Output to Multiple Audio Devices with Alsa

I like to have the option of playing music on my headphones or outputting to the stereo – it really improves family life when they don’t always have to hear my dubstep ;) Unfortunately, the sound cards and devices you have available don’t always fit together nicely. For Linux users, ALSA gives you a lot of control and flexibility over audio devices. However, it is usually difficult to figure out exactly what needs to be done to output to multiple audio devices.

In my case, I have an onboard audio device with optical digital out. But onboard audio usually receives a lot of noise from the rest of the components on the motherboard – your headphones buzz when a hard drive seeks or you move your mouse pointer. I have broad musical tastes and orchestras sound pretty awful with static in the background. By adding an old-school Creative Labs SoundBlaster Live card, I can get exceptional quality output for headphones.

What caused difficulty was getting audio output to both the onboard and SoundBlaster audio devices at the same time. By default, Linux applications will send audio directly to one specific device. All you need is a properly structured ~/.asoundrc file to configure ALSA.

I had to do a bit of hunting on the web and some trial-and-error. No two configurations seem to be the same, and ALSA configuration can be a bit daunting. You have to get into the nitty gritty of each audio device to see exactly which inputs and outputs it provides. Then you determine which are connected to your stereo, headphones, etc. Take the time to completely read and understand the output of aplay -l and aplay -L – they make it much easier to get a handle on what devices are in your system. You’ll also want to use alsamixer to change settings and may need to use iecset to enable or disable digital outputs.

Once you have your devices figured out and know which output plays where, you instruct ALSA to duplicate the audio output to both of the sound cards. This is the configuration that worked for me (install in ~/.asoundrc or /etc/asound.conf) – the 2-channel settings are for the headphones (SoundBlaster Live) and the 4-channel settings are for the stereo (onboard optical out):

# 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"
}