Also, what do you think isolation transformers do then?
Video baluns are not the same as audio baluns. Audio baluns ARE isolation transformers. Your typical video balun is not.
Typical audio balun schematic (though they have the impedences reversed - the RCA side is hi-Z while the XLR side is 600 ohm):
Typical video balun schematics:
If what you were saying was true then there would not be any ground loops with one camera and recorder.
There aren't... at least, not due to the use of video baluns (there are plenty of other possible causes). As I already stated, one camera - no problem. As you add cameras, the problem becomes worse. And it ONLY happens (in my experience) on cheap cameras with a shared video and audio ground, and a shared power supply. Dual-voltage cameras, no problem. AC-only cameras, no problem. 12VDC cameras with regulators, no problem. Anything that isolates your power connection's ground from the video connection's ground, eliminates this issue.
See, I came from an audio background, live and studio. I was very familiar with audio "isolation-type" baluns. I've also used isolation transformers in car audio to break ground loops.
So when I discovered this problem on a system using baluns and a buttload of cheap cameras, I couldn't figure out why it was happening, because I figured video baluns were isolation transformers too. But I also noticed, on this particular site, that when I took cameras off the power can and gave them their own 12V wall wart, they cleaned right up. Cameras still on the can - noise; cameras on separate power, no noise. Two cameras on a wall wart gave a little noise. Three got even worse. In short, any time multiple cameras shared a power connection, they got noise. Anything not on that same power was fine.
Later I bought a couple dozen super-cheap "baluns" my buddy came across... they didn't work, so I took one apart to see how they were wired: it essentially matched the first diagram I have under video baluns. I metered it... yup, resistance from one balanced pin to the coax center pin, but infinite resistance measured between pins, and between center pin and shield on the coax side - not what you would expect from an isolation transformer. So I looked up video balun schematics, and there they were...
Look, in a perfect world, you DON'T want DC from the power supply traveling in the coax.
Of course you don't. But when the camera uses the same ground for power and video, that's what happens, whether you want it or not. And that's why CHEAP cameras have this problem.
Its the DC in the coax that gives you the artifacting you see in the screen.
You are literally looking at the current flowing between the two different ground potentials.
Yes you are. Differing potentials caused by the (relatively) massive DC resistance introduced by the balun designs shown above, where a coil is inserted in-line with the signal.