rPi 4 as USB Audio Interface

usb_audio.md

How to enable USB Audio Gadget for rPi 4

Update the device (kernel should be >= 6.1)

sudo apt update && sudo apt upgrade -y

Enable USB Gadget and g_audio

echo "dtoverlay=dwc2" | sudo tee -a /boot/config.txt
echo "dwc2" | sudo tee -a /etc/modules
echo "g_audio" | sudo tee -a /etc/modules
sudo reboot

Validate the result

dmesg | grep -iE "g_audio|dwc2"

# Expected output:
[    2.981896] dwc2 fe980000.usb: supply vusb_d not found, using dummy regulator
[    2.982209] dwc2 fe980000.usb: supply vusb_a not found, using dummy regulator
[    3.190674] dwc2 fe980000.usb: EPs: 8, dedicated fifos, 4080 entries in SPRAM
[    3.191356] dwc2 fe980000.usb: DWC OTG Controller
[    3.191410] dwc2 fe980000.usb: new USB bus registered, assigned bus number 3
[    3.191483] dwc2 fe980000.usb: irq 39, io mem 0xfe980000
[    3.191919] usb usb3: Manufacturer: Linux 6.1.0-rpi7-rpi-v8 dwc2_hsotg
[    5.661701] g_audio gadget.0: Linux USB Audio Gadget, version: Feb 2, 2012
[    5.661755] g_audio gadget.0: g_audio ready
[    5.661785] dwc2 fe980000.usb: bound driver g_audio
[    5.838324] dwc2 fe980000.usb: new device is high-speed
[    5.871874] dwc2 fe980000.usb: new address 51
[    6.045460] dwc2 fe980000.usb: dwc2_hsotg_ep_sethalt(ep 00000000745eb641 ep1in, 0)

Set g_audio params on boot

sudo echo "options g_audio c_srate=96000 c_ssize=4 p_srate=96000 p_ssize=4 iManufacturer="Linux Foundation" iProduct="Multifunction Composite Gadget" idProduct=0x0104 idVendor=0x1d6b" > /etc/modprobe.d/g_audio.conf

Install pipewire and bluetooth packages

sudo apt install pipewire wireplumber libspa-0.2-bluetooth

Connect your bluetooth device

MAC_ADDRESS=<MAC_ADDRESS>
echo "Pairing and connecting to $MAC_ADDRESS"
coproc bluetoothctl
echo -e 'agent on\npair $MAC_ADDRESS\ntrust $MAC_ADDRESS\nconnect $MAC_ADDRESS\nexit' >&p
output=$(cat <&p)
echo $output

List pipewire inputs

pw-link -i

List pipewire outputs

pw-link -o

Connect an input (USB interface) to an output (bluetooth device)

pw-link alsa_input.platform-fe980000.usb.stereo-fallback:capture_FR bluez_output.$MAC_ADDRESS.1:playback_FR
pw-link alsa_input.platform-fe980000.usb.stereo-fallback:capture_FL bluez_output.$MAC_ADDRESS.1:playback_FL

The same can be done to connect an input (USB mic) to an output (USB interface)

pw-link alsa_input.usb-Microphone-00.mono-fallback:capture_MONO alsa_output.platform-fe980000.usb.stereo-fallback:playback_FL
pw-link alsa_input.usb-Microphone-00.mono-fallback:capture_MONO alsa_output.platform-fe980000.usb.stereo-fallback:playback_FR

Make a simple shell script called pw-connect.sh

#!/bin/bash

while true; do
  # Send audio from USB to BT
  pw-link alsa_input.platform-fe980000.usb.stereo-fallback:capture_FL <YOUR_OUTPUT_DEVICE>
  pw-link alsa_input.platform-fe980000.usb.stereo-fallback:capture_FR <YOUR_OUTPUT_DEVICE>

  # Send audio from MIC to USB
  pw-link <YOUR_INPUT_DEVICE> alsa_output.platform-fe980000.usb.stereo-fallback:playback_FL
  pw-link <YOUR_INPUT_DEVICE> alsa_output.platform-fe980000.usb.stereo-fallback:playback_FR
  sleep 1
done

Make a user service to run on boot with sudo nano /etx/systemd/user/pw-connect.service

[Unit]
Description=Pipewire auto link audio streams

[Service]
Type=simple
#User=
#Group=
ExecStart=<PATH_TO_FILE>/pw-connect.sh
Restart=on-failure
StandardOutput=file:%h/log_file

[Install]
WantedBy=default.target

Enable and start the user service

systemctl --user daemon-reload
systemctl --user --now enable pw-connect.service

Just reboot and it should be working!

sudo reboot