Something I learned not long ago on this site: low quality/too long HDMI cables can make some monitors go black when the user stands or sits on a certain kind of office chair. [1] [2]
Apparently it's a known issue at Dell: [3]
>Surprisingly, we have also seen this issue connected to gas lift office chairs. When people stand or sit on gas lift chairs, they can generate an EMI spike which is picked up on the video cables, causing a loss of sync. If you have users complaining about displays randomly flickering it could actually be connected to people sitting on gas lift chairs. Again swapping video cables, especially for ones with magnetic ferrite ring on the cable, can eliminate this problem. There is even a white paper about this issue.
This is an issue at our office. We have some cheaper LG monitors that flicker when someone stands up from his gas spring chair. The Dell monitors we have don’t suffer this issue.
A paper on the issue: https://www.emcesd.com/pdf/uesd99-w.pdf
Apparently a dangling bag of coins can cause similar issues, but haven’t encountered that so far.
> Apparently a dangling bag of coins can cause similar issues
A dangling bag of coins is a famous cheap EMC test for products. All of this sounds like monitor vendors cheaped out on their EMC and need to have their ESD and Immunity certifications reevaluated....
Or maybe it's just the fact that they were dumb enough to ship an I²C interface over a cable as part of the industry spec. Never ship I²C off-board or this kind of crap happens.
My own favorite trick was to put a 1st gen cellphone near the device and call it, or to fire a piezo based gas lighter with the shroud removed against device ground. This was for devices that stood a very good chance of being deployed in a hostile environment so being able to withstand that kind of interference wasn't a luxury.
Why are the monitor vendors at fault when the cables have the poor shielding?
We don't know who produced these cables nor where they came from. I'd wager it's mostly an issue with cheaper products, and you often don't get any cables if you're buying a cheap monitor, so they're likely from a third party.
> Why are the monitor vendors at fault when the cables have the poor shielding?
Because part of hardware engineering is making devices that function even in poor environments, and that tolerate foreseeable misuse. (This is a major part of medical device development, for example.) Cable shielding is always iffy on consumer-grade cables. If other vendors can produce monitors that tolerate its absence, because that seems to be reasonably common!, then if you can't, your products will be seen as inferior. Exactly as is happening here.
Cable shielding often is subpar on industry equipment too. Yes, they have bought shielded cables, no they have not connected the shielding to the expected locations on patch up. Or just wrapped it loosely around, so everytime somebody patches up a new RJ45, you get a new quality situation, cause the vibrations shake the roulett box. Finally, ground.. ground is the yellow green cable and it goes to ground - and ground should be something professional done. Not a network of suffering machinery, pumping hovering potential into one another.
Also dont trust all electricians. Lots of them just try and error, nod in terror. Some do not even speak the language of the e-plan they are cabling up.
A bit of a problem is that HDMI is very sensitive to EMC thanks to DVI (which forms its base) being essentially VGA minus the DAC step - there's even porches and overscan included in the signal. But this meant that you could make very cheap TV displays, which thanks to very limited set of frequencies involved in cables could be made to work "stable enough".
So imagine all the problems you could have, and multiply it by variable frequency cable that thanks to increases in resolution has to run way higher clocks than it used to, combined with being cheapest possible design, and that unlike it's analog origins an error in bitstream can now crash the connection instead of just causing temporarily off-colour area.
Yeah, the simplest possible circuitry for driving a flat panel TV from HDMI (except the HDCP block, which AFAIK can be placed inline before the display logic) would be to declare the 720i/p capability and then just... pump the data straight onto display interface with some buffers here and there meaning to drop the porches, driven all by HSync/VSync signal recovered from the HDMI clock. :V
That solves a lifelong mystery for me. I have twice had monitors that blink off right when I shift position or get up from my desk. I can tell it's caused by me moving, but I can never reproduce it. When it first started happening, I would aggressively shake the monitor cables and nothing happened. But I never considered that it could be my chair.
I put ferrite rings on a ton of stuff to reduce interference, so much I questioned how reasonable it was to do so. Thank you for providing additional confirmation bias for the things I do.
On the subject of interference, this is why a good amount of shop equipment doesn't use USB. Especially for vinyl cutters, serial/RS-232 tend to have less issues than USB. USB tends to be more sensitive to the static buildup caused by moving and cutting vinyl.
Similar but different: I have a 200m Ethernet run (PoE repeater midway), outdoors, FTP, going to a mast atop a hill.
There’s lightning protection etc. on the circuit.
It’s worked fine for several years.
Last week, we had a huge electrical storm - and every time lightning flashed within 8km, the switch at the cabin would reboot. Unplugged, stuck a scope on the Ethernet cable - and lo and behold, 30V+ flashes every time lightning struck anywhere even remotely nearby. The cable is acting like a great big antenna.
Have you looked at shielded twisted pair or optical fiber for that run? I have a similar project planned but at the moment I’m not looking forward to trenching the cable into the ground so it hasn’t moved forward yet.
I didn’t trench mine as I anticipated it might not be the ideal solution… plus it goes up a 45 degree hill, threads past cliffs, etc. - ran it inside a little 10mm slitted duct, at least, to protect it from friction, UV, and curious teeth.
I need it to be able to deliver PoE, which is why I went for Ethernet - but yeah, probably going to replace it with STP, although not sure that’s going to stop lightning induction in the earth/shielding. Other option is splashing out for SWA STP, which should definitely do the job.
For the other run, where I don’t need power, I’m using 802.11n with a pair of Yagis to go about a kilometre. No problems except the usual headaches around layers and discovery a Wi-Fi bridge provides.
I had a motherboard (an ASUS ROG Strix B550-E) that would frequently freeze when I stood up from my chair. I always figured it was something to do with static electricity discharging when I stood up, but no amount of fiddling with grounding helped. An EMI pulse from the gas lift getting picked up somewhere insufficiently shielded would explain it.
Interestingly, I also have a Steelcase Leap chair, as mentioned by someone else in this thread; maybe the gas lift they use is particularly prone to this? Other people also complained about freezes with this motherboard model, so perhaps the combination was particularly bad.
Holy crap, this is amazing. I've wondered about this a lot when it kept happening, and so far my most likely seeming theory had been 'static discharge through the carpet', or something like that... But the chair generating an EMI pulse? Hilarious
Apparently it's a known issue at Dell: [3]
>Surprisingly, we have also seen this issue connected to gas lift office chairs. When people stand or sit on gas lift chairs, they can generate an EMI spike which is picked up on the video cables, causing a loss of sync. If you have users complaining about displays randomly flickering it could actually be connected to people sitting on gas lift chairs. Again swapping video cables, especially for ones with magnetic ferrite ring on the cable, can eliminate this problem. There is even a white paper about this issue.
[1] https://www.youtube.com/watch?v=voW5kEI7JKE
[2] https://www.youtube.com/watch?v=D6pY4t0k1hk
[3] https://support.displaylink.com/knowledgebase/articles/73861...