How can I use ALSA dmix and multi plugins together?

Question

I'm playing with a console sound visualiser which wants its own ALSA device to listen on. I'm editing ~/.asoundrc. I know I'll need the multi plugin to split the sound data onto separate devices. However, my soundcard has no hardware mixing, so I'll also need a dmix plugin in there somewhere, for software mixing.

If I try to add a multi as a slave of the dmix, I get this error _{(line-wrapped)}:

ALSA lib pcm_direct.c:1525:(_snd_pcm_direct_get_slave_ipc_offset)
Invalid type 'multi' for slave PCM

I tried creating a "dummy middleman" plug with the multi as a slave, and pointing the dmix to that, but still get the same error. Looks like dmix wants the whole chain of stuff to be plug or hw…

If I try to add a dmix module as a slave of a multi instead, I get

Device or resource busy

As you'd expect when software mixing isn't the first step in the pipeline, but multiple programs try to grab the sound card.

Both of these things (dmixing, and multi-ing to a loopback device) work well separately.

Why do dmix and multi not work together? How can I make this work?

---

Here's my ~/.asoundrc, with options that give Invalid type 'multi' for slave PCM:

# thx
# http://wiki.ubuntuusers.de/.asoundrc
# http://alsa.opensrc.org/Dmix
# http://forums.linuxmint.com/viewtopic.php?f=196&t=94877

pcm.snd_card {
    type hw
    card 1
    device 0
}

# allows multiple programs to output sound simultanously ("software mixing")
pcm.dmixer {
    type dmix
    ipc_key 1024
    ipc_perm 0666 # allow other users
    slave.pcm "out"
    slave {
        period_time 0
        period_size 1024
        buffer_size 4096
        ### if having problems
        # rate 44100
        ### some sound cards need the exact data format
        # format S32_LE
        ### Available formats: S8 U8 S16_LE S16_BE U16_LE U16_BE S24_LE S24_BE
        ###                    U24_LE U24_BE S32_LE S32_BE U32_LE U32_BE
        ###                    FLOAT_LE FLOAT_BE FLOAT64_LE FLOAT64_BE
        ###                    IEC958_SUBFRAME_LE IEC958_SUBFRAME_BE MU_LAW
        ###                    A_LAW IMA_ADPCM MPEG GSM
        channels 2 # must match bindings
    }
    bindings {
        0 0
        1 1
    }
}

# allows multiple programs to capture simultaneously
pcm.dsnooper {
    type dsnoop
    ipc_key 2048
    ipc_perm 0666 
    slave.pcm "snd_card"
    slave 
    {
        period_time 0
        period_size 1024
        buffer_size 4096
        channels 2 
    }
    bindings {
        0 0
        1 1
    }
}

pcm.!default {
    type asym
    playback.pcm "dmixer"
    capture.pcm "dsnooper"
}

pcm.out {
    type plug
    slave.pcm {
        type multi
        slaves {
            a { channels 2 pcm "snd_card" }
            b { channels 2 pcm "hw:Loopback,0,0" }
        }
        bindings {
            0 { slave a channel 0 }
            1 { slave a channel 1 }
            2 { slave b channel 0 }
            3 { slave b channel 1 }
        }
    }
    ttable [
        [ 1 0 1 0 ]   # left  -> a.left,  b.left
        [ 0 1 0 1 ]   # right -> a.right, b.right
    ]
}

# In case I ever want to use PulseAudio, for bluetooth speakers or such.
#pcm.!default {
#    type pulse
#}
#ctl.!default {
#    type pulse
#}

Answer 1

Turns out each output device needs its own dmix:

[!default] → multi → dmix → hw [normal]
                   ↳ dmix → hw [loopback]

I was missing a second dmix between the multi and loopback-hw, so although my usual card would have been fine, the loopback card had no mixing.

Many thanks to CL. for patience and expertise.

---

For the technical details, here's my ~/.asoundrc now:

pcm.snd_card { # my usual sound card
    type hw
    card 2
}

ctl.!default { # default control; alsamixer and such will use this
    type hw
    card 2
}

# software mixer for sound card
pcm.dmixer {
    type dmix
    ipc_key 1024
    ipc_perm 0666 # allow other users
    slave.pcm "snd_card"
    slave {
        period_time 0
        period_size 1024
        buffer_size 4096
        channels 2 # must match bindings
    }
    bindings {
        0 0
        1 1
    }
}

# software mixer for loopback device
pcm.dmixerloop {
    type dmix
    ipc_key 2048
    ipc_perm 0666 # allow other users
    slave.pcm "hw:Loopback,0,0"
    slave {
        period_time 0
        period_size 1024
        buffer_size 4096
        channels 2 # must match bindings
    }
    bindings {
        0 0
        1 1
    }
}

# allows multiple programs to capture simultaneously
pcm.dsnooper {
    type dsnoop
    ipc_key 2048
    ipc_perm 0666 
    slave.pcm "snd_card"
    slave 
    {
        period_time 0
        period_size 1024
        buffer_size 4096
        channels 2 
    }
    bindings {
        0 0
        1 1
    }
}

pcm.!default {
    type asym
    playback.pcm "out"
    capture.pcm "dsnooper"
}

# Multi, splitting onto usual card and loopback
pcm.out {
    type plug
    slave.pcm {
        type multi
        slaves {
            a { channels 2 pcm "dmixer" }
            b { channels 2 pcm "dmixerloop" }
        }
        bindings {
            0 { slave a channel 0 }
            1 { slave a channel 1 }
            2 { slave b channel 0 }
            3 { slave b channel 1 }
        }
    }
    ttable [
        [ 1 0 1 0 ]   # left  -> a.left,  b.left
        [ 0 1 0 1 ]   # right -> a.right, b.right
    ]
}