LE 11.0.1 & 4K TV screen flicker, no problem 4K Monitor

Nahannidog · September 7, 2023 at 4:15 AM

Hello,

recently installed LE 11.0.1 on a new RPi4 4GByte. Works great on my 4K monitor, but when I moved it to the 4K TV (LG brand)in the living room there is strange/annoying screen behavior: Screen flickers blank for ~ 1 second at a few random second intervals. Flicker seems worse in the GUI, or settings but also happens when viewing a video.

I saw in the release notes "50/60fps H264 HW decoding may need force_turbo=1 or core_freq_min=500 in config.txt to avoid AV-sync-issues/skipping", doesn't sound exactly like my issue, but tried it anyway. Doesn't seem to make a difference, even viewing H264 video

Following wiki recommendations, I enabled logging then, used SSH with the pastekodi command to get this URL:

http://ix.io/4FzK

I hope this means something to you pros...its about 4-5 minutes of wandering around GUI and settings and playing a short video, with ~ 4 reboots. (Pls ignore the portion with wrong datecode, my WiFi barely reaches inside the passive cooling aluminum enclosure, so timeserver sometimes not reached)

Another very consistent symptom on the 4K TV, on reboot the screen display is consistently this sequence: 1. colorful Raspberry Pi square, 2. LibreElec logo, 3. blank ~ 1 second, 4. LE logo, 5. blank ~ 1 second, 6. LE logo, 7. Kodi GUI (usually flashing). In contrast on the 4K monitor, I get 1. colorful RPi square, 2. LE logo, 3. Kodi GUI, never flashing. and less boot time.

BTW, Settings>System>Display>Resolution set to 1920x1080 for the logging. Behavior seems the same when I previously had 3840x2160.

Hardware difference between 4K TV and 4K monitor is only the display...used the same microHDMI cable, same wireless Keyboard+Mouse, same 3Amp PSU. WiFi distance is closer with Monitor.

Hope this isn't too deep a quagmire. BTW, my RPI2 and LE 9.X.Y works fine with this 4K TV...but kinda slow, thus the upgrade attempt.

Thanks!!

**chewitt** · September 7, 2023 at 10:19 AM

I'd advise against trying to run the Kodi GUI at 4K, Please have a read: https://wiki.libreelec.tv/configuration/4k-hdr

**popcornmix** · September 7, 2023 at 10:58 AM

My guess would be the HDMI cable. 4kp60 requires a very high clock rate and that needs a high quality cable.

Just because it works on another TV doesn't guarantee the cable is perfect (the other TV may just be better and handling a low level noisy signal).

But as chewitt says, follow the recommended instructions and you'll rarely be running at 4kp60 (most video content is 24fps) so are much less likely to hit the issue.

Nahannidog · September 7, 2023 at 10:08 PM

Just bought a new uHDMI cable (JSAUX brand), same length as before. All the screen flicker problems are gone on 4K TV! no flicker on reboot, GUI, video. Thanks!

So I'm a little wiser, some related questions:

1. Settings>System>Display> Resolution explicitly sets the resolution (I've been using 1920x1080). >Refresh rate explicitly sets refresh rate...so what is the meaning of the resolution/refresh whitelist? Whitelist sorta implies there's some negotiating of parameters between OS and Display device...are explicit setting of resolution/refresh just a starting point???

2. (Depending on the answer to #1), are the explicit settings for resolution/refresh enforced during the boot up procedure?

3. With old cable and 4KTV, I had flicker problems until resolution dropped all the way down to 720x576!!!! Any guesses as to why the old cable performed so much more poorly (poorly only with RPi4 & 4KTV, fine with RPi4 & 4K Monitor)? The microHDMI connector seems like a weak link, being so tiny. Or maybe 4KTV spews alot more EMI and old cable was poorly shielded?

**chewitt** · September 8, 2023 at 5:27 AM

I suggest reading the link I already posted.

**popcornmix** · September 8, 2023 at 11:41 AM

Quote from Nahannidog

3. With old cable and 4KTV, I had flicker problems until resolution dropped all the way down to 720x576!!!! Any guesses as to why the old cable performed so much more poorly (poorly only with RPi4 & 4KTV, fine with RPi4 & 4K Monitor)? The microHDMI connector seems like a weak link, being so tiny. Or maybe 4KTV spews alot more EMI and old cable was poorly shielded?

The usual reason is thin wires, which means high resistance, which means significant voltage drop (and that is more significant with the higher resolution, higher clock frequency hdmi modes).

This video may be illuminating showing just how many random hdmi cables are sub-par.