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I have a Logitech C720 webcam which I use for video chats, in particular with Zoom. Also, I'm sitting in a room with a small amount of sunlight, and light from a 48W, 3000K light, led-based, ceiling-mounted luminaire.

Now, when I turn my camera on and "test video" in Zoom, I see a sort of a vertical wave pattern on the video image, i.e. 

luminosity of pixel at position x,y at time t = 
average luminosity of pixel at position x,y  *
(1 + a_1 * sin(a_2 * y + a_3 * t) ).

If I turn the inside LED-based light off, the phenomenon goes away.

My question:

  1. (Less on-topic) Why is this happening? Does this have something to do with the difference between the camera's sample frequency and my house's electricity frequency (50 Hz)?
  2. What, other than turning the light off, can I do to cancel or weaken this pattern - if anything?

Note: I'm using Devuan GNU/Linux 3 (~= Debian 10 Buster without systemd).

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    I don't know that particular camera or it's control panel, but you're right it's a strobe effect of the video frame-rate being a close multiple of your mains frequency. Best guess on 50Hz is that you'e shooting PAL, 25fps. See if you can switch to NTSC 30 [or 29 & a bit] fps, which ought to eliminate it.
    – Tetsujin
    Commented Apr 22, 2020 at 15:09
  • @Tetsujin: Ok, so, how do I switch to NTSC? Zoom doesn't let me make that choice.
    – einpoklum
    Commented Apr 22, 2020 at 15:55
  • Probably from your actual camera control panel, if it isn't in the current app you're trying to change. I'm afraid I have neither Zoom, that camera, nor even a Windows machine to test on - this is pure theory ;)
    – Tetsujin
    Commented Apr 22, 2020 at 16:57
  • @Tetsujin: I don't have a camera control panel. And - I'm on Linux, not Windows. The Cinnamon "System Settings" dialog doesn't have an entry for cameras.
    – einpoklum
    Commented Apr 22, 2020 at 17:41
  • Then I'm even further out of my depth. I have a vague working knowledge of Win, but almost nada on nix, sorry.
    – Tetsujin
    Commented Apr 22, 2020 at 18:23

1 Answer 1

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(Thanks goes to Tetsujin for suggesting the appropriate course of action.)

The problem is, as you suspect, due to the electric power line frequency to your home: 50 Hz. In fact, the relevant OS driver - Linux USB Video Class - has specific functionality to account for alternate-current power effects on ambient lighting, which can be set to (at least) 50 Hz, 60 Hz or Disable. You likely have one of the latter values set, and need to change that setting.

As to how to change that setting - since Zoom doesn't let you do that, you need to find an app that will, or some hardware configuration interface of your desktop environment, or a utility for your specific webcam. For your specific case, see this answer on Unix.SX.

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