I am stuck at 30Hz refresh rate with Intel MacBook Pro and Dell S2725QC monitor

Dell – Dell 27 Plus 4K USB-C Monitor - S2725QC

This is a 4K monitor with a USB-C port that supports DisplayPort Alt Mode, using DisplayPort version 1.4. It has three downstream USB ports (two USB-A, one USB-C) that can run at up to 5 Gbps.

I wonder if what we are seeing is a bandwidth issue. I could be mistaken, but I believe that when you are using "plain" USB-C to connect a display, there are two choices:

• Carry USB traffic only on pins reserved for USB 2.0 data. This limits USB hub port speed, but allows DisplayPort traffic to use ALL of the "flexible" pins.
• Use some of the "flexible" pins to carry USB 3.0 data and some to carry DisplayPort traffic. This reduces the bandwidth available to carry a video signal. How much this hurts you depends on the version of DisplayPort that's involved. DisplayPort version 1.4 gets higher bandwidth out of the pins available to it, and so you don't encounter the same degree of tradeoffs with it, that you would with DisplayPort version 1.2.

My guess is that

• Your Intel-based MacBook Pro supports DisplayPort version 1.2.
• Your M1 MacBook Pro supports DisplayPort version 1.4.
• Your Dell monitor is set up to use some of the "flexible" pins to carry USB 3.0 data. My Dell monitor has an on-screen menu item (Display > USB-C Prioritization) with two choices: "High Resolution" and "High Data Speed". The setting is on "High Data Speed" (which appears to be the default). This is a different monitor from yours – but it is also one that supports DisplayPort version 1.4, and that is connected to an Apple Silicon Mac.

So … if you go into your monitor menu, you may find that changing the setting to "High Resolution" will let you get 60 Hz on the older MacBook Pro.

that Dell S2725QC appears to be a 4K display that accepts 10 bits/color BUT

uses FRC to attain billions of colors

2x HDMI 2.0 and

USB-C with support for DisplayPort 1.4

The problem with supporting that display on a USB-C cable may be down to 8 bits/color Vs 10 bits color (HDR=ON) over USB-C. You did not show which setting is active in your screenshot.

if your USB-C cable is not the absolute highest quality, it is possible that at 10 bits/color, it misses the maximum supported by recent MacOS, which is 60 Hz -- PROVIDED the transmission to the display is completely error-free.

Cables 'shipped in the box' are notorious for being 'lowest bidder' cables, and are too frequently found to be inadequate. If you try a new cable, the length is limited to ONE meter, maximum. Look for a cable that is certified and has the USB logo/speed rating embossed on both ends.

If you set HDR=OFF to drop back to 'only' millions of colors, the nominally-supported refresh rate jumps to 120 Hz, but still subject to error-free transmission in recent MacOS.

EDIT: I did not see Servant of Cats posting before I began composing. Nothing I wrote is intended to diminish what they wrote in any way.

(Continued)

Arikai wrote:

I have the same issues as the original poster on a 2020 iMac. I tried changing the setting to high resolution and now it tells "no USB-C signal from your device" and the option to change the setting back has been greyed out and is inaccessible. It gives me the same error when trying to use an hdmi cable. I'm completely locked out of the menu options to change this setting back, or to even factory reset the monitor.

Did you try turning 10 bits/color (HDR) OFF?

That reduces the bandwidth required considerably, and unless you are editing magazine cover art, does not make a lot of difference is the presentation of colors.

HDMI -- does not seem to be a contender

ONE cable:

To connect that display on a Mac that does NOT have a DIRECT HDMI port is troublesome. The highest that can be supported is HDMI 1.4, [limited to 4K at 30 Hz] because although you MIGHT be able to FIND an off-brand adapter to HDMI 2.0, these get 'burn your skin off' HOT and fail early. Not recommended.

Connecting that display to your M1 MacBook Pro on its DIRECT HDMI port is also disappointing, because it only supports HDMI 1.4 [limited to 4K at 30 Hz] directly. (More-capable HDMI first appeared in M2 and later.)

TWO cables:

You could connect certain displays at full resolution and full color support using Two cables. Then each cable carries far less data for only the left or right half-display, and the display use Picture-by-Picture (cousin of Picture-In-Picture) to put the halves back together in a seamless way.

Unfortunately, that particular display does not appear to have Picture-by-Picture built in, so that will not work for THIS display.

I have the same issues as the original poster on a 2020 iMac. I tried changing the setting to high resolution and now it tells "no USB-C signal from your device" and the option to change the setting back has been greyed out and is inaccessible. It gives me the same error when trying to use an hdmi cable. I'm completely locked out of the menu options to change this setting back, or to even factory reset the monitor.

Arikai wrote:

I have the same issues as the original poster on a 2020 iMac. I tried changing the setting to high resolution and now it tells "no USB-C signal from your device" and the option to change the setting back has been greyed out and is inaccessible. It gives me the same error when trying to use an hdmi cable. I'm completely locked out of the menu options to change this setting back, or to even factory reset the monitor.

There is no reason why the Mac would have control over that monitor menu – but you could try going into it when the Mac is turned off. If the monitor still balks, you may want to contact Dell Support and ask them why.

As for HDMI, that menu has no bearing on it. HDMI does not support USB data.

Just wondering, are you in Mirror or Extended mode?