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Based on this post: https://raspberrypi.stackexchange.com/a/58174 I was able to get 4 usb microphones to behave as 1 device with 4 channels. What I am looking to do is down mix the 4 microphones into a 2 channel device. I thought this was achievable by doing the following:

pcm.multi {
    type multi;
    slaves.a.pcm "hw:0,0";
    slaves.a.channels 1;
    slaves.b.pcm "hw:1,0";
    slaves.b.channels 1;
    slaves.c.pcm "hw:2,0";
    slaves.c.channels 1;
    slaves.d.pcm "hw:3,0";
    slaves.d.channels 1;
    bindings.0.slave a;
    bindings.0.channel 0;
    bindings.1.slave b;
    bindings.1.channel 0;
    bindings.2.slave c;
    bindings.2.channel 0;
    bindings.3.slave d;
    bindings.3.channel 0;
}

ctl.multi {
    type hw;
    card 0;
}

pcm.ttable {
    type route;
    slave.pcm "multi";
    slave.channels 2;   #### Set it to 2 channels
    ttable.0.0 0.5;     #### input 0 to output 0
    ttable.1.1 0.5;     #### input 1 to output 1
    ttable.2.0 0.5;     #### input 2 to output 0
    ttable.3.1 0.5;     #### input 3 to output 1
}

ctl.ttable {
    type hw;
    card 0;
}

This doesn't work when recording in Audacity. I can record 4 channels in Audacity when I set the ttable to 4 channels and use the following:

ttable.0.0 1;    
ttable.1.1 1;     
ttable.2.2 1;
ttable.3.3 1;

My ultimate goal is to be able to use all four mics as one input in chromium, which apparently will not allow you to use a device with more than 2 channels. Alsa is completely new to me so any advice would be appreciated.

Many Thanks,

S

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The plugin parameters are not inputs and outputs, but master and slaves. So for capture, you have to exchange them:

pcm.ttable {
    type route
    slave.pcm "multi"
    slave.channels 4    # the slave has 4 channels
    ttable.0.0 0.5      # master ch 0 - slave ch 0
    ttable.1.1 0.5      # master ch 1 - slave ch 1
    ttable.0.2 0.5      # master ch 0 - slave ch 2
    ttable.1.3 0.5      # master ch 1 - slave ch 3
}
  • Thanks for the quick response. I think this may have done it except Audacity keeps freezing when I try to record and for some reason at least 1 microphone each time refuse to record at all (using the ttable or just recording them individually) which is strange. I will do some more tests but I think your suggestion is where I was going wrong. The only other change I have made is pcm.!default { type hw card 0 } and ctl.!default { type hw card 0 } so I'm not sure why the mics that work on other machines and have previously worked on this pi have suddenly stopped. Thanks, S. – user1961104 May 16 '18 at 18:42
  • I don't know if the Pi has enough USB bandwidth to handle all devices, or if they all run at exactly the same speed. – CL. May 16 '18 at 20:38
  • I was wondering about bandwidth, but the fact that it is happy with all 4 mics when I'm outputting 4 channels made me think otherwise. Speed could be more likely. I will run some tests tomorrow. Thanks again - big help. S – user1961104 May 16 '18 at 21:38

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