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I have a few webcam devices connected over USB and I want to record the audio from them using arecord. The device list is below (Cards 1 through 4):

t:~$ arecord --list-devices
**** List of CAPTURE Hardware Devices ****
card 0: PCH [HDA Intel PCH], device 0: ALC Analog [ALC Analog]
  Subdevices: 1/1
  Subdevice #0: subdevice #0
card 1: TT [CA TT], device 0: USB Audio [USB Audio]
  Subdevices: 1/1
  Subdevice #0: subdevice #0
card 2: TT_1 [CA TT], device 0: USB Audio [USB Audio]
  Subdevices: 1/1
  Subdevice #0: subdevice #0
card 3: TT_2 [CA TT], device 0: USB Audio [USB Audio]
  Subdevices: 1/1
  Subdevice #0: subdevice #0
card 4: TT_3 [CA TT], device 0: USB Audio [USB Audio]
  Subdevices: 1/1
  Subdevice #0: subdevice #0

When I list the devices that PulseAudio sees, I have the following (partial list only showing the Webcams):

t:~$pactl list
Module #0
        Name: module-device-restore 
....
Card #11
    Name: alsa_card.usb-VS_Inc_CA_TT-02
    Driver: module-alsa-card.c
    Owner Module: 38
    Properties:
        alsa.card = "1"
        alsa.card_name = "CA TT"
        alsa.long_card_name = "VS Inc CA TT at usb-0000:00:14.0-3"
        alsa.driver_name = "snd_usb_audio"
        device.bus_path = "pci-0000:00:14.0-usb-0:3:1.2"
        sysfs.path = "/devices/pci0000:00/0000:00:14.0/usb2/2-3/2-3:1.2/sound/card1"
        udev.id = "usb-VS_Inc_CA_TT-02"
        device.bus = "usb"
        device.vendor.id = "1a1a"
        device.vendor.name = "Technology Inc."
        device.product.id = "2a2a"
        device.product.name = "CA TT"
        device.serial = "VS_Inc_CA_TT"
        device.form_factor = "webcam"
        device.string = "1"
        device.description = "CA TT"
        module-udev-detect.discovered = "1"
        device.icon_name = "camera-web-usb"
    Profiles:
        input:analog-stereo: Analog Stereo Input (sinks: 0, sources: 1, priority: 60, available: yes)
        input:iec958-stereo: Digital Stereo (IEC958) Input (sinks: 0, sources: 1, priority: 55, available: yes)
        off: Off (sinks: 0, sources: 0, priority: 0, available: yes)
    Active Profile: input:analog-stereo
    Ports:
        analog-input: Analog Input (priority: 10000, latency offset: 0 usec)
            Part of profile(s): input:analog-stereo
        iec958-stereo-input: Digital Input (S/PDIF) (priority: 0, latency offset: 0 usec)
            Part of profile(s): input:iec958-stereo

Card #12
    Name: alsa_card.usb-VS_Inc_CA_TT-02.2
    Driver: module-alsa-card.c
    Owner Module: 39
    Properties:
        alsa.card = "2"
        alsa.card_name = "CA TT"
        alsa.long_card_name = "VS Inc CA TT at usb-0000:00:14.0-4"
        alsa.driver_name = "snd_usb_audio"
        device.bus_path = "pci-0000:00:14.0-usb-0:4:1.2"
        sysfs.path = "/devices/pci0000:00/0000:00:14.0/usb2/2-4/2-4:1.2/sound/card2"
        udev.id = "usb-VS_Inc_CA_TT-02"
        device.bus = "usb"
        device.vendor.id = "1a1a"
        device.vendor.name = "Technology Inc."
        device.product.id = "2a2a"
        device.product.name = "CA TT"
        device.serial = "VS_Inc_CA_TT"
        device.form_factor = "webcam"
        device.string = "2"
        device.description = "CA TT"
        module-udev-detect.discovered = "1"
        device.icon_name = "camera-web-usb"
    Profiles:
        input:analog-stereo: Analog Stereo Input (sinks: 0, sources: 1, priority: 60, available: yes)
        input:iec958-stereo: Digital Stereo (IEC958) Input (sinks: 0, sources: 1, priority: 55, available: yes)
        off: Off (sinks: 0, sources: 0, priority: 0, available: yes)
    Active Profile: input:analog-stereo
    Ports:
        analog-input: Analog Input (priority: 10000, latency offset: 0 usec)
            Part of profile(s): input:analog-stereo
        iec958-stereo-input: Digital Input (S/PDIF) (priority: 0, latency offset: 0 usec)
            Part of profile(s): input:iec958-stereo

Card #13
    Name: alsa_card.usb-VS_Inc_CA_TT-02.3
    Driver: module-alsa-card.c
    Owner Module: 40
    Properties:
        alsa.card = "3"
        alsa.card_name = "CA TT"
        alsa.long_card_name = "VS Inc CA TT at usb-0000:00:14.0-1"
        alsa.driver_name = "snd_usb_audio"
        device.bus_path = "pci-0000:00:14.0-usb-0:1:1.2"
        sysfs.path = "/devices/pci0000:00/0000:00:14.0/usb2/2-1/2-1:1.2/sound/card3"
        udev.id = "usb-VS_Inc_CA_TT-02"
        device.bus = "usb"
        device.vendor.id = "1a1a"
        device.vendor.name = "Technology Inc."
        device.product.id = "2a2a"
        device.product.name = "CA TT"
        device.serial = "VS_Inc_CA_TT"
        device.form_factor = "webcam"
        device.string = "3"
        device.description = "CA TT"
        module-udev-detect.discovered = "1"
        device.icon_name = "camera-web-usb"
    Profiles:
        input:analog-stereo: Analog Stereo Input (sinks: 0, sources: 1, priority: 60, available: yes)
        input:iec958-stereo: Digital Stereo (IEC958) Input (sinks: 0, sources: 1, priority: 55, available: yes)
        off: Off (sinks: 0, sources: 0, priority: 0, available: yes)
    Active Profile: input:analog-stereo
    Ports:
        analog-input: Analog Input (priority: 10000, latency offset: 0 usec)
            Part of profile(s): input:analog-stereo
        iec958-stereo-input: Digital Input (S/PDIF) (priority: 0, latency offset: 0 usec)
            Part of profile(s): input:iec958-stereo

Card #14
    Name: alsa_card.usb-VS_Inc_CA_TT-02.4
    Driver: module-alsa-card.c
    Owner Module: 41
    Properties:
        alsa.card = "4"
        alsa.card_name = "CA TT"
        alsa.long_card_name = "VS Inc CA TT at usb-0000:00:14.0-2"
        alsa.driver_name = "snd_usb_audio"
        device.bus_path = "pci-0000:00:14.0-usb-0:2:1.2"
        sysfs.path = "/devices/pci0000:00/0000:00:14.0/usb2/2-2/2-2:1.2/sound/card4"
        udev.id = "usb-VS_Inc_CA_TT-02"
        device.bus = "usb"
        device.vendor.id = "1a1a"
        device.vendor.name = "Technology Inc."
        device.product.id = "2a2a"
        device.product.name = "CA TT"
        device.serial = "VS_Inc_CA_TT"
        device.form_factor = "webcam"
        device.string = "4"
        device.description = "CA TT"
        module-udev-detect.discovered = "1"
        device.icon_name = "camera-web-usb"
    Profiles:
        input:analog-stereo: Analog Stereo Input (sinks: 0, sources: 1, priority: 60, available: yes)
        input:iec958-stereo: Digital Stereo (IEC958) Input (sinks: 0, sources: 1, priority: 55, available: yes)
        off: Off (sinks: 0, sources: 0, priority: 0, available: yes)
    Active Profile: input:analog-stereo
    Ports:
        analog-input: Analog Input (priority: 10000, latency offset: 0 usec)
            Part of profile(s): input:analog-stereo
        iec958-stereo-input: Digital Input (S/PDIF) (priority: 0, latency offset: 0 usec)
            Part of profile(s): input:iec958-stereo

Now, I am writing a shell script that reads the card number from a fixed config file and I don't want the card numbers in the arecord --list-devices to change if one of the webcams is unplugged. I want to make sure that the webcam connected to first USB port (/devices/pci0000:00/0000:00:14.0/usb2/2-3/2-3:1.2) is always mapped to card 1, the second one to card 2 and so on. In other words, I want the mapping between Webcams/USB-port to the card number is fixed.

Is this possible?

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1 Answer 1

Reset to default
1

Changing the card number for devices with identical USB IDs is not possible.

But it is possible to use udev rules to change the card ID (TT_2 etc. above), which can also be used with arecord. See "Identify two identical audio devices" on https://alsa.opensrc.org/Udev:

SUBSYSTEM!="sound", GOTO="my_usb_audio_end"
ACTION!="add", GOTO="my_usb_audio_end"

DEVPATH=="/devices/pci0000:00/0000:00:13.2/usb2/2-6/2-6.2/2-6.2:1.0/sound/card?", ATTR{id}="UA1A_A"
DEVPATH=="/devices/pci0000:00/0000:00:13.2/usb2/2-6/2-6.3/2-6.3:1.0/sound/card?", ATTR{id}="UA1A_B"

LABEL="my_usb_audio_end"

$ arecord -D default:UA1A_A something.wav
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  • So here's what I actually did. First found a list of all my pcm devices ls -la /dev/snd/pcm* . Then looked up the udev info using udevadm info -a -p $(udevadm info -q path -n /dev/snd/pcmC1); Then added the id using the corresponding kernel SUBSYSTEM=="sound", KERNELS=="2-3:1.2", ATTR{id}="UA1A_A" Commented Sep 14, 2020 at 20:28

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