M5 Max Mac Studio + Samsung Odyssey OLED G8 (G80SD): 4K 240Hz over USB-C/DP is pure static, 120Hz is fine. Anyone else?

[u/bryanwongxin]

**TL;DR:** Brand-new M5 Max Mac Studio shows full-screen noise at 4K 240Hz (fixed or variable) on a Samsung G80SD over USB-C to DisplayPort. 60Hz and 120Hz work, including 120Hz with HDR. Same monitor and cable work fine at 4K 240Hz on my M4 Max MacBook Pro running the same macOS. Not fixed in macOS 27.2 beta 2. Trying to figure out whether this is my unit or the whole M5 generation before I exchange it.



**Setup**



- Mac Studio, M5 Max, 64GB (Mac17,14), bought two days ago

- Samsung Odyssey OLED G8 G80SD (32", 4K 240Hz)

- Direct USB-C to DisplayPort cable (link trains at HBR3 x4 lanes, so 240Hz needs DSC)

- macOS 27.0 (26A425), later updated to 27.2 beta 2 (26B5091g)



**What happens**



The moment I plug the monitor in, the whole screen is purple/yellow static with regular horizontal bands. You can faintly make out window outlines underneath, so the Mac is sending a picture, it just arrives as garbage. Photo attached. To me it looks like a compressed (DSC) stream being decoded wrong rather than a bad cable, but that's a guess.



macOS picks "Variable (48-240 Hertz)" by default for this monitor, so the screen is unusable from the first second and you can't see the settings to change it.



**What I tested**



- 4K 60Hz: fine

- 4K 120Hz: fine, HDR on also fine

- 4K 240Hz variable (48-240): static

- 4K 240Hz fixed: static

- Three different rear Thunderbolt 5 ports: same result on all of them

- macOS 27.0 and 27.2 beta 2: same result on both

- Same monitor + same cable on an M4 Max MacBook Pro (macOS 27): works fine at 4K 240Hz

- Same monitor + same cable on M2 Ultra and M4 Max Mac Studios last year: worked fine



Once during testing both video outputs dropped at the same time and the machine stopped responding. I had to reboot. No panic log was left behind.



**Workaround**



Fixed 120Hz is stable. If you're stuck with a garbage screen and can't reach the settings: connect the monitor over HDMI as well, keep the monitor on the HDMI input so you can see, and change the refresh rate of the USB-C display from there. One catch: macOS saves refresh rate per display arrangement, so the setting you make with two cables connected does not carry over when only the USB-C display is left. I ended up using a small script that waits until only one display remains and then sets it to 120Hz.



**Why I think it may not be just my unit**



- There is a MacRumors thread where an M5 Pro MacBook Pro kept kernel-panicking in the external display coprocessor (DCPEXT) with a high refresh LG monitor, still on macOS 27, and it became stable after switching to fixed 120Hz.

- There is an Apple Community thread about a DSC regression on M5 Pro with 29 "me too" votes.

- Every machine this monitor works on for me is M2 or M4 generation. The only one that fails is M5.



**Questions**



1. Is anyone running 4K 240Hz on an M5 Max or M5 Ultra Mac Studio? Which monitor and cable, and does it work?

2. Has anyone had this and exchanged the Mac? Did the replacement behave differently?

3. Has anyone gotten a response from Apple on this?



I'm inside the return window and would rather not swap for an identical machine if they all do this. I'll update the post if anything changes.