Back in the old Cathode Ray Tube (CRT) days, monitor images were scanned to a screen one horizontal line of “pixels” at a time. An electron gun would fire a spot onto the screen, and the location of the spot scanned across horizontally and then it would move down vertically and do another line. The old analog electronic circuitry that accomplished this had to have the horizontal and vertical scan rates synchronize with the signal frequency. The screen would go “wonky” when the vertical and/or horizontal sync circuit went out of sync, which could easily happen with electrical or magnetic disturbance. As far as I know, CRT screens aren’t being made anymore, and most of the monitor circuitry is digital and shielded, so this effect doesn’t really happen in modern equipment, but it still looks pretty cool.
You could simulate it with a setup like this where you animate a few parameters. A much better simulation could bring in snowy static (signal loss), a blanking rate, etc., but this should get you started.
And here’s the animation: http://youtu.be/91gCFLIfITE