How can I downmix stereo audio output to mono in PipeWire (on 22.10)?

Question

I need a way to set mono audio output on my system.
I had used this solution before: Can I downmix stereo audio to mono?

pacmd load-module module-remap-sink sink_name=mono master=NAME_OF_AUDIO_SINK \
 channels=2 channel_map=mono,mono

However, it stopped working when I upgraded to Kubuntu 22.10, presumably because it now uses PipeWire instead of PulseAudio. When I do pacmd list-sinks I get:
No PulseAudio daemon running, or not running as session daemon.

How can I do the same (downmix stereo audio output to mono) on PipeWire?

I found here a solution to a similar problem, but if I understand correctly that's for getting mono input. I want mono output.
Even better if I can easily switch it to mono/stereo using GUI, or set it to mono only when my stupid earphones are connected.

PS: They should really implement Mono Sound as a profile/configuration on System Settings > Audio, but oh well...

Answer 1

I managed to achieve true mono output by combining the stereo stream into a mono sink Pipewire - Module Combine Stream:

use $ wpctl status to get the sink id of your desired sink and $ wpctl inspect [ID] to get the node.name variable to use on the configuration file.

$ wpctl status
PipeWire 'pipewire-0' [0.3.56, user@hostname, cookie:1163266174]
Audio
 ├─ Devices:
 │      42. HD Audio Controller                 [alsa]
 │     105. USB PnP Audio Device                [alsa]
 │
 ├─ Sinks:
 │  *   48. HD Audio Controller Analog Stereo   [vol: 0.50]
 │
 ├─ ...
 │
 ├─ Sources:
 │  *  101. USB PnP Audio Device Mono           [vol: 0.74]
 │
 └─ ...

$ wpctl inspect 48

id 48, type PipeWire:Interface:Node
    ...
    ...
  * factory.id = "18"
    factory.mode = "merge"
    factory.name = "api.alsa.pcm.sink"
    ...
  * media.class = "Audio/Sink"
  * node.description = "HD Audio Controller Analog Stereo"
  * node.name = "alsa_output.pci-0000_08_00.4.analog-stereo"
  * node.nick = "ALC1220 Analog"
    ...
  * object.path = "alsa:pcm:1:front:1:playback"
  * object.serial = "49"
    ...

• In your distribution folder of pipewire(usually /usr/share/pipewire), find the pipewire.conf file and add this configuration with the modifications that suits your needs. You can either just copy the part inside context.modules=[] or create a new file and add to the ~/.config/pipewire/pipewire.conf.d/ or ~/.config/pipewire/pipewire-pulse.conf.d directories.

• At the context.modules section:

context.modules = [
{   name = libpipewire-module-combine-stream
        args = {
            combine.mode = sink
            node.name = "combine_stereo_to_mono_sink" # Can be any name of your choice
            node.description = "Mono Speakers" # This is the name that will appear in the sound settings
            combine.latency-compensate = false
            combine.props = {
            audio.position = [ MONO ]
            }
            stream.props = {
                stream.dont-remix = true
            }
            stream.rules = [
            {   matches = [
                    {   media.class = "Audio/Sink"
                        node.name = "alsa_output.pci-0000_08_00.4.analog-stereo" # Your node.name here
                    } ]
                    actions = { create-stream = {
                            audio.position = [ FL FR ]
                            combine.audio.position = [ MONO ] # These both are what combine both channels into one
                    } } }
            ]
        }
    }
]

After saving the file, either reboot your system or restart pipewire and wireplumber:

$ systemctl --user restart pipewire pipewire-pulse wireplumber

This is my sound settings: Mono Speakers Mono Speakers2

Answer 2

Maybe you could use qpwgraph and link both L and R channels in a manner similar to this:

Profiles can be saved, but for that I redirect you to the docs. Here is a bit about saving and loading profiles with pipewire on ArchWiki: https://wiki.archlinux.org/title/PipeWire#PipeWire_patch_sets_for_command_line

Answer 3

I managed to make mono audio output work in PipeWire by installing EasyEffects equalizer.

sudo apt install easyeffects

1. Output > Effects > Add Effect > Stereo Tools
2. On Stereo Matrix tab, choose "Mono Sum L+R". "Stereo to Mid-Side" seems to work too.
3. In Preferences, make sure to enable "Launch Service at System Startup".
4. Create presets for "Mono" and "Stereo" if desired (optional).

Answer 4

• Another solution without the need to install another program would be creating a mono output configuration file for PipeWire.

• For example the following configuration can be done per user session:

  • Create a mono-playback.conf in ~/.config/pipewire/pipewire.conf.d with the following:

    context.modules = [
        { name = libpipewire-module-loopback
            args = {
                node.description = "Mono Playback Device"
                capture.props = {
                    node.name      = "mono_output"
                    media.class    = "Audio/Sink"
                    audio.position = [ MONO ]
                }
                playback.props = {
                    node.name      = "playback.mono_output"
                    audio.position = [ MONO ]
                    node.passive   = true
                }
            }
        }
    ]
    

  • Restart pipewire service:

    systemctl --user restart pipewire{,-pulse}.{service,socket}
    

  • This will create a virtual device (a sink in this case) named Mono Playback Device

  • The device can be selected by GUI from an audio applet or system settings. Example using KDE Plasma audio applet.

• For Further reading to understand the options you can refer to PipeWire Wiki, Virtual Devices section.