Split multi-input USB audio interface (Focusrite Vocaster One) with pulseaudio

oliverguenther

2/23/24, 12:29 PM

I have a Focusrite Vocaster One USB audio interface that I operate with an XLR microphone on my Ubuntu 22.04. It pops up as a Microphone input with the name "Vocaster One USB Multichannel".

What I noticed was that in web conferences, my audio interface would output sounds from PC audio even though the microphone was muted through the hardware button. It seems the default channel used for input is the mixing channel for recording, which contains both the XLR input and the loopback from the computer audio.

How can I select the individual inputs with pulseaudio which doesn't include the mix?

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Ubuntu

oliverguenther

2/23/24, 12:29 PM

The pulseaudio configuration allows to remap partial sources of the multi-input device, however the exact channel used for different functions is guesswork. It's not entirely clear to me what values the master_channel_map can receive, but I had success in selecting front-center which gives me the mic input without any mixes, and hardware mute button correctly silences everything.

First, find out the device name of the multinput audio interface:

$ pacmd list-sources | grep name:
    name: <alsa_output.usb-Focusrite_Vocaster_One_USB_V1AR19B2601E24-00.analog-surround-40.monitor>
    name: <alsa_input.usb-Focusrite_Vocaster_One_USB_V1AR19B2601E24-00.multichannel-input>

In my case, the master name is alsa_input.usb-Focusrite_Vocaster_One_USB_V1AR19B2601E24-00.multichannel-input

Then add this line to the /etc/pulse/default.pa at the bottom:

load-module module-remap-source source_name=vocaster-mic source_properties=device.description=Vocaster master=alsa_input.usb-Focusrite_Vocaster_One_USB_V1AR19B2601E24-00.multichannel-input remix=no channels=1 master_channel_map=front-center channel_map=mono

This adds a new input called "Vocaster" with the front-center channel of the multi-input device.

Optionally, mark this device as the default so you don't have to switch on boot time:

set-default-sink alsa_output.usb-Focusrite_Vocaster_One_USB_V1AR19B2601E24-00.analog-surround-40.monitor
set-default-source vocaster-mic

Afterward, restart or use pulseaudio -k to kill the previous pulseaudio daemon. Note that doing this breaks audio in apps until they are restarted.

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