Mono/stereo output toggle

[langmario]

tl;dr: is there a way of enabling mono output for single-speaker setups? Could this be added as a simple mono/stereo switch?

Hi,

I'm running a Raspberry Pi 3B+ with a Raspberry Pi DigiAMP+ connected to a single custom 3D-printed passive speaker.

I have tried almost all the popular audio OSes and software stacks like Volumio, moOde, and piCorePlayer but was never really satisfied with any of them. They often feel sluggish, the setups are overly complex, and achieving a mono output for a single speaker setup was sometimes terribly convoluted (looking at you, moOde and CamillaDSP...). With every option, there was always something that kept me searching for a better solution.

Then I stumbled upon odio, and I really, really like it so far. It just works exactly the way I want it to. Bluetooth worked immediately, Spotify connected right away, and everything I needed was effortless to set up.

The only missing piece to make it absolute perfection for my use case is an easy way to enable a mono downmix, as I am not running a stereo pair. Is this something that could be added, maybe as a simple mono/stereo toggle anywhere?

Also, outside of a potential future UI toggle, is there a workaround I could use to achieve mono output in the meantime?

Thanks a lot for all your work on this project!

[b0bbywan]

Hi! Thanks for your message, glad you like odio!

Good news, it looks totally doable in the pulseaudio backend of odio-api, and before I add it, there's an easy workaround possible.

A simple command to test:

pactl load-module module-remap-sink master=@DEFAULT_SINK@ sink_name=mono channels=1 channel_map=mono sink_properties=device.description=Mono
pactl set-default-sink mono # This can also be done directly from the audio section of the dashboard

If that works, you can add a simple script and systemd user service to service to enable this on boot, as it's done with the pulse-tcp module. Assuming you're using odio user from the rpi image, adapt to your needs if you used the script and configured another user:

/home/odio/.local/bin/pulse-mono.sh

#!/bin/bash -e
# Mono downmix for single-speaker setups: loads a mono remap sink on top of
# the current default sink and makes it the default. stop restores stereo.

STATE_DIR="/run/user/$(id --user)/pulse"
MODULE_ID_FILE="${STATE_DIR}/mono-module-id"
MASTER_FILE="${STATE_DIR}/mono-master"
SINK_NAME="mono"
MODULE_ARGS="sink_name=${SINK_NAME} sink_properties=device.description=Mono channels=1 channel_map=mono"

wait_for_pulseaudio() {
    for _ in {1..20}; do
        pactl info >/dev/null 2>&1 && return 0
        sleep 0.5
    done
    logger "pulse-mono: pulseaudio not reachable, exiting"
    exit 1
}

loaded_module_id() {
    while IFS=$'\t' read -r module_id name args; do
        if [[ "${name}" == "module-remap-sink" && "${args}" == "${MODULE_ARGS}" ]]; then
            echo "${module_id}"
            return 0
        fi
    done < <(pactl list modules short)
    return 1
}

start() {
    wait_for_pulseaudio
    local master
    master=$(pactl get-default-sink)
    if [[ "${master}" == "${SINK_NAME}" ]]; then
        logger "pulse-mono: already default"
        return 0
    fi
    echo "${master}" > "${MASTER_FILE}"

    local module_id
    if module_id=$(loaded_module_id); then
        logger "pulse-mono: remap sink already loaded (${module_id})"
    else
        logger "pulse-mono: loading remap sink on ${master}"
        module_id=$(pactl load-module module-remap-sink master="${master}" ${MODULE_ARGS})
    fi
    echo "${module_id}" > "${MODULE_ID_FILE}"
    pactl set-default-sink "${SINK_NAME}"
}

stop() {
    if [[ -f "${MASTER_FILE}" ]]; then
        pactl set-default-sink "$(cat "${MASTER_FILE}")" || true
    fi
    local module_id
    if [[ -f "${MODULE_ID_FILE}" ]]; then
        module_id=$(cat "${MODULE_ID_FILE}")
    else
        module_id=$(loaded_module_id) || module_id=""
    fi
    if [[ -n "${module_id}" ]]; then
        logger "pulse-mono: unloading remap sink (${module_id})"
        pactl unload-module "${module_id}" || true
    fi
    rm --force "${MODULE_ID_FILE}" "${MASTER_FILE}"
}

case "$1" in
    start) start ;;
    stop) stop ;;
    *) echo "Usage: $0 {start|stop}"; exit 1 ;;
esac

/home/odio/.config/systemd/user/pulse-mono.service

[Unit]
Description=PulseAudio mono downmix sink
After=pulseaudio.service
Requires=pulseaudio.service
PartOf=pulseaudio.service
StartLimitIntervalSec=60
StartLimitBurst=15

[Service]
Type=oneshot
RemainAfterExit=yes
Restart=on-failure
RestartSec=2
ExecStart=%h/.local/bin/pulse-mono.sh start
ExecStop=%h/.local/bin/pulse-mono.sh stop

[Install]
WantedBy=default.target

Then to enable at boot:

systemctl --user daemon-reload
systemctl --user enable --now pulse-mono

Let me know how that works for you!

[langmario]

Worked like a charm, thank you very much for your fast response and solution!! Looking forward to seeing it integrated natively down the road.