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This is the best I can do to describe what happened on my monitor (HP EliteDispaly E242, to be exact). The OS is Windows 10. When it occurs, wavy lines appear in a 2-inch wide vertical section (from top to bottom) on the monitor. The wavy lines are just like those wavy lines I see above a hot grill or an asphalt road in the middle of the summer. That section slowly moves across the screen, from left to right, and then it disappeared. It does not cause any trouble. I am just curious about this. Anyone had the same experience? Or anyone can help me explain this?

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    Quick test - take a photo, take a screenshot. if the photo has lines & the screenshot doesn't, it's a hardware failure. – Tetsujin Feb 14 at 18:32
  • Make sure to use HDMI or DisplayPort and not VGA connector of your monitor. – Robert Feb 14 at 18:40
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In the days of CRT displays, it would certainly be due to propinquity to a transformer, which produces a 50 or 60 Hz AC magnetic field. Since line current (mains) frequency is just a fraction of a Hz different from monitor sweep frequency, it would cause a slow wiggle.

That said, a similar effect could appear in an LCD display if any cable picked up electrical noise synchronized with the power line. For example, light dimmers may create high-frequency harmonics of the line frequency to many MHz, called radio frequency interference or RFI.

So, try this:

  • First, change the display refresh rate. If your AC mains is 60 HZ, for example, try switching between 59 and 60 Hz refresh. The jagged band should at least stop moving, if not disappear entirely.
  • Move any other electrical device away from the PC and display.
  • If there's a light dimmer or speed control in the room, turn it off or to maximum.
  • If a device outside your control is generating RFI, e.g. in an industrial environment, you might consider putting a power line RFI filter or power strip in the PC and display lines, and/or ferrite beads on all cables to the display.

One final possibility: a filter capacitor in the display or PC is dying, allowing line-frequency interference through. If that's the case, repair is needed.

  • Thanks. I will try the display refresh rate change next time. Beyond that, I cannot do much about my work environment. Just FYI, this is a flat panel and I am using Display Port. – Seaport Feb 19 at 21:43

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