'no audio recorded from alsa loopback device

I've been trying to get an alsa loopback device working (to test an application that reads audio) following the instructions here:

https://sysplay.in/blog/linux/2019/06/playing-with-alsa-loopback-devices/

The device is enabled using:

modeprobe snd-aloop

As can be seen from:

>aplay -l
**** List of PLAYBACK Hardware Devices ****
card 0: PCH [HDA Intel PCH], device 0: ALC269VB Analog [ALC269VB Analog]
  Subdevices: 1/1
  Subdevice #0: subdevice #0
card 0: PCH [HDA Intel PCH], device 3: HDMI 0 [HDMI 0]
  Subdevices: 1/1
  Subdevice #0: subdevice #0
card 0: PCH [HDA Intel PCH], device 7: HDMI 1 [HDMI 1]
  Subdevices: 1/1
  Subdevice #0: subdevice #0
card 1: Loopback [Loopback], device 0: Loopback PCM [Loopback PCM]
  Subdevices: 8/8
  Subdevice #0: subdevice #0
  Subdevice #1: subdevice #1
  Subdevice #2: subdevice #2
  Subdevice #3: subdevice #3
  Subdevice #4: subdevice #4
  Subdevice #5: subdevice #5
  Subdevice #6: subdevice #6
  Subdevice #7: subdevice #7
card 1: Loopback [Loopback], device 1: Loopback PCM [Loopback PCM]
  Subdevices: 7/8
  Subdevice #0: subdevice #0
  Subdevice #1: subdevice #1
  Subdevice #2: subdevice #2
  Subdevice #3: subdevice #3
  Subdevice #4: subdevice #4
  Subdevice #5: subdevice #5
  Subdevice #6: subdevice #6
  Subdevice #7: subdevice #7

I am supposed to be able to play and record audio using:

aplay -D hw:1,0 foo.wav

&

arecord -D hw:1,0 bar.wav

However the audio recorded is completely silent. This is more visible when I use sox:

AUDIODEV=hw:1,0 play -V foo.wav
AUDIODEV=hw:1,0 rec -V -c 1 -r 8000 bar.wav

As it provides a convenient ascii-art volume indicator:

In:100%  00:10:00.00 [00:00:00.00] Out:4.80M [     =|=     ]        Clip:0
alsa


Solution 1:[1]

The loopback device actually creates pairs of devices. One device is the read end and a separate device is the write end. What you actually need is:

aplay -D hw:1,0 foo.wav

&

arecord -D hw:1,1 bar.wav

or using sox:

AUDIODEV=hw:1,0 play -V foo.wav
AUDIODEV=hw:1,1 rec -V -c 1 -r 8000 bar.wav

I discovered this by accident when I copied an .asoundrc configuration that worked as below. The asound configuration is not required however:

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

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

See also https://stackoverflow.com/a/58998959/1569204

Sources

This article follows the attribution requirements of Stack Overflow and is licensed under CC BY-SA 3.0.

Source: Stack Overflow

Solution Source
Solution 1

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