ALSA: send audio to two audio devices

Question

On my laptop, I have an onboard sound card, and also a connected bluetooth headset. I have configured the bluetooth device in /etc/asound.conf:

# cat /etc/asound.conf

pcm.bluetooth {
    type bluetooth
    device 12:34:56:78:9a:bc
    profile "auto"
}

ctl.bluetooth {
    type bluetooth
}

Now I can play audio to my headset by specifying the new audio device, such as:

mplayer -ao alsa:device=bluetooth file.mp3

If I want to play to my default device, I simply omit the device:

mplayer file.mp3

However, I need to configure ALSA, so that all sound is sent to both devices by default, without having to explicitly set this per application.

ie:

mplayer file.mp3

should play both on the laptop soundcard, as well as in the bluetooth headset.

How can I do that ?

Answer 1

Here's one way to do it from ~/.asoundrc ; example shows an on-board and soundblaster live card united under the default PCM.

# duplicate audio to both devices
pcm.!default plug:both

ctl.!default {
  type hw
  card SB
}

pcm.both {
  type route;
  slave.pcm {
      type multi;
      slaves.a.pcm "sblive";
      slaves.b.pcm "onboard";
      slaves.a.channels 2;
      slaves.b.channels 4;
      bindings.0.slave a;
      bindings.0.channel 0;
      bindings.1.slave a;
      bindings.1.channel 1;

      bindings.2.slave b;
      bindings.2.channel 0;
      bindings.3.slave b;
      bindings.3.channel 1;
      bindings.4.slave b;
      bindings.4.channel 2;
      bindings.5.slave b;
      bindings.5.channel 3;
  }

  ttable.0.0 1;
  ttable.1.1 1;

  ttable.0.2 1; # front left
  ttable.1.3 1; # front right
  ttable.0.4 1; # copy front left to rear left
  ttable.1.5 1; # copy front right to rear right
}

ctl.both {
  type hw;
  card Live;
}

pcm.onboard {
   type dmix
   ipc_key 1024
   slave {
       pcm "hw:0,1"
       period_time 0
       period_size 2048
       buffer_size 65536
       buffer_time 0
       periods 128
       rate 48000
       channels 4
    }
    bindings {
       0 0
       1 1
       2 2
       3 3
    }
}

pcm.sblive {
   type dmix
   ipc_key 2048
   slave {
       pcm "hw:1,0"
       period_time 0
       period_size 2048
       buffer_size 65536
       buffer_time 0
       periods 128
       rate 48000
       channels 2
    }
    bindings {
       0 0
       1 1
    }
}

ctl.onboard {
   type hw
   card "SB"
}

ctl.sblive {
   type hw
   card "Live"
}

(Source)

Answer 2

You want to use the multi-plugin.

Several well documented examples of its use exist:

From the alsa people: https://alsa.opensrc.org/TwoCardsAsOne

From somebody with a similar question here: https://unix.stackexchange.com/a/194631/127903

Answer 3

If pulseaudio is also an option, there is an elegant solution that creates a multi output device which copies the output to all other outputs.

In short:

• install and run paprefs or
• pactl load-module module-combine-sink

See https://askubuntu.com/questions/78174/play-sound-through-two-or-more-outputs-devices