loopback_spectrum.conf

pcm.dac {
  type hw
  card 0
}

pcm.both {
    type route;
    slave.pcm {
        type multi;
        slaves.a.pcm "plughw:0,0"
        slaves.b.pcm "plughw:Loopback,1,0"
        slaves.a.channels 2;
        slaves.b.channels 2;
        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;
    }

    ttable.0.0 1;
    ttable.1.1 1;

    ttable.0.2 1;
    ttable.1.3 1;
}

pcm.!default {
        type plug
        slave.pcm "both"
}

ctl.!default {
        type hw
        card SB
}