To reduce video noise, it is often recommended to add a common ground between VTX and camera to avoid ground loops. What is a ground loop, how does it add video noise and how can a common ground negate its effects. Thanks.
If enough current flows and two ground pads are far enough apart, there might be a voltage difference between them because of the resistance in the PCB:s traces. That's called a ground loop.
As for why that causes noise; if the camera and the VTX have different reference points, a signal sent from the camera won't be received correctly by the VTX, and since a PAL or NTSC (The types of video that is used for analog FPV systems) signals use less than one volt, even a small in potential can cause video noise that will appear as horizontal bars moving vertically.
Here is an some more information if you would like to read more about it
$\begingroup$ Excellent, thank you for that explanation $\endgroup$ Apr 14, 2020 at 21:06
$\begingroup$ No problem. I'd be happy if you marked this as the answer if you think it is! $\endgroup$ Apr 14, 2020 at 21:12
$\begingroup$ I’m going to leave this question open for a little longer to invite more answers, however I feel that you have explained it very well so will be happy to mark this as the answer shortly. Thanks! $\endgroup$ Apr 14, 2020 at 21:15