asoundrc

# this upmix 2channels to 3, one for LFE
pcm.upmix2021 {
    type plug
    slave.pcm lowpass2121
    slave.channels 3
    ttable {
        0.0    1
        1.1    1
        0.2    1
        1.2    1
    }
}

# this should apply low pass filter for LFE channel
pcm.lowpass2121 {
    type ladspa
    slave.pcm upmix2121
    path "/usr/lib/ladspa"
    channels 3
    plugins {       
        0 {
            id 1672 # 4 Pole Low-Pass Filter with Resonance (FCRCIA) (1672/lp4pole_fcrcia_oa)
            policy none
            input.bindings.2 "Input";
            output.bindings.2 "Output";
            input { controls [ 200 0 ] }
        }
        1 {
            id 1098
            policy duplicate
            input.bindings.0 "Input";
            output.bindings.0 "Output";        
        }         
    }
}


pcm.upmix2121 {
    type plug
    slave.pcm surround21
    slave.channels 3
    ttable {
        0.0 1
        1.1 1
        2.2 1
    }
}

pcm.!default upmix2021