Hot plug USB DAC --- Auto Switch Audio Output with udev

Help and Support Raspberry Pi
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With udev
(Tested on RPi3)

  • Plug in / Turn on USB DAC --- auto switch audio output to USB DAC

  • Unplug / Turn off USB DAC --- auto swith audio output to HDMI

Get /device/path…

sudo udevadm monitor --udev

  • plug in / turn on USB DAC

  • copy /devices/path.../card1

Get device parameter

sudo udevadm info --path=/devices/path.../card1 --attribute-walk
  • copy ‘KERNEL’ and ‘SUBSYSTEM’ for rules

Create hot plug rules
/etc/udev/rules.d/usbsound.rules

ACTION=="add", KERNEL=="controlC1", SUBSYSTEM=="sound", RUN+="/home/osmc/usbsound.sh"
ACTION=="remove", KERNEL=="controlC1", SUBSYSTEM=="sound", RUN+="/home/osmc/hdmisound.sh"

Create json-rpc command files

  • change ‘audiooutput’ in ‘guisettings.xml’

/home/osmc/usbsound.sh

#!/bin/bash

# auto switch for single usb sound only

# get http 'port' >>> as may be changed by user
port=$(sed -n 's:.*>\(.*\)</webserverport>.*:\1:p' /home/osmc/.kodi/userdata/guisettings.xml)

# get usb 'id' >>> /dev/snd/controlC1 == /sys/devices/platform/soc/3f980000.usb/usb1/1-1/1-1.[1 to 4]/...
# (get usb path | sed path to '...card1/id' | prepend '/sys' dir to path) >>> get file content
path=$(udevadm info -n /dev/snd/controlC1 -q path | sed 's/controlC1/id/' | sed 's/^/\/sys/')
id=$(< $path)

# [{set audio output to usb 'id'}, {notify}] http 'port'
curl -H "Content-type: application/json" -X POST -d '[
	{"jsonrpc":"2.0","method":"Settings.SetSettingValue", "params":{"setting":"audiooutput.audiodevice","value":"ALSA:@:CARD='$id',DEV=0"},"id":1},
	{"jsonrpc":"2.0","method":"GUI.ShowNotification", "params":{"title":"AUDIO OUTPUT", "message":"Switched to USB Sound"}, "id":1}
]' http://localhost:$port/jsonrpc

Create unplug notification
/home/osmc/hdmisound.sh

#!/bin/bash

# auto swith back to hdmi by system default, notify only

# get http 'port'
port=$(sed -n 's:.*>\(.*\)</webserverport>.*:\1:p' /home/osmc/.kodi/userdata/guisettings.xml)

# notify with http 'port'
curl -H "Content-type: application/json" -X POST -d '{"jsonrpc":"2.0","method":"GUI.ShowNotification", "params":{"title":"AUDIO OUTPUT", "message":"Switched to HDMI"}, "id":1}' http://localhost:$port/jsonrpc

Set files permission

sudo chmod +x /home/osmc/hdmisound.sh usbsound.sh
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Nice