Macbook Pro M1 Max + Thunderbolt 4 Dock + 2 Spectrum Monitors not working correctly

I’m currently running a Macbook Pro M1 Max and a Sonnet Echo 11 Thunderbolt 4 Dock. To this dock I connect my two Spectrum monitors. The first monitor works correctly at 4k144hz. The second one, however, is only able to run at 4k30hz.

It was suggested to me by the Sonnet support to change the resolution directly on the monitor to something lower than 144. I can’t try this though, since there’s no such option on the Spectrum.

The dock supports up to two Apple Pro XDR displays so one would think bandwidth shouldn’t be an issue.

Currently on the 105 firmware.

Although you mention you don’t think bandwidth is an issue, here is a great post from the community that might explain why you are hitting this limit.

I can’t see anything on the Echo site that indicates what expected refresh rate this dock would support running two XDR displays. Are you trying to get both displays to 144Hz or both to the same refresh rate but lower than 144Hz?

There are reports that these can help when macOS is not allowing the selection of a refresh rate higher than 30Hz or 60Hz

  • Change the resolution to Scaled, then change it back to Default for Display
  • Unplug and replug the thunderbolt cable after toggling the resolution
  • Change to USB Type-C to display port cable to take advantage of DCS.

Also check that both Spectrums have the USB bandwith set to highest refresh rate.

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I assume it’s 60 Hz. Though if it was lower it would be at the least 47,95 Hz (this is the lowest frequency the XDR will run at according to the specifications) which should still be plenty enough for at least running the monitors at 90 Hz each.

Looking at the calculator here it’s cutting it close running at 144 Hz with DSC at a 2:1 ratio. Should for sure be OK with a 3:1 ratio.

From looking at the available display settings on MacOS using cscreen, both directly connected to the computer and through the dock, it looks like there aren’t actually any options between 144 and 60 when running the native 3840x2160. I’m just guessing here but could this be an issue?

None of the suggestions there help, unfortunately. The USB4 (TB4) cables I’m using support DSC and so does the 105 firmware. I can run the monitors separately just fine at 144 hz, it’s a matter of getting them to run via the dock that is the issue.

Edit: Missed the question in there. Ideally I’d like to run them both at 144 through the dock. But even just running them at 120 would be fine.

An update: I’ve gotten a response from the Sonnet Support and according to them the Apple Pro Display XDR connects at exactly half the pixel density and does some magic pixel interpolation to make it “full”. This is the reason that you can connect two XDR displays but not two 5k monitors :thinking:

I’ll just give up and hook it up to the Macbook directly.

I wanted to add a little bit more info in here, in case others find this thread.

From what I’ve been able to find, Thunderbolt 4 doesn’t increase the amount of supported bandwidth compared to Thunderbolt 3. It only increases the minimum bandwidth to 40Gbps.

This means that a Thunderbolt 4 connection is able to support up to 4 lanes of DP alt mode data (32Gbps signal bandwidth, of which 25.92Gbps is usable). This amount of bandwidth is enough for a single 4K @ 120Hz signal or dual 4K @ 60Hz signals.

DSC helps to push more data down the connection, but the amount of compression varies based on the source image. Video drivers might choose a relatively low amount of compression when showing available video modes, to make sure they’re able to compress the image enough in all cases.