Northern Utah WebSDR Configuring high-rate (and normal-rate) audio loopback devices

sudo apt-get --purge remove linux-sound-base alsa-base alsa-utils

sudo apt-get install linux-sound-base alsa-base alsa-utils

sudo apt-get install dkms build-essential linux-headers-`uname -r` alsa-base alsa-firmware-loaders alsa-oss alsa-source alsa-tools alsa-utils

sudo usermod -a -G audio username

#
pcm.!default {
    type hw
    card 0
}

ctl.!default {
    type hw
    card 0
}
#
# Direct audio from receiver to "loop#_in" and point WebSDR at "loop#_out"
#
# Input from receiver - Loopback 0
#
pcm.a_loop0_in {
    type plug
    slave {
        pcm "hw:lp0,0,0"
        rate "unchanged"
        format "unchanged"
        channels "unchanged"
    }
}
#
# output to WebSDR - Loopback 0
#
pcm.a_loop0_out {
    type plug
    slave {
        pcm "hw:lp0,1,0"
        rate "unchanged"
        format "unchanged"
        channels "unchanged"
    }
}
#

#
# Input from receiver - Loopback 1
#
pcm.a_loop1_in {
    type plug
    slave {
        pcm "hw:lp1,0,0"
        rate "unchanged"
        format "unchanged"
        channels "unchanged"
    }
}
#
# output to WebSDR - Loopback 1
#
pcm.a_loop1_out {
    type plug
    slave {
        pcm "hw:lp1,1,0"
        rate "unchanged"
        format "unchanged"
        channels "unchanged"
    }
}
#

1)  Typically starting from the "websdr" directory of your system create a "fastloop-src" directory ("md fastloop-src"), go into it ("cd fastloop-src)" and place the "fastloop.tar.gz" file there.  In that location, extract the contents of fastloop.tar.gz and change to the resultant fastloop directory (e.g., "tar xf fastloop.tar.gz", "cd fastloop").

2)  Build the snd-fastloop.ko module:   Do this by typing "make". This will automatically download the vanilla snd-aloop source, apply the necessary changes to make it support 768 kHz sampling, and then compile it to "snd-fastloop.ko".

Note:  You may get an error (particularly if you are doing this under Unbuntu 18.x) in which case you may try deleting the first three lines of the file "fastloop.patch" and trying again.

Do not proceed until you have successfully built "snd-fastloop.ko" and note that its size is approximately 30k.  This file may still be created even if the "make" fails, but it will have either a zero length, or be much smaller than 30k.

3)  If the compilation succeeded, install the newly built module:  "sudo make modules_install". This will save the module under /lib/modules/[x.y.z]/extra/, where modprobe will find it.

It is possible - or even likely - that this step will throw a few errors related to certificates and/or keys and the installation of the module will fail.  If you like, you can go through the hassle of creating your own certificate/key (I'll let you figure out how to do this...) but if you aren't bothered about keys and things, do the following at the command line:

    sudo depmod -a

After a brief pause, the module "snd-fastloop.ko" should be installed.

If you wish to verify the installation of this module, go to /lib/modules ( cd /lib/modules ).  The sub-directory that you want will likely be the one with the highest version number:  Go into that directory where you should find the "extra" subdirectory, and in that directory you should see "snd-fastloop.ko" and its size will be somewhere around 30k.

4)  Create a file "/etc/modprobe.d/fastloop.conf", containing the  module options you want to apply. For instance, you could use something like "options snd-fastloop index=8 id=fl0 enable=1".  This is the same procedure as above, except, of course, that we are using "snd-fastloop" instead of "snd-aloop".  You can specify up to eight devices.

Comment:  You can have both "snd-aloop" and "snd-fastloop" devices at the same time, but remember that you can have only up to 32 total sound devices (0-31) - including any audio hardware that may already be installed on your system.

5)  Load the fastloop kernel module: "sudo modprobe snd-fastloop".  If all goes well, you should be able to do "aplay -l" and see the loopback devices with names like "fl0", "fl1", etc.  If you do not see that, carefully review the above steps and check for errors.

To have the module loaded automatically on every boot, you can add the line "snd-fastloop" to the file "/etc/modules" in a manner similar to that described above.

If you upgrade your operating system, you will probably have to re-install "snd-fastloop".  Because it isn't part of the standard package, it won't appear in the appropriate /lib/modules subdirectory after an upgrade.  It's likely that you'll need only do step 3, above - but be prepared to do steps 1-3.