Bypass Apple HDR restriction at high refresh rates/large screen sizes

[heyjme]

BetterDisplay has allowed me to pair my LG C2 42" with my MacBook Pro M1 Max, giving me 120hz via the Cable Matters USB-C to HDMI adapter. I'm loving this setup, and so glad there's a workaround in place to give me a proper monitor-like experience with this external display (TV).

The only limitation that I face is the lack of HDR support with 4k resolution @ 120hz. I've done some digging and the following reddit discussion contains a comment from Apple on the matter. This essential states that they've deliberately disabled HDR for larger screen sizes/higher refresh rates on external displays on macOS 14.1+:

Hello. Unfortunately, there is a frontend bandwidth limitation that prevents us from reliably supporting HDR at high refresh rates beyond a certain screen size, so we have had to gate the feature slightly more conservatively than in earlier builds of the OS.

I wonder if there's a way to bypass this restriction, as has been done with the refresh rate?

[waydabber]

No, this limitation is not about the connection but the framebuffer size an framebuffer pixel clock. Apple can limit this in the DCP firmware. There is no way around it. Not sure why Apple did it especially as it worked just fine for 2+ years. Interestingly same limitation applied to M2 initially while (probably due to fix the M2 green screen of death bug) while M1 was unlimited (which was awkward as the older chip seemingly did better than the new one) - but when they limited M1 they removed this limit from the M2 and fixed the green death bug. A more transparent explanation would be helpful from Apple. :)

[heyjme]

Thanks for the detailed explanation, as ever @waydabber. Not the answer I was hoping for, though ;) So essentially I can choose between 60hz and HDR or 120hz and no HDR. Or the third option, which presumably Apple would prefer: upgrade to M2 or M3 - ho hum.

[waydabber]

120Hz HDR should work up to 2432x1368 HiDPI (with 4K output - weird cut-off resolution from Apple there). Limit is lower for 144Hz and higher for 100Hz. I assume you wanted to use 2560x1440 so this might be an acceptable tradeoff.

[heyjme]

You assume, right. Thank you again - this is indeed a better tradeoff than lowering the refresh rate 👍

[dbacks2196]

Thanks so much, @waydabber, and sorry you're facing that issue, @mustdobetter. I can confirm that I have the same problem with an M3 Max, and I can't figure out a way to get HiDPI resolutions > 2432x1368, refresh rates > 60Hz, and HDR to work simultaneously. I submitted a report through Feedback Assistant asking if there is or will be an option to re-enable it from the command line in the recovery partition (or whatever they're calling that for Apple silicon Macs). It seems like getting enough people to complain to Apple about it is the only way, since it's not something that can be fixed with any third-party tools like BetterDisplay.

[nexuscan]

I have mbpr 2018 and cable matter adapter and benq ex3210u monitor. and I cannot get 4k 120hz.....

[JESSUSA809]

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