Document video chat/streaming/recording options with the Pi 4

[geerlingguy]

I have a couple Logitech C920 webcams, and I wanted to see if I could use them while streaming and/or recording clips at 1080p... the problem is "it's complicated", and unlike macOS where there are literally a dozen or so free options with fancy/simple GUIs for the purpose, on Linux/RPi, it's a lot more complex.

The most reliable way to capture video is with ffmpeg, but its usage is inscrutable for beginners. And maybe not impossible for me, but it still requires a lot of futzing around.

After reading this article about streaming 1080p content to YouTube directly (without killing your Pi's poor CPU), it was apparent I was in luck that my Logitech C920 has a built-in H.246 encoder that puts out a pre-compressed stream.

Using this incantation gets the video and dumps it into an mp4 file, with minimal CPU usage:

ffmpeg -f v4l2 -codec:v h264 -framerate 30 -video_size 1920x1080 -itsoffset 0.5 -i /dev/video0 -copyinkf -codec:v copy -codec:a aac -ab 128k -g 10 -f mp4 test.mp4

Using a simpler/more standard example from ffmpeg's documentation, however, results in the Pi's CPU struggling to maintain 5 fps:

ffmpeg -f v4l2 -framerate 30 -video_size 1920x1080 -i /dev/video0 output.mkv

You might wonder, 'how do you know the webcam is at /dev/video0? Well, find it with:

$ v4l2-ctl --list-devices
bcm2835-codec-decode (platform:bcm2835-codec):
    /dev/video10
    /dev/video11
    /dev/video12

bcm2835-isp (platform:bcm2835-isp):
    /dev/video13
    /dev/video14
    /dev/video15
    /dev/video16

HD Pro Webcam C920 (usb-0000:01:00.0-1.2.3):
    /dev/video0
    /dev/video1

Now, onto finding a way to get my audio interface into the stream...

[geerlingguy]

For audio, I can get my audio device using:

$ aplay -l
**** List of PLAYBACK Hardware Devices ****
card 0: b1 [bcm2835 HDMI 1], device 0: bcm2835 HDMI 1 [bcm2835 HDMI 1]
  Subdevices: 4/4
  Subdevice #0: subdevice #0
  Subdevice #1: subdevice #1
  Subdevice #2: subdevice #2
  Subdevice #3: subdevice #3
card 1: Headphones [bcm2835 Headphones], device 0: bcm2835 Headphones [bcm2835 Headphones]
  Subdevices: 4/4
  Subdevice #0: subdevice #0
  Subdevice #1: subdevice #1
  Subdevice #2: subdevice #2
  Subdevice #3: subdevice #3
card 2: U192k [UMC202HD 192k], device 0: USB Audio [USB Audio]
  Subdevices: 1/1
  Subdevice #0: subdevice #0

$ arecord --list-devices
**** List of CAPTURE Hardware Devices ****
card 2: U192k [UMC202HD 192k], device 0: USB Audio [USB Audio]
  Subdevices: 1/1
  Subdevice #0: subdevice #0
card 3: C920 [HD Pro Webcam C920], device 0: USB Audio [USB Audio]
  Subdevices: 1/1
  Subdevice #0: subdevice #0

That seems to correspond to -i hw:2,0, but I could be wrong...

[geerlingguy]

This thread helped me figure out the audio issue; it has to do with alsa not being able to load the device correctly, it seems. I had to create an .asoundrc file with my preferences:

$ cat ~/.asoundrc
pcm.!default {
        type hw
        card 2
}

ctl.!default {
        type hw
        card 2
}

See docs: https://www.alsa-project.org/main/index.php/Asoundrc

But I was still getting the following error:

[alsa @ 0x55b31e38c0] cannot set sample format 0x10000 2 (Invalid argument)
hw:2,0: Input/output error

[geerlingguy]

So I can get a successful A/V stream all from the camera using:

ffmpeg -ar 44100 -ac 2 -f alsa -i hw:3,0 -f v4l2 -codec:v h264 -framerate 30 -video_size 1920x1080 -itsoffset 0.5 -i /dev/video0 -copyinkf -codec:v copy -codec:a aac -ab 128k -g 10 -f mp4 test.mp4

That grabs both audio and video from the attached Logitech C920... and works well, actually. Though I was getting random clicks in the audio every now and then. Note that I was NOT touching anything, and my desk was perfectly still. So maybe something in the chain that ffmpeg sets up for the recording stream :-/

[geerlingguy]

After searching, searching, and searching some more, I finally found this email which had the correct suggestion:

ffmpeg -ar 44100 -ac 2 -f alsa -acodec pcm_s32le -i hw:2,0 -f v4l2 -codec:v h264 -framerate 30 -video_size 1920x1080 -itsoffset 0.5 -i /dev/video0 -copyinkf -codec:v copy -codec:a aac -ab 128k -g 10 -f mp4 test-webcam-audio.mp4

I mean... I worked in radio engineering, and have done live audio at a zillion events, and know my way around most any GUI-based tools. It's no wonder people give up on Linux audio :-(

[geerlingguy]

So... VLC lets you 'open a capture device', but the interface for that, surprising as it may seem, is even more inscrutable than the CLI options for FFmpeg! I couldn't quickly figure out a way to record a 1080p stream using the external USB audio interface, and I've already spent way more time on this part of the experiment than I originally though I'd do.

So... putting a pin in this.

[geerlingguy]

Screenshots of the behavior of BlueJeans and Zoom not seeing any audio source even when two were identified and usable via FFmpeg...

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