Arch Linux

accy

Member

Registered: 2019-12-23

Posts: 5

Recording Speakers Through ALSA Loopback

Hi, I've been trying to record system sound through an ALSA loopback to no success. I am trying to use ffmpeg, though I suppose arecorder should work as well.

I followed this guide, which for convenience I will reproduce below: https://trac.ffmpeg.org/wiki/Capture/ALSA

First add the aloop module

sudo modprobe snd-aloop pcm_substreams=1

This should create a loopback device. Here's the output of aplay -l:

card 0: PCH [HDA Intel PCH], device 0: CX20590 Analog [CX20590 Analog]
  Subdevices: 1/1
  Subdevice #0: subdevice #0
card 1: NVidia [HDA NVidia], device 3: HDMI 0 [HDMI 0]
  Subdevices: 1/1
  Subdevice #0: subdevice #0
card 1: NVidia [HDA NVidia], device 7: HDMI 1 [HDMI 1]
  Subdevices: 1/1
  Subdevice #0: subdevice #0
card 1: NVidia [HDA NVidia], device 8: HDMI 2 [HDMI 2]
  Subdevices: 1/1
  Subdevice #0: subdevice #0
card 2: Loopback [Loopback], device 0: Loopback PCM [Loopback PCM]
  Subdevices: 1/1
  Subdevice #0: subdevice #0
card 2: Loopback [Loopback], device 1: Loopback PCM [Loopback PCM]
  Subdevices: 1/1
  Subdevice #0: subdevice #0

then set ~/.asoundrc to the following:

# .asoundrc
pcm.multi {
    type route;
    slave.pcm {
        type multi;
        slaves.a.pcm "output";
        slaves.b.pcm "loopin";
        slaves.a.channels 2;
        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;
    }

    ttable.0.0 1;
    ttable.1.1 1;
    ttable.0.2 1;
    ttable.1.3 1;
}

pcm.!default {
	type plug
	slave.pcm "multi"
} 

pcm.output {
	type hw
	card 0       # replace with your output card, if needed
}

pcm.loopin {
	type plug
	slave.pcm "hw:Loopback,0,0"
}

pcm.loopout {
	type plug
	slave.pcm "hw:Loopback,1,0"
}

Finally, record with

ffmpeg -f alsa -ac 2 -ar 44100 -i loopout out.wav

However, my out.wav file gets created, but just remains an empty file, even though ffmpeg appears to successfully record. I'm not sure what the problem is.

I've also tried this guide from stackexchange, no success:
https://unix.stackexchange.com/question … undrc-file
I've also tried the solution from this old thread
https://bbs.archlinux.org/viewtopic.php?id=147852

Does anyone know what's going wrong here?

Last edited by accy (2019-12-23 04:40:59)

teckk

Member

Registered: 2013-02-21

Posts: 548

Re: Recording Speakers Through ALSA Loopback

Haven't done that in a while. I have a script that does this. If this does you any good.

mv ~/.loop_asoundrc ~/.asoundrc
modprobe snd-aloop

cat ~/.asoundrc

pcm.!default {
  type asym
  playback.pcm "LoopAndReal"
  #capture.pcm "looprec"
  capture.pcm "hw:0,0"
}

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:0,0"
    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
  }
}

ffmpeg -f alsa -i hw:1,1 -c:a libmp3lame -b:a 32k -vn capture.mp3

ffprobe ~/capture.mp3
...
Input #0, mp3, from '~/capture.mp3':
  Metadata:
    encoder         : Lavf58.29.100
  Duration: 00:00:15.07, start: 0.023021, bitrate: 32 kb/s
    Stream #0:0: Audio: mp3, 48000 Hz, stereo, fltp, 32 kb/s

   
And capture.mp3 plays ok.