Debian – Create virtual device in .asoundrc file

alsaaudiodebianlinux

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.

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

Best Answer

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:

hw:0,0 (PCH is my main sound card).
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.

Related Solutions

Trying to hear audio coming from LINE IN using ALSA on Debian

The simplest way is to connect aplay and arecord by a pipe. There's no arecord -L information for PCM sources, but assuming it looks similar to the PCM sinks:

arecord -t au -r 44100 -D front:CARD=Intel,DEV=0 | aplay -t au -D front:CARD=Intel,DEV=0

There's a noticable delay until the output is played, because a pipe isn't intended for real-time audio processing.

The -t au options select the Sun Audio format. This is important, because e.g. the WAV format contains a header with the length of the file, so it can't be used across a pipe.

The default rate for arecord is 8000 samples/s, which is usually not what you want, so the -r option is also important.

The PCM sources and sinks may not support some combinations of rates/format/channels, so you either may have to pick valid combinations for your hardware using more options, or use plughw instead of front. For more modern ALSA installations, plughw entries are automatically generated, and they put a plug plugin in front of the real hardware to do format conversion. If your ALSA doesn't generate those automatically, you have to add them manually to your .asoundrc.

There are other ways to do it, for example with a chain of various ALSA plugins, if you want to have this feature permanently. It's not necessary for the hardware to be able to route audio directly.

Debian – How to enable both built-in audio output and HDMI audio output with PulseAudio

I just managed to configure exactly this thanks to the instructions in the ArchLinux wiki :

Set the one sink - HDMI - in pavucontrol/Configuration, and the other - analog output - you specify in default.pa :

load-module module-alsa-sink device=hw:0,0
load-module module-combine-sink sink_name=combined
set-default-sink combined

Maybe you were missing the last two lines?

Addendum: This is OK to add the analog output to HDMI, but what if you want it the other way around? (I want to keep analog input, Pulse calls that "duplex"...) Unfortunately, load-module module-alsa-sink device=hw:0,3 did not work to add HDMI output to Pulse's "Analog Stereo Duplex" configuration. Pulse would start, but no HDMI output.

I compared the output of pacmd list-sinks in the two situations. When I added device=hw:0,3, that one would show up without ports. When HDMI output was chosen in pavucontrol, there would be ports. Looking at the output details, I came up with device=hdmi:0.

Choose "Analog Stereo Duplex" as your pavucontrol configuration, and add to default.pa :

load-module module-alsa-sink device=hdmi:0
load-module module-combine-sink sink_name=combined
set-default-sink combined

Works like a charm. I keep my microphone input, and on pavucontrol/Output Devices I have all options for analog output and for HDMI.

Best Answer

Related Solutions

Trying to hear audio coming from LINE IN using ALSA on Debian

Debian – How to enable both built-in audio output and HDMI audio output with PulseAudio

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