Linux: virtual audio support via ALSA Loopback card

[aramg]

NOTE: This PR includes the changes in #8008, which is expected to get merged first.

Description

Adds virtual audio support via the ALSA loopback module, snd_aloop. As implemented, audio is optional and the virtualcam depends on v4l2loopback still. snd_aloop is not auto-loaded and an audio-only mode is not available.

Motivation and Context

Allows sharing audio output with other programs as a virtual microphone.

How Has This Been Tested?

1. Load the loopback modules: modprobe v4l2loopback modprobe snd-aloop
2. Start OBS and click [Start Virtual Cam]. Both audio and video are activated:

info: v4l2-output: Using device 'Dummy video device (0x0000)' at '/dev/video0'
info: ALSA output: hw:Loopback,1,0
info: Virtual camera starting: audio=1 video=1

3. Add a media source, and observe video output as before (in this case, via '/dev/video0' **).
4. Record audio: arecord -D hw:Loopback,1,0 --duration=30 -fS16_LE -c2 -r48000 /tmp/test.pcm
5. Replay audio: aplay -fS16_LE -c2 -r48000 /tmp/test.pcm
6. This varies by distro, but to load the device into PluseAudio:
   pacmd load-module module-alsa-source source_properties=device.description=OBS device=hw:Loopback,1,0

** ideally the audio and video device names will be added to the OBS status bar, a to-do.

Types of changes

• New feature (non-breaking change which adds functionality)

Checklist:

• My code has been run through clang-format.
• I have read the contributing document.
• My code is not on the master branch.
• The code has been tested.
• All commit messages are properly formatted and commits squashed where appropriate.
• I have included updates to all appropriate documentation.

[david672orford]

I have been testing this PR. Here are some initial observations:

• This PR includes linux-v4l2: Create & use an OBS specific loopback card #8008 which is not really related. Worse, it changes behavior and is broken. When the virtual camera is started, it attempts to create a new V4L2 loopback interface called "OBS Virtual Camera". This produces nuisance pkexec authorization popup for a command which ultimately fails do to the use of a non-existent "add" subcommand.
• While it makes a kind of sense that since the video output goes to an in-kernel loopback devices, the audio should too, I am not sure this is the best decision. The only monitoring output for Linux I see in the code goes to Pulseaudio. Shouldn't this use Pulseaudio as well?
• When I play a video file I hear frequent clicking. I do not hear these clicks on the monitor output for the same videos.

[aramg]

Thanks for the feedback.

I think you are correct, the fact that video goes through a kernel loopback device does not mean audio should, too.
There have been some alternative options posted, which seem easier [1][2] (have not tested myself).

I am open to closing this PR, or refactoring if this approach is chosen after all.

https://obsproject.com/forum/threads/quick-and-easy-virtual-microphone-for-linux.158340/
https://www.reddit.com/r/obs/comments/n1e9j6/comment/gwco2ec/

[david672orford]

I am really interested in getting the main audio output working. You can get by with the monitor output, but it bypasses the built-in mixer in OBS and there are no fades for scene transitions.

Audio for the virtual camera was requested nearly three years ago:

https://ideas.obsproject.com/posts/1415/obs-virtual-camera-audio

I don't really understand the response from Lain about why a satisfactory implementation is not currently possible. When I asked for details of what needed to be done, I was told that "someone willing, and available, to spend time to do it". I found that completely uninformative.

Your patch is the closest thing we have to an implementation. I wish we could find a way to move it forward. I can program in C, but if nobody is willing to discuss whatever is blocking a properly-working implementation, I'm stuck. Maybe you have some insights.

[david672orford]

Wanted to follow up on the clicking. After more tests I find that when I read audio directly from the Alsa loopback device there is no clicking. I get the clicking when I read the audio through PipeWire.

[david672orford]

Just posted #10495 which implements audio output through PulseAudio. Look a Lua script in the PR decription which starts and stops a virtual audio cable when the virtual camera starts and stops.