Create virtual device in .asoundrc file

Question

I have been trying to create a virtual device to record the internal audio of my sound card using pure ALSA. After googling a lot, I found a .asoundrc file¹ that is very near of what I am looking for:

pcm.mycard {
    type hw
    card 0
}

ctl.mycard {
    type hw
    card 0
}

pcm.myconvert {
    type plug
    slave {
        pcm "myrecord"
        format S16_LE
    }
}

pcm.myrecord {
    type file
    format "raw"
    slave.pcm "mycard"
    file "| oggenc -Q -q6 -r -B %b -C %c -R %r -o /tmp/record-$(date +%%Y-%%m-%%d_%%H%%M%%S).ogg -"
}

pcm.!default{
   type asym
   playback.pcm "myconvert"
   capture.pcm "mycard"
}

This configuration makes possible to record a ogg file with all audio played in my computer. What I would like to do now, is to modify the pcm.myrecord part of the configuration, so that the audio is "streamed" in a virtual card that I could capture later with let's say, arecord. I would be glad if someone could help me with this.

I tried this: How do I create virtual ALSA device from which I can record everything that is played? but that did not work for me. Below, the output from arecord -L:

muammar@zarathustra /tmp 
  % arecord -L                                                                                                                                         !10015
null
    Discard all samples (playback) or generate zero samples (capture)
default:CARD=PCH
    HDA Intel PCH, CS4208 Analog
    Default Audio Device
sysdefault:CARD=PCH
    HDA Intel PCH, CS4208 Analog
    Default Audio Device
front:CARD=PCH,DEV=0
    HDA Intel PCH, CS4208 Analog
    Front speakers
dmix:CARD=PCH,DEV=0
    HDA Intel PCH, CS4208 Analog
    Direct sample mixing device
dsnoop:CARD=PCH,DEV=0
    HDA Intel PCH, CS4208 Analog
    Direct sample snooping device
hw:CARD=PCH,DEV=0
    HDA Intel PCH, CS4208 Analog
    Direct hardware device without any conversions
plughw:CARD=PCH,DEV=0
    HDA Intel PCH, CS4208 Analog
    Hardware device with all software conversions
default:CARD=Loopback
    Loopback, Loopback PCM
    Default Audio Device
sysdefault:CARD=Loopback
    Loopback, Loopback PCM
    Default Audio Device
front:CARD=Loopback,DEV=0
    Loopback, Loopback PCM
    Front speakers
surround21:CARD=Loopback,DEV=0
    Loopback, Loopback PCM
    2.1 Surround output to Front and Subwoofer speakers
surround40:CARD=Loopback,DEV=0
    Loopback, Loopback PCM
    4.0 Surround output to Front and Rear speakers
surround41:CARD=Loopback,DEV=0
    Loopback, Loopback PCM
    4.1 Surround output to Front, Rear and Subwoofer speakers
surround50:CARD=Loopback,DEV=0
    Loopback, Loopback PCM
    5.0 Surround output to Front, Center and Rear speakers
surround51:CARD=Loopback,DEV=0
    Loopback, Loopback PCM
    5.1 Surround output to Front, Center, Rear and Subwoofer speakers
surround71:CARD=Loopback,DEV=0
    Loopback, Loopback PCM
    7.1 Surround output to Front, Center, Side, Rear and Woofer speakers
dmix:CARD=Loopback,DEV=0
    Loopback, Loopback PCM
    Direct sample mixing device
dmix:CARD=Loopback,DEV=1
    Loopback, Loopback PCM
    Direct sample mixing device
dsnoop:CARD=Loopback,DEV=0
    Loopback, Loopback PCM
    Direct sample snooping device
dsnoop:CARD=Loopback,DEV=1
    Loopback, Loopback PCM
    Direct sample snooping device
hw:CARD=Loopback,DEV=0
    Loopback, Loopback PCM
    Direct hardware device without any conversions
hw:CARD=Loopback,DEV=1
    Loopback, Loopback PCM
    Direct hardware device without any conversions
plughw:CARD=Loopback,DEV=0
    Loopback, Loopback PCM
    Hardware device with all software conversions
plughw:CARD=Loopback,DEV=1
    Loopback, Loopback PCM
    Hardware device with all software conversions

Thanks.

1. https://bbs.archlinux.org/viewtopic.php?id=167830

Solution

I solved this issue as follows:

First, load the snd-aloop module:

sudo modprobe snd-aloop

This will create a new device called Loopback:

± % cat /proc/asound/cards                                                                                                                             !10017

 0 [PCH            ]: HDA-Intel - HDA Intel PCH
                      HDA Intel PCH at 0xc1814000 irq 60
 1 [HDMI           ]: HDA-Intel - HDA Intel HDMI
                      HDA Intel HDMI at 0xc1810000 irq 61
 2 [Loopback       ]: Loopback - Loopback
                      Loopback 1

From the information shown above we have to create two hw devices:

1. hw:0,0 (PCH is my main sound card).
2. hw:2,1 (The Loopback virtual device).

Normally, hw devices have the form: hw:X,Y. For our purposes, it seems that Y for the main card is always 0 while for the Loopback we have Y equals 1.

Now, in order to make all of this work, we need a ~/.asoundrc file with the following:

pcm.!default {
  type asym
  playback.pcm "LoopAndReal"
  #capture.pcm "looprec"
  capture.pcm "hw:X1,Y1"
}

pcm.looprec {
    type hw
    card "Loopback"
    device 1
    subdevice 0
}


pcm.LoopAndReal {
  type plug
  slave.pcm mdev
  route_policy "duplicate"
}


pcm.mdev {
  type multi
  slaves.a.pcm pcm.MixReale
  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
}


pcm.MixReale {
  type dmix
  ipc_key 1024
  slave {
    pcm "hw:X1,Y1"
    rate 48000
    #rate 44100
    periods 128
    period_time 0
    period_size 1024 # must be power of 2
    buffer_size 8192
  }
}

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

Modify hw:X1,Y1 with the values for your main card (in my case hw:0,0).

You can test that this configuration works by playing something in your computer. If you hear it, so it is fine. Now let's play something in the computer, and record it from this virtual device with ffmpeg:

ffmpeg -f alsa -i hw:X2,Y2 -c:a libmp3lame -b:a 256k -vn capture.mp3

Where hw:X2,Y2 has to be replaced by the Loopback device, in this case hw:2,1.

I suppose that using arecord should work too.

Comments

[Admin]

Possible duplicate of How do I create virtual ALSA device from which I can record everything that is played?

[Admin]

@CL as I wrote above, I tried it and it did not work. However, I think I have come with a solution. I will post it later when I am 100% sure.

[Akhilesh Dhar Dubey]

Thank you so much for this. I've been trying for years to get this to work, and your example worked for me as-is.

[gm_]

@Pistos I am glad it could help.