mjhjmtu

I'm using my Ubuntu 14.04 LTS Laptop toplay music at a party. I'm using Mixxx and is performs very well for that task. Now I'd like to add some light effects and plan to use Qlcplus for that. So I need to send the sound from Mixxx to the soundcard and Qlcplus.

I've managed to configure Mixxx to send sound to the ALSA loopback and recover it from there in Qlcplus. But then I have no sound output. How can I route sound from the ALSA loopback to speakers ?

After lots of searching I've found the answer to my question. You need to appropriately configure ALSA and it will provide a new device which duplicates sound sent to it to two or more devices like soundcards and the loopback device.

Here the .asoundrc file I use:

# If you want this to be the default, then you
# need to override the default device and provide
# a playback path to the CardAndLoop and a capture
# path to whatever soundcard you have (here the 1st card)
#pcm.!default 
# type asym
# playback.pcm "CardAndLoop"
# capture.pcm "hw:0,0"
#

# This is the interface you use for sound output
# It will send the output to the soundcard and loopback device
pcm.CardAndLoop 
 type plug
 slave.pcm MultiCh
 route_policy "duplicate"


# Virtual multichannel device with four channels
# two the for the soundcard, two for the loopback
pcm.MultiCh 
 type multi
 slaves.a.pcm pcm.MixCard
 slaves.a.channels 2
 slaves.b.pcm pcm.MixLoopback
 slaves.b.channels 2
 bindings.0.slave a
 bindings.0.channel 0
 bindings.1.slave a
 bindings.1.channel 1
 bindings.2.slave b
 bindings.2.channel 0
 bindings.3.slave b
 bindings.3.channel 1


# Mixer for the soundcard
pcm.MixCard 
 type dmix
 ipc_key 1024
 slave 
# pcm "hw:Conectiv,0"
 pcm "hw:PCH,0"
# rate 48000
 rate 44100
 periods 128
 period_time 0
 period_size 1024 # must be power of 2
 buffer_size 8192
 


# Mixer for the loopback
pcm.MixLoopback 
 type dmix
 ipc_key 1025
 slave 
 pcm "hw:Loopback,0"
# rate 48000
 rate 44100
 periods 128
 period_time 0
 period_size 1024 # must be power of 2
 buffer_size 8192
 

This file will provide a new ALSA sound device 'CardAndLoop'. If you choose this device in your sound application the sound will be sent to the 'PCH' sound card and the loopback device. You can start a second application and use the loopback device as input and it will obtain the sound played by the 1st application.

I used names to denominate the sound devices. These names can be taken from the /proc/asound/cards file:

$ cat /proc/asound/cards
0 [PCH ]: HDA-Intel - HDA Intel PCH
 HDA Intel PCH at 0xf5330000 irq 44
2 [Loopback ]: Loopback - Loopback
 Loopback 1

Have fun !