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I want to run four identical USB audio devices, but ALSA shows only two of them on one machine (all four operate properly when plugged in alone).

It seems they are detected (and initialised) properly:

cat /proc/asound/modules 
 0 snd_usb_audio
 1 snd_usb_audio
 2 snd_usb_audio
 3 snd_usb_audio

cat /proc/asound/cards
 0 [Device         ]: USB-Audio - USB PnP Sound Device
                      C-Media Electronics Inc. USB PnP Sound Device at usb-0000:00:12.2-3.1, full spe
 1 [Device_1       ]: USB-Audio - USB PnP Sound Device
                      C-Media Electronics Inc. USB PnP Sound Device at usb-0000:00:12.2-3.2, full spe
 2 [Device_2       ]: USB-Audio - USB PnP Sound Device
                      C-Media Electronics Inc. USB PnP Sound Device at usb-0000:00:12.2-3.3, full spe
 3 [Device_3       ]: USB-Audio - USB PnP Sound Device
                      C-Media Electronics Inc. USB PnP Sound Device at usb-0000:00:12.2-3.4, full spe

cat /proc/asound/devices 
  0: [ 0]   : control
 16: [ 0- 0]: digital audio playback
 24: [ 0- 0]: digital audio capture
 32: [ 1]   : control
 33:        : timer
 48: [ 1- 0]: digital audio playback
 56: [ 1- 0]: digital audio capture
 64: [ 2]   : control
 80: [ 2- 0]: digital audio playback
 88: [ 2- 0]: digital audio capture
 96: [ 3]   : control
112: [ 3- 0]: digital audio playback
120: [ 3- 0]: digital audio capture

aplay however shows only two of them.

./aplay -l
**** List of PLAYBACK Hardware Devices ****
card 0: Device [USB PnP Sound Device], device 0: USB Audio [USB Audio]
  Subdevices: 1/1
  Subdevice #0: subdevice #0
card 1: Device_1 [USB PnP Sound Device], device 0: USB Audio [USB Audio]
  Subdevices: 1/1
  Subdevice #0: subdevice #0

What could be the limiting factor here?

1
  • 1
    Do you have four controlCx devices in /dev/snd/? When you run strace -e open aplay -l, do all four get opened?
    – CL.
    Jan 27, 2016 at 8:01

1 Answer 1

0

The comment of @CL solved it. As this setup runs on a Synology without udev running, only two character devices were in place.

ls -lA
crw-r--r--    1 root     root      116,   0 Nov 11  2009 controlC0
crw-r--r--    1 root     root      116,  32 Nov 11  2009 controlC1
crw-r--r--    1 root     root      116,   4 Nov 11  2009 hwC0D0
crw-r--r--    1 root     root      116,  36 Nov 11  2009 hwC1D0
crw-r--r--    1 root     root      116,  16 Jan 26 20:51 pcmC0D0p
crw-r--r--    1 root     root      116,  48 Jan 26 16:27 pcmC1D0p
crw-r--r--    1 root     root      116,   1 Nov 11  2009 seq
crw-r--r--    1 root     root      116,  33 Nov 11  2009 timer

Creating them following the output of /proc/asound/devices solved it.

mknod controlC2 c 116 64
mknod controlC3 c 116 96
mknod pcmC2D0p  c 116 80
mknod pcmC3D0p  c 116 112

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