Green or red pixels on TV screen over long HDMI cable

Arcticold · June 16, 2021

TV: LG OLED65CX3LA 4K OLED (over multiple software updates)

OS: Windows 10 (over multiple versions)

HDMI cable length: 15m (4K capable, but also tried with a shorter 10m cable).

Issue: I'm getting green or red pixels on my TV when connecting it to my PC over HDMI.

I've tried two different PCs with GPUs: GTX 1080 Ti and GTX 1660 Super, and two different HDMI cables - the issue remains. This occurs usually only in the beginning when I open TV and/or extend (same issue when used as the only display) my desktop to the TV

It's rarely as bad as it's on the picture below - usually affected area is half of what can be seen below. Green pixels occur more often than red. Usually it happens on pixels that are suppose to be fully black (my guess is that they should be turned off), but not always. 15m cable has been tested with another screen at 1080p and no such issues have been detected.

My guess is signal deterioration, because it only happens with 4K resolution and when TV is receiving input from other sources (such as PS4 Pro) no green/red pixels occur (PS4 sits at a much shorter cable). However, this doesn't explain why the problem goes away eventually if I switch between "PC screen only" and "Extend" regimes often enough on Windows 10. Around half the times it works from the start perfectly fine. Sometimes the TV says that no signal can be detected, despite desktop being extended over the cable.

Do I need signal booster? Or would optical HDMI make more sense here? Or is there something else going on?

I can say that I had an HDMI switch (USB powered) between 15m cable and my PC for a while and that seemingly only increased the occurrence of the issue.

Arcticold · September 27, 2021

If anyone ever finds this and is facing the exact same issue then I have an update. I went for it and invested into an optical HDMI cable and since installing it there have been no issues whatsoever. I guess the lesson here is that even if the cable is certified for 4K by the manufacturer, it might not be able to do so if it's just too long.