Using aplay and choose which output on card to use

Question

I have an xmos-200 board that is able to record 8 channels and push output through 8 channels. I'm going to focus on the output.

For outputs, there are 4 3.5mm jacks with 2 channels each. The board is recognized by Linux, and aplay -L shows

sysdefault:CARD=x20
    xCORE USB Audio 2.0, USB Audio
    Default Audio Device
front:CARD=x20,DEV=0
    xCORE USB Audio 2.0, USB Audio
    Front speakers
surround40:CARD=x20,DEV=0
    xCORE USB Audio 2.0, USB Audio
    4.0 Surround output to Front and Rear speakers
surround41:CARD=x20,DEV=0
    xCORE USB Audio 2.0, USB Audio
    4.1 Surround output to Front, Rear and Subwoofer speakers
surround50:CARD=x20,DEV=0
    xCORE USB Audio 2.0, USB Audio
    5.0 Surround output to Front, Center and Rear speakers
surround51:CARD=x20,DEV=0
    xCORE USB Audio 2.0, USB Audio
    5.1 Surround output to Front, Center, Rear and Subwoofer speakers
surround71:CARD=x20,DEV=0
    xCORE USB Audio 2.0, USB Audio
    7.1 Surround output to Front, Center, Side, Rear and Woofer speakers
iec958:CARD=x20,DEV=0
    xCORE USB Audio 2.0, USB Audio
    IEC958 (S/PDIF) Digital Audio Output
dmix:CARD=x20,DEV=0
    xCORE USB Audio 2.0, USB Audio
    Direct sample mixing device
dsnoop:CARD=x20,DEV=0
    xCORE USB Audio 2.0, USB Audio
    Direct sample snooping device
hw:CARD=x20,DEV=0
    xCORE USB Audio 2.0, USB Audio
    Direct hardware device without any conversions
plughw:CARD=x20,DEV=0
    xCORE USB Audio 2.0, USB Audio
    Hardware device with all software conversions

I can play audio files from all the output ports and all the channels using

aplay --device plughw:CARD=x20,DEV=0 camera.wav

I want to choose which of the 4 stereo outputs is used. Any help on how I can go about doing that?

Answer 1

The four jacks are separated physically, but the software just sees a single eight-channel device.

You can split this device using ALSA plugins:

pcm_slave.eightchannels {
    pcm "hw:x20,0"
    channels 8
    rate 48000     # or whatever
}

pcm.stereo1 {
    type plug
    slave.pcm {
        type dshare
        ipc_key 20160316     # any random but unique number
        slave eightchannels
        bindings [ 0 1 ]
    }
}
pcm.stereo2 {
    type plug
    slave.pcm {
        type dshare
        ipc_key 20160316
        slave eightchannels
        bindings [ 2 3 ]
    }
}
pcm.stereo3 {
    type plug
    slave.pcm {
        type dshare
        ipc_key 20160316
        slave eightchannels
        bindings [ 4 5 ]
    }
}
pcm.stereo4 {
    type plug
    slave.pcm {
        type dshare
        ipc_key 20160316
        slave eightchannels
        bindings [ 6 7 ]
    }
}

To allow multiple application to use the same stereo device, replace dshare with dmix.