# Unnamed definition of the input card.

      # The "multi" plugin merges several cards into one. This is necessary,
      # because the "route" plugin can only handle one card.
      type multi
  
      slaves {	# Definition of the child cards.
 
        a {	# Definition of child card "a".
          pcm {

            # The "hw" plugin communicates directly with the ALSA kernel
            # driver.
            type hw
            
            card "Generic"	# Card name as identified by alsa. See output of "aplay -l".
	    device 3		# Card device as identified by alsa. See output of "aplay -l".
            channels 2
          }
          channels 2
        }

        b {
          pcm {
            type hw
            card "Intel"
	    device 0
            channels 2
          }
          channels 2
        }

      }

      # Definition of the card's channels.
      bindings {
        # Channel 0...
        0 {
          # ... bound to child card "a"'s...
          slave a
          # ... channel 0.
          channel 0
        }
        1 {
          slave a
          channel 1
        }
        2 {
          slave b
          channel 0
        }
        3 {
          slave b
          channel 1
        }
      }
    }
  }

  # Definition of transfer table for the card's output.
  ttable {
    # Channel 0...
    0 {
      # ... bound to child card's channel 0 at full volume...
      0 1.0
      # ... AND child child card's channel 1 at full volume.
      1 1.0
    }
    1 {
      2 1.0
      3 1.0
    }
  }
}

# This defines the controls for the card "both", which is what tools such as
# alsamixer refer to.
ctl.both {
  type hw
  card "Generic"
}