Dual monitors, one that blinks

TidBITS Talk
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I have a Dell U2713HM monitor running on a MacBook Pro, M1, 15.7.1. I’ve recently reinstalled the system following a logic board replacement by Apple. A problem I had until a year or two ago on the same Mac has returned: The monitor blinks at random.

When I had the problem before, I managed to cure it by adjusting the resolutions and refresh rate in the Mac’s Settings>Displays panel.

Now the problem is back. I’ve tried repeating the old solution, adjusting the resolution and refresh rate in just about every combination or the two. I’ve also tried two different connections (straight into the Mac via an Apple adapter and through a hub, CalDigit TS3 Plus, with proper cable adapters.

Nothing seems to work. It still blinks. Today I sent an inquiry to CalDigit and one to Dell.

Any ideas?

Displays:

DELL U2719DX:
Resolution: 2560 x 1440 (QHD/WQHD - Wide Quad High Definition)
UI Looks like: 2560 x 1440 @ 60.00Hz
Main Display: Yes
Mirror: Off
Online: Yes
Rotation: Supported

DELL U2713HM:
Resolution: 1600 x 900
UI Looks like: 1600 x 900 @ 60.00Hz
Mirror: Off
Online: Yes
Rotation: Supported

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I have had a similar problem for a long tiime, with a 3-monitor setup. I have not been able to figure out a cure. I have read that if only 1 monitor is connected by HDMI, it should be the monitor furthest to the left. (Something to do with the timing of which monitor powers on first when booting up the Mac.) Haven’t yet gotten around to switching my cables to test that theory. (A bit of a reach - not the simplest thing to do.)

But when mine blinks, it does so for about 2 seconds - more than a blink. Is that what yours does? Is the monitor that blinks the one with the HDMI connector?

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That kind of blinking is usually caused by the monitor losing timing/sync with the video signal. Assuming the hardware isn’t broken, I’d check your cables.

According to Wikipedia your resolution (2560x1440) at 60 Hz requires HDMI’s 340 MHz TMDS data rate. This means a “high speed” HDMI cable. Most cables sold today should support that data rate, but you might have an old cable or one that may be just a bit too long to reliably carry that data rate.

The other display (1600x900) is lower resolution than 1080p (1920x1080), and should work with any HDMI cable. But again, some cables are not designed well and may be too long to carry the kind of signal they’re expected to carry.

If you have other cables, preferably ones that are certified as high speed or better, give them a try and see if it helps.

If adjusting refresh rates and resolutions used to work (do you remember what worked?), then it may be that the default parameters auto-detected by macOS are too high for your cables, and lower resolutions/refresh rates are compatible.

You might also want to see if it may be an issue with EMI. Most HDMI cables are not shielded. If another nearby signal is interfering, that might do it. I know that at home, I sometimes lose the picture on my TV in the winter if I get a shock near the TV.

I’m not sure what around your computer might interfere, but you might want to see if moving the HDMI cables away from other cables is possible, and if it helps. I remember, from years ago, that SuperSpeed USB cables would interfere with Bluetooth connectivity, so the concept is not a novel one, but I don’t know what might interfere with an HDMI cable.

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@Shamino, Thank you for that substantive answer.

First, please clarify this:
”…default parameters auto-detected by macOS…”
How do those parameters relate to the parameters I’ve set manually?

I should clarify three things.

  1. This occurs only on the second display, the 1600x900.
  2. No, I do not remember what rates and resolutions used to work, but I’m virtually certain that I’ve tried every possible combination – though it might be worth another try along with listing the combinations tried.
  3. The cable feeding that monitor is the same as in the previous setup. In fact, it’s the one that came with the monitor – 11 years ago. On the other hand, who knows, this might be worth a try.

The EMI interference idea sounds like it’s definitely worth a look.

Thank you!

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@m3cutl3r

Thank you for your reply. I said “blinking,” but it doesn’t quite describe it. It turns off and on for two or four seconds, occasionally only a second or less.

What do you mean, “the monitor furthest to the left”? To the left of what? If you mean as viewed on the desk, that sounds like it wouldn’t have anything to do with its connection.

Thanks!

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Yes - furthest to the left as you view them from your seat. As I recall, the issue was that a monitor connected with an HDMI cable will power on faster than a monitor connected by another kind of cable, and the one that powers on first should be the furthest to the left. I didn’t fully understand the principle when I read it, but that should be close to the description I read.

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We have single Dell 27” monitors attached to the Mac Studios on campus, they wake only sporadically with the Mac if you tap the keyboard. Frequently I have to reach around the back and hit their power button. Connected with the cable they shipped with as well. But they don’t temporarily cut out once up. I’ve presumed that there’s a mismatch somewhere on the monitoring/sync/timing front. I might get a higher end cable and test that, it could be Dell’s cables.

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A variant of @tommy advice: When you have booted and the problem is present disconnect the power on the monitor by removing the plug at the wall or at the back of the monitor. Count to 10 and connect again. This has helped me many times when I worked with support.

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I don’t want to be “that guy” but you should replace that model. Its a model that came out in 2013, and was only supported on Windows 7 and Windows 8. I had the same issue, “repurposing” Dell displays with newer macs and the flickering was consistent with Dell displays older than the model of mac, that were find on an older Dell PC.

Doing the disconnect dance and resolution changes, etc, will wear you down. Trust me. A decent LG or “Mac-Compatible” display will not have this issue.

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Interesting, I have this issue too. My blinking monitor is also a Dell, 24". It’s on the right of a 2 monitor setup.

I honestly can’t remember if my issue started before I got the M4 Mini, but I think it was (at that time it was the rightmost monitor hooked up to my laptop)

I bought all new cables when I got the Mini.

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Every modern display supports DDC and related standards where it can tell the attached computer what its capabilities are.

In the absence of explicit configuration (e.g. by you, via the control panel), most operating systems will generate a signal corresponding to the display’s preferred resolution and refresh rate (typically matching the panel’s native capabilities), as reported by DDC.

According to the manual, a Dell U2713HM has a native resolution of 2560x1440. So that’s what macOS is going to default to until you configure it for something else. I assume the 1600x900 resolution you selected was manually configured.

OK. So the kind of cable is not an issue. Any HDMI cable should be able to support that resolution.

But…

If macOS thinks this is a “HiDPI”/Retina display, then it will be putting out a 2560x1440 signal, with the image scaled to be be equivalent to 1600x900. If that’s happening, then it will still need a high speed cable.

The cables that come with monitors aren’t always the best quality, but if it’s been working fine in the past, then it’s not going to be a manufacturing defect.

The fact that it is an 11 year old cable may mean that it is no longer up to spec. Conductors and contacts can oxidize over time. So you might want to try swapping its cable.

Another possibility: You’ve got two HDMI monitors. You said you are using adapters (docking station?). Maybe there’s an issue there?

The manual says that its inputs are DVI and VGA.

DVI, at the display’s native resolution, will require a dual-link interface, which a basic DVI-HDMI adapter won’t support. A lower resolution (like 1600x900) will have no problem with single-link, but not if HiDPI scaling is used. So double-check the settings and see if you can disable HiDPI operation.

For instance, my display shows this in the system settings (macOS 15.7.1):

image
image714×747 99.5 KB

Note that some resolutions (e.g. 1024x640) have both normal and HiDPI variants. The normal ones will produce a signal with that resolution. The HiDPI ones will produce the panel’s native resolution (1920x1200), with the image scaled (by the computer’s GPU) to be an equivalent size to the specified resolution.

If you’re using a HiDPI resolution, try switching it to the normal version of the same resolution.

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Fixed?

I replaced the cable with a new one (Amazon description: “Anker HDMI Cable, 4K@60Hz Ultra-Fast Nylon-Braided USB C to HDMI Cord with Advanced Shielding, Thunderbolt 5/4/3”) and I haven’t seen a single blink since yesterday about noon.

Assuming this wraps up this discussion, thank you to everyone who replied. The quality of these responses is why I post such questions here.

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Glad to hear it. So it may well have been a case of the old cable just wearing out over time.

So consider my next comment to be purely academic. I made an error in my comment above:

I didn’t read that manual closely enough. The display comes bundled with DVI and VGA cables, but it has more inputs than that (which you, of course, already knew):

  • DisplayPort
  • VGA
  • Dual-Link DVI
  • HDMI

So my comment about DVI needing dual-link for the native resolution is absolutely correct, but I missed the part about HDMI.

But I also notice that you have a DisplayPort connector. That would actually be even better than HDMI, because DisplayPort is the format supported by USB-C/TB3 when used for video.

HDMI works because DisplayPort and HDMI (and single-link DVI) share a common ancestry and are compatible with each other if you restrict yourself to features common to all three protocols.

I don’t think you should go buy yet another cable, since what you’ve got now is working, but if the subject comes up in the future, you might find easier/better results using a USB C to DisplayPort cable.

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I have two monitors and one of them blinks off some of the time. I have an Acer X203H which I picked up off the street, and a ViewSonic VX2703 that I bought some time ago. The ViewSonic is the larger one (that I use as the primary one). Every so often, the VS screen goes blank. Its power light stays on, and nothing on the Acer changes (so, I mean, my Mac doesn’t think the VS is gone and doesn’t try to throw all the windows and icons onto the Acer; I assume it would do that given enough time).

I assumed this was most likely because the VS is a bit old, maybe it is starting to fail, but now I see maybe that is not the problem. Maybe it is a cable, or a setting – I recently bought a Mac Studio to replace my old, old Mac Pro (trashcan!), and I don’t recall if the problem happened with the Pro so maybe changing to the Studio is the relevant issue. It doesn’t happen enough to be a problem and I don’t do anything like online gaming where it would be a disaster to have no screen for like 2-3 second.

Anyways, good thread as usual here and some thoughts to ponder; thanks everyone!

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As written above, several-second “blinks” generally mean the display is losing sync with the video stream. Cables are always the first thing I check in these cases.

For an old/junk display, another possibility might be the backlight failing. To test for that, shine a flashlight onto the screen and look closely. If you see an image in the reflection, then the LCD is working, but the backlight has failed.

Depending on the display, a failed backlight might be repairable, but it may be more work than you’re prepared to do. Do some web searches, including iFixit, to see if you can figure out how and decide if it’s something you want to try.

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That’s good to know in case any blinking recurs. I’m more in the mood to recruit angels to stand guard over the so-far-well-functioning monitor.

Thank you again, @Shamino.

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I think both monitors are on HDMI cables (although there is a dongle or two back there), I don’t recall the exact setup at the moment. But I can try swapping the two HDMI cables and see what happens! I don’t recall how old the cables are, or where they are from, or anything. (I do have some other HDMI cables around, I could try one of those if needed.) Thank you! Good thread.

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If you swap cables, make sure the ones you use are suitable for the resolutions you’re displaying. These days, there are five types of HDMI cables:

  • Standard. Guaranteed up to 2.2275 Gbit/s.
    • This is up to 74.25 MHz TMDS. This is about 720p60 or 1080p30 or 1080i60 resolutions.
    • While this might be OK for older TVs, it is unacceptable for computer use.
  • High speed. Guaranteed up to 10.2 Gbit/s
    • This is up to 340 MHz TMDS. This is about 1080p144 or 1440p75
  • Premium high speed. Guaranteed up to 18.0 Gbit/s
    • This is up to 600 MHz TMDS. This is about 1080p240, 1440p144 or 4K60
  • Ultra high speed. Guaranteed up to 48.0 Gbit/s
    • Up to about 4K144 with HDR
  • Ultra96. Guaranteed up to 96 Gbit/s
    • up to about 4K288 with HDR

Additionally, the standard, high-speed and premium high-speed cables come in versions with or without Ethernet. If you use HDMI ARC (e.g. with a hi-fi amplifier), you need the Ethernet version.

I couldn’t find the X203H manual on Acer’s web site, but I found a copy here: User manual Acer X203H (English - 28 pages)

Ditto for the ViewSonic VX2703. But I found a copy here: https://www.bhphotovideo.com/lit_files/95897.pdf

Based on those manuals:

  • Acer X203H
    • VGA or single-link DVI input
    • The native resolution is undocumented, but the highest resolution listed in its standard timing table is 1600x900 at 60 Hz, which is probably it.
    • Since modern Macs don’t have DVI connectors, you’ll need an adapter/cable. Either a USB-C or HDMI cable. Be sure to get one certified for 1080p60 (HDMI high-speed or an equivalent)
  • ViewSonic VX2703mh-LED
    • VGA, single-link DVI or HDMI input
    • Native resolution is 1080p, supported up to 60 Hz
    • Use an HDMI high speed cable in order to ensure support for the native resolution at 60 Hz.

See also: Wikipedia: HDMI: Refresh frequency limits for common resolutions.

In particular, note the top row of the table. It shows that 1080p at 24-bit color can go up to 73 Hz refresh with a 160 MHz TMDS data rate. This is beyond the capability of standard HDMI, but well within the limits of high-speed HDMI.

So make sure any HDMI cable you get is designed for at least high-speed (preferably certified for it, but inexpensive cables often are not).

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Wow I did not know that about the different standards for HDMI!!! Yes I have very likely just grabbed any old HDMI cable lying around….. When I have some time I will work on identifying cables (since I have several in the old milk crate of computer cables and miscellaneous). This would actually be really good to know (since I have two HDMI cables in use up here in the office, one in my GF’s office, and maybe 2 on the TV), which cable is which type (and label them if needed). Nice!!! (I will also check the resolutions and refresh rates I am running these two monitors at.)

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Note that cables rarely have the speed ratings printed on the connectors or insulation. If you didn’t retain the original packaging, you may not be able to figure it out without very expensive cable-testing equipment.

But since we’re not writing an academic paper or working in a controlled environment, the only important thing is that you get a stable picture. So if it works, you’re good to go, regardless of the certifications that the cable may or may not have. But if you need to shop for a new cable, it’s important to know of the five types in order to make sure you get one sufficient for your needs.

These days, I’d suggest only buying “Ultra high speed”, because they will support 4K with HDR at pretty much any refresh rate you require (unless you’re a gamer with high-end equipment capable of driving a 4K screen at 240 Hz) and they don’t cost much more than lower-speed cables.

Using CableMatters (a brand I trust) as a reference:

These prices are similar to most of the prices I’ve seen on Amazon, from a wide variety of brands.

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