When trying to use a “big” USB audio interface (i.e. one with many in- and outputs) on Linux, you might be experiencing problems like only some of your in- and outputs working, none working at all, etc. This is because, while PulseAudio works very well with commodity hardware, it just doesn’t know your way too expensive interface. However, in a few minutes you can tell it how to deal with it.

First, you need to know the vendor and product ID of your interface. For that, you can use lsusb:

$ lsusb | grep Focusrite
Bus 001 Device 006: ID 1235:820b Focusrite-Novation Clarett+ 4Pre

Then you can tell udev that you want special treatment for your special interface. For that you need to create a rules file in one of its configuration directories, like /lib/udev/rules.d/91-pulseaudio.rules. It should look like this, substituting 1235 and 820b for your IDs:

SUBSYSTEM!="sound", GOTO="pulseaudio1_end"
ACTION!="change", GOTO="pulseaudio1_end"
KERNEL!="card*", GOTO="pulseaudio1_end"

SUBSYSTEMS=="usb", ATTRS{idVendor}=="1235", ATTRS{idProduct}=="820b", ENV{PULSE_PROFILE_SET}="big-interfaces.conf"

LABEL="pulseaudio1_end"

Now, PulseAudio will look for profiles in /usr/share/pulseaudio/alsa-mixer/profile-sets/big-interfaces.conf. This is, finally, where you can have some fun. A simple example to just get stereo output and four inputs, with the first two being one stereo input, looks like this:

[General]
auto-profiles = no

[Mapping analog-out]
description = Analog Outputs
device-strings = hw:%f
channel-map = left,right
direction = output

[Mapping analog-in]
description = Analog Inputs
device-strings = hw:%f
channel-map = left,right,aux0,aux1
direction = input

[Profile output:analog-out+input:analog-in]
description = Analog Stereo
output-mappings = analog-out
input-mappings = analog-in

The exact configuration of course depends on your specific hardware and needs. Once you’ve configured it to your liking, you need to tell udev to re-setup your interface and restart PulseAudio:

$ sudo udevadm trigger -ssound
$ pulseaudio -k