ALSA Virtual Soundcard : Signal Source

Based on the 'minivosc' project here.

This is an extension of the above that can provide a sine-wave source at various sample-rates and in various formats. It is intended as a stepping-stone to a Virtual Soundcard that will be able to receive proprietry network audio formats.

Building

make clean && make

Optionally copy it somewhere sensible, for example only:
sudo cp snd-vss.ko /lib/modules/5.4.0-80-generic/kernel/sound/drivers/

Installing / Removing

To load the module

sudo insmod ./snd-vss.ko

To try and reload the module during development

systemctl --user stop pulseaudio
sudo alsa force-unload

[do whatever with the module]

sudo alsa force-reload
systemctl --user start pulseaudio

To unload the module (tricky - better to develop on a VM!)

sudo rmmod snd_vss (will probably fail because snd-pcm,snd depend on it)

To find out what's using sound so that you can stop/restart those...

lsof | grep pcm
modinfo -F depends snd-vss.ko

Testing

These commands are useful when checking if the module is loaded and recognised, and for checking naming.
Here are some example results:

lsmod | grep snd_vss :

snd_vss               16384  1
snd_pcm               102400  8 snd_hda_intel,snd_oxygen_lib,snd_ice1712,snd_vss

cat /proc/asound/cards :

 0 [STX            ]: AV200 - Xonar STX
                      Asus Virtuoso 100 at 0x4000, irq 16
 1 [M2496          ]: ICE1712 - M Audio Audiophile 24/96
                      M Audio Audiophile 24/96 at 0x5040, irq 19
 2 [HDMI           ]: HDA-Intel - HDA ATI HDMI
                      HDA ATI HDMI at 0x50260000 irq 17
 3 [Source         ]: VIRTUAL - Signal Source
                      Virtual Signal Source (no h/w)

cat /proc/asound/card3/snd_vss :

VIRTUAL vss version 1.0

cat /proc/asound/hwdep :

03-00: VIRTUAL vss version 1.0

cat /proc/asound/card3/pcm0c/sub0/status :

closed

cat /proc/asound/card3/pcm0c/sub0/info :

card: 3
device: 0
subdevice: 0
stream: CAPTURE
id: VIRTUAL
name: Signal Source
subname: subdevice #0
class: 0
subclass: 0
subdevices_count: 1
subdevices_avail: 1

arecord -l :

**** List of CAPTURE Hardware Devices ****
card 0: STX [Xonar STX], device 0: Multichannel [Multichannel]
  Subdevices: 1/1
  Subdevice #0: subdevice #0
card 0: STX [Xonar STX], device 1: Digital [Digital]
  Subdevices: 1/1
  Subdevice #0: subdevice #0
card 1: M2496 [M Audio Audiophile 24/96], device 0: ICE1712 multi [ICE1712 multi]
  Subdevices: 1/1
  Subdevice #0: subdevice #0
card 3: Source [Signal Source], device 0: VIRTUAL [Signal Source]
  Subdevices: 1/1
  Subdevice #0: subdevice #0

Troubleshooting

If you see something like this:

insmod: ERROR: could not insert module ./snd-vss.ko: Unknown symbol in module

First check which symbols are missing by doing dmesg | tail. You might discover something like [ 2529.380716] snd_vss: Unknown symbol snd_pcm_new (err -2). You can then check which symbols the kernel does know about using cat /proc/kallsyms | grep snd_.

If the kernel does not include (EXPORT) the snd_xxx_xxx symbols, try loading the required modules first, for example:

sudo modprobe snd_pcm
sudo modprobe snd_hwdep

Loaded modules can be checked with lsmod | grep snd.

If that doesn't work, try installing the ALSA components themselves and trying again:

sudo apt update
sudo apt install alsa-base
sudo apt install alsa-tools