Cannot get 60 Hz over USB-C

I recently received a new Dell U2720Q 4K display to use with my 2020 13" Intel MacBook Pro. I planned to use the display at 2560x1440, basically the same res as on my old 27" but with better image quality.

When I attached the display with the included USB-C to USB-C cable, I was not able to get this resolution at 60 Hz, the max offered was 30 Hz which made mouse pointer movement jumpy. When I used a DP cable instead, the desired 60 Hz started working. The display supports DP 1.4 alt mode over USB-C as does my rather recent Mac. The USB-C cable I was using was the one included.

I would really like to use the display over USB-C since that way it will also charge my Mac. I don’t care about USB bandwidth downstream of the display because I’m not planning to use any USB devices attached to the screen. I did make sure to select the screen option to prioritize high resolution over data speed, which according to the manual should allow for 60 Hz up to 3840x2160. Any ideas how I can get this to work over USB-C?

This Wirecutter article on USB-C cables might be helpful. They divide USB-C cables between best for charging and best for monitors:

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I’m also a bit suspicious of the cable, but as I said, this is the cable that came with the monitor. Dell makes no mention at all about special 3rd-party cables being required for 60 Hz.

I’m seeing if our IT department can loan me an Apple TB3 cable for testing. That should have both the b/w and the PD requirements covered. If that works, I can still worry about if I should try to save a few bucks by finding a suitable USB-C or if I’ll just stick with a more expensive TB3 cable.

I tried two other cables I had. Both were rated for USB 3.1 Gen2 and PD up to 100 W. The only difference was 1’ vs. 3’. Neither cable allowed for 60 Hz. I’m back to DP now while I wait for an Apple TB3 cable to arrive.

Everything I’ve experience about USB-C leads me to think that different connectors for different things is probably a good idea.

So apparently nobody around here can find an Apple TB3 cable and they’re on backorder until whenever. But I was able to order this USB4/TB3 cable off of Amazon just to check if it’s just an issue with the USB-C cable included with the monitor.

But no dice. Even with this fancy 40 Gbps cable, 30 Hz was absolutely the highest I could achieve at 2560x16001440. So the cable goes back to Amazon and I conclude that I absolutely must use DP to connect this so-called “USB-C display”. I’m now pondering if it’s just the display’s USB-C implementation that’s, or rather if it’s related to the crummy Intel graphics on my 2020 13" MBP. I can’t wait to swap this thing for a new M1X 14" MBP.

Although USB-C has a DisplayPort alternate mode, Apple does not say anywhere what version of DisplayPort a 2020 MBP actually supports. At least I couldn’t find anything. I did find a B&H Photo product page that says (emphasis mine):

Your Dell display says that it uses DisplayPort 1.4, but if the B&H product page is correct, your Mac can’t output version 1.4.

But it still seems strange to me, because according to Wikipedia, the HBR2 encoding (defined by DisplayPort 1.2) has enough bandwidth to support 4K (3840x2160 - the display’s native resolution) at 60 Hz.

I wonder if it may simply be a matter of 2560x1600 being an unsupported resolution. According to the user’s guide (page 12), 2560x1600 is not on the list of preset display modes (probably because it’s a different aspect ratio). 2560x1440 is and 3840x2160 are, and are supported at 60 Hz.

What happens if you try to use one of these two resolutions? (Obviously, you won’t be mirroring the built-in display to use them.)

If 3840x2160 works, then I would use that and use the Mac’s Retina display scaling to make the content large enough for you to find comfortable.

If MacOS doesn’t see the display as a “HiDPI” screen (enabling Retina scaling in the Display preferences), there are ways to fake-out macOS to make these preferences appear. Or if you’d prefer something more polished, I’ve read many good things about SwitchResX ($16 per computer with a 10-day free trial).

1600 was a typo. 2560x1440 is supported both by the Mac and the screen natively. But indeed, the only way to get the Mac to display that at 60 Hz—at least so far—is over DP.

Well holy cow. Got an 80-cm Apple TB3 cable this morning and sure enough, with this cable for the first time I am seeing 60 Hz at 2560x1440 on the Dell U2720Q over its USB-C port.

I really wonder what’s going on here. The screen is not TB, it’s clearly USB-C. And I already tested several 40 Gbps USB-C cable (3.2 gen 2) from different manufacturers to no avail. And of course the same behavior was already noted with the included USB-C cable, which really gets you wondering what Dell is doing here. :man_shrugging:


Very interesting Simon. Looking at getting a 27” external monitor to replace my ageing pair of 24” I have attached to my iMac 5k and possibly a new MBPro (ahem). I have a Caldigit TB hub with two HDMIs out and I was curious to evaluate its actual output. How did you evaluate the frame rate? Just set it in Display Preferences and see?

Although I could never stand Dell as a company, in the past I would have always recommended their screens. I’ve had several excellent displays from Dell over many years—note the panels are great, the design is of course plastic garbage (at least it’s adjustable) and the hardware controls are meh. This is the first time I’m left thinking Dell butchered what should have been a slam dunk screen.

It sounds like the Dell display is fine, they just cheaped out on the included USB-C cable, using one that fails to fully support DisplayPort 1.2 over USB’s DisplayPort Alt Mode. Or its a manufacturing defect in that exact cable (maybe Dell Support would send a replacement). Apple’s Thunderbolt 3 cables, or likely most Thunderbolt cables, have to be manufactured to more exacting standards.

It’s not just the Dell cable. Or this specific cable. A second U2720Q came with a cable that showed the exact same 30 Hz baloney. As did several different serious USB-C cables (3.2 gen 2) and a “USB4” cable from various manufacturers.

I wonder if there’s an issue with USB-C Alt DP mode on Big Sur. I should be testing the screen with the included USB-C cable with a Windows and a Linux box. Not sure I’m that into figuring it out though. Maybe I’ll just wait until somebody else figures it out. :stuck_out_tongue: