Screen flicker started during Monterey upgrade

Question

Screen flicker started during Monterey upgrade

My spouse just updated her 6 month old Mac Mini to Monterey. During the update, and most of the time after, the screen flickers. The only monitor appears to be rapidly alternating between two different screens: what it should be seeing, and, the screen as it appeared a while ago while she was on Facebook reading about Star Wars. The Facebook/Star Wars image never changes. It's possible to use, because it is often showing what it should, but it certainly isn't pleasant.

I looked around online, and have tried booting, resetting PRAM or NVRAM with command-p-q, unplugging the monitor cable, power cycling the monitor, reducing the refresh rate, turning off transparency in Accessibility, and whatever else I found that doesn't require getting new equipment.

Other ideas I've found: replace the monitor cable with a different brand, use a different port on the Mini, add a port adapter and use different ports at one end or the other with a different cable, and replace the whole monitor with a different brand.

These all seem to be just guesses at all things that might change how the display works.

What is going wrong?

How do I settle this???

I have to start pulling things out of her crowded and complex furniture setup to find more details about the cable, monitor, and port being used. Don't even know the monitor brand at the moment. I wish I had some idea what was malfunctioning.

Thanks!

Mac mini 2018 or later

Posted on Nov 22, 2021 10:57 AM

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Answer 1

Top-ranking reply

TrafGib

Level 2

409 points

Nov 22, 2021 12:07 PM in response to christianfrommd

You might well win the award for strangest problem this week! What strikes me most, among all your reported details, is that the odd Star Wars screen content remains consistent, despite all of your other actions. Given that you have taken all of those steps on the Mac mini, including reboot, the idea of the Star Wars image remaining intact is too strange.

With that said, what comes to mind on how that might happen is if the monitor has more than one input connected. For example, the single monitor on my desk is connected to two different computers. Using the monitor's menu controls, I can toggle back and forth between the two different inputs. Is there any chance that you have two inputs to the monitor? If not two different computers connected, maybe two different cable types, connecting the monitor to the same computer? For example, and HDMI cable and a DisplayPort cable at the same time?

Why the Star Wars Facebook image is being retained, even through a reboot, is VERY odd unless it is originating from a different source than the Mac mini, which wasn't rebooted. I have never heard of a monitor, with a buffer of some type, retaining an image, but you never know. Have you tried unplugging the monitor from the wall, for an extended period, such that it has a chance to forget that second stuck image?

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Answer 2

Nov 22, 2021 1:49 PM in response to TrafGib

Thank you TrafGib! I think that may have solved the problem. At least, for now, it looks much better!

More than one input? This monitor and this Mac Mini are the only monitor and the only computer. No shenanigans.

I removed power for 5 minutes. Now the Star Wars image is gone.

There's still a faint flicker visible when viewing some things, such as the sunset picture she has as background, but not on others, such as a large window containing text on a big white background. Changing the refresh rate from 60 Hz to 50 Hz makes this a little more noticeable, so we're back on 60. The thing is, this flicker is faint enough she's not sure she wouldn't have seen it months ago if she looked as carefully as we are looking now. Maybe all we are seeing is the innate flicker it has when functioning as intended.

By the way, she has an LG model 24MP88 monitor and I found the manual. It uses a joystick setup interface that I bet neither of us can operate, and has advice like "It is a mode that the screen is adjusted to the best for the newspaper." "It is a mode that the screen is adjusted to the best for the cartoon." "Optimizes the screen to improve the visual effects of a video."

There's a setting called Response Time with this instruction: "Sets a response time for displayed pictures based on the speed of the screen. For a normal environment, it is recommended that you use Normal. For a fast-moving picture, it is recommended that you use High." This sounds backwards, but maybe the setting is Response Speed, or maybe a fast moving picture really wants Low.

Ominously, they observe "Setting to High may cause image sticking."

There was troubleshooting advice for flickering: "If the selected resolution is HDMI 1080i 60/50 Hz, the screen may be flickering. Change the resolution to the recommended resolution 1080P." It sounds like 1080 is the default, and we certainly never changed any of the settings with the joystick. I hunted around for this joystick and couldn't even find it. I guess there are leads to pursue if this faint flicker becomes more bothersome or the alternate image returns.

I guess this solved my problem, or answered my question, so I am going to click the "This solved my question" button.

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Answer 3

TrafGib

Level 2

409 points

Nov 22, 2021 3:59 PM in response to christianfrommd

The “i” in 1080i means interlaced. That dates back to the CRT (cathode ray tube) days. That will tend to flicker as the monitor draws every even numbered scan line, returns to the top of the screen, then draws every odd number scan line. Lots of activity for a single frame. Old phosphor tubes would glow a while, retaining the image, while the balance of the lines were drawn. Can’t say why that would still even be an option for settings.

Regardless, you don’t want 1080i set. Regarding refresh rate settings, that matters most for things like video, movies, fast paced games, etc. when the picture on the screen is changing rapidly. An often used example is a football game, with the players running down the field. If your monitor (or TV screen) has a low refresh rate, you may see streaking as the picture on the screen doesn’t update fast enough to keep up with the action.

On the opposite end of the spectrum, you can have a high refresh rate turned on, while looking at a still photo, and notice a subtle (or maybe not so subtle) flickering. Basically, no action moving on the screen yet the monitor keeps redrawing the screen at a high refresh rate and your eye picks up on it.

Solutions such as monitors and GPUs with variable refresh rates, that are adaptive as necessary, help minimize the issue. As you say, your current hardware may have been flickering all along, but now you are hyper-focused on it.

Proper settings, and a quality video cable, can certainly help. For example, many people are not aware that HDMI cables have speed ratings. Trying to push a high refresh rate through an old low speed HDMI cable does not produce good results.

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Answer 4

Nov 24, 2021 8:42 AM in response to TrafGib

>"The “i” in 1080i means interlaced. [...] Regardless, you don’t want 1080i set."

The "i" means interlaced, huh? Yes, I know about interlacing. And the front porch and the back porch, the data encoded during the flyback, how they added color to the original "B&W" signal, and on and on.

There was some amazing engineering optimization already done by the time I was watching TV in the early 60's. Did you know they derived the CRT acceleration voltage from a transformer driven by the horizontal scan signal, which was high frequency and had very high di/dT on the retrace or flyback, which allowed a smaller transformer? That's why the high voltage transformer is called a "flyback transformer". Not only that, but since it has such a high frequency, the flyback transformer makes an ideal source of DC to bias the tubes in the audio amplifier, because you can get away with a smaller filter capacitor. So much optimization. Tube television sets were anything but primitive!

But I was already thinking about interlacing as a possible culprit in the problem we observed. One way to implement interlacing would be to have two screen buffers that were offset by one interlaced pixel (or one half of one noninterlaced pixel) in the vertical direction, and display them alternately. Perhaps they'd be rewriting one while the other was being displayed. Aaaanyhoo, what if for some reason this scheme halted at some point, so the Facebook StarWars screen was left in one of the two buffers, and the system was only updating the other one -- and yet, it continued to alternately display one then the other? THAT would be how the old image got "stored" somehow.

Could such an alternate screen buffer interlace scheme have been used before but stopped by Monterey?

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Answer 5

Nov 24, 2021 8:55 AM in response to christianfrommd

More on this: "One way to implement interlacing would be to have two screen buffers that were offset by one interlaced pixel (or one half of one noninterlaced pixel) in the vertical direction, and display them alternately."

Hunting around online, these buffers seem to be a thing in some designs, and are called "frame buffers" as they store a video frame. I haven't found out whether this happens on Minis and monitors sold this year, or indeed on computers at all (maybe they're just a television set feature). Still, it's an interesting lead.

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Answer 6

TrafGib

Level 2

409 points

Nov 24, 2021 9:27 AM in response to christianfrommd

"the Facebook StarWars screen was left in one of the two buffers," - Very interesting theory.

"Could such an alternate screen buffer interlace scheme have been used before but stopped by Monterey?" - Certainly not my area of expertise, but I would think that the whole interlace scheme would be more a function of the monitor, and GPU, than the OS itself. Since we are so far beyond the days of CRT monitors, I am not sure why you were even seeing the 1080i option.

Flyback transformers, and their associated capacitor, always hold a special place in my heart. Back in the mid 70s, my college electronics courses included labs on TV repair. Despite our professor's warnings, one of our particularly not-too-bright students failed to discharge the capacitor before diving into his repair. Rather humorous when he was blown about 6 feet off his stool, and onto the floor!

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Answer 7

Nov 24, 2021 10:29 AM in response to TrafGib

"Rather humorous when he was blown about 6 feet off his stool, and onto the floor!"

Hmm... maybe that's the real reason they were called "flyback" transformers....

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Screen flicker started during Monterey upgrade

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