Minimal ALSA loopback device config

.asoundrc

# ${HOME}/.asoundrc

# =========================================
Base ALSA device targetting loopback device
# =========================================
pcm.SDR_IN {
	type hw
	card "SDRLB"
	device 0
	subdevice 0
	format "S16_LE"
	channels 1
	rate 48000
	nonblock 1
	hint.show on
	hint.description "Send nc packets here"
}

pcm.SDR_OUT {
	type hw
	card "SDRLB"
	device 1
	subdevice 0
	format "S16_LE"
	channels 1
	rate 48000
	nonblock 1
	hint.show on
	hint.description "SDR Rx audio"
}

# =========================================
Plug devices for adapting to application requirements
# =========================================
pcm.SDR_IN_AUTO {
	type plug
	slave {
		pcm "SDR_IN"
	}
	hint.show on
	hint.description "Send nc packets here"
}

pcm.SDR_OUT_AUTO {
	type plug
	slave {
		pcm "SDR_OUT"
	}
	hint.show on
	hint.description "Connect decoder here"
}

alsa-base.conf

# /etc/modprobe.d/alsa-base.conf

# This is a configuration file for modprobe to force specific device ordering
# and indexing. This then exposes the devices to ALSA in the proper order
# regardless of the number of devices plugged in or the order they were connected.

# id=SDRLB is the name ALSA will show for the loopback device, which can be referenced in ~/.asoundrc
options snd-aloop enable=1 index=1 pcm_substreams=4 id=SDRLB

modules.conf

# /etc/modules-load.d/modules.conf

# List of modules to load at boot
snd-aloop