Windows is famous for its Blue Screen of Death, but macOS has a Green Screen of Death… that’s right, team Apple will not be outdone!
I can’t believe I am making yet another video about HDMI 2.1 so soon, but there are just so many hidden gems to discover when it comes to these M2 Pro Macs. Today, I’ll talk about some of the new things I’ve learned since I made the limitations in HDMI 2.1 video, including how the restriction preventing scaled 1440p @ 120Hz with HDR on may be a software limitation, giving us hope that it could be enabled in the future.
Initial Setup Shock
I did my initial testing of the HDMI 2.1 port on a M2 Pro MacBook Pro 14-inch, but recently decided to return that computer in favor of a M2 Pro Mac mini instead. You can also check out a previous video that I made about the reasons why.
When I set up the MacBook Pro for the first time, it prompted me to update to the latest macOS version, Ventura 13.2, during the initial setup wizard, which I did. That means I did the original HDMI tests on macOS 13.2.
However, when I set up my M2 Pro Mac mini for the first time, the setup wizard did NOT prompt me to upgrade the OS. Therefore, after initial setup, I immediately checked the display settings and found that I actually could enable scaled 1440p resolution at 120Hz with HDR on! This was not possible previously with the MacBook Pro, which forces you down to 100Hz at scaled 1440p, and 60Hz at higher scaled resolutions if you want to use HDR. Another video I made goes more into detail about that.
Green Screen of Death
So at first I thought maybe there was a difference between the MacBook Pro and Mac mini’s HDMI 2.1 support, and I decided to see how high of a resolution I could go and still get 120Hz with HDR. On my LG C1 TV, it turns out that you can go up as high as 3200×1800, but if you try to go up one more to 3360×1890, you get… a Green Screen of Death. The system crashes and it reboots. So this is proof! Macs do have a death screen after all, not just spinning beech balls!
At this point, I decided to check the version of macOS that I was running. It turns out the M2 Pro Mac mini came with macOS 13.0 out of the box, so I immediately suspected that the behavior differences I saw compared to my previous testing was due to the macOS version. I decided to upgrade to version 13.2, and indeed the same exact restrictions returned. I could not longer use scaled 1440p resolution or higher at 120Hz with HDR.
However, what this experience suggests is that the limitation preventing the use of 120Hz HDR at higher scaled resolutions may be a software one, since it was apparently allowed in a previous version of macOS. Now, it’s a good thing that Apple fixed the bug that allowed selecting options that could cause the system to crash. However, was Apple a bit overzealous in the restrictions?
At this point, there were 2 possibilities as I saw it. One possibility is because macOS scaling uses the retina display principle of rendering at 4 times the pixels of the chosen resolution, the HDMI 2.1 port bandwidth cannot support higher scaled resolutions. However, this discovery that previous versions of macOS did allow some higher scaled resolutions to work suggests that perhaps the HDMI 2.1 port bandwidth wasn’t the limiting factor after all.
Display Stream Compression
So to check this further, I found a table listing the bandwidth requirements for different resolutions, refresh rates, and HDR. Now keep in mind that HDMI 2.1 supports up to 48 Gbps of bandwidth, and HDR generally requires 10-bit color depth. If this chart is accurate, UHD 4K @ 120Hz with HDR and 10-bit color only requires 40.1 Gbps, which is within the limits of HDMI 2.1 as expected. However, 5K 120Hz with the same color parameters requires about 50 Gbps of bandwidth, which is just above the HDMI 2.1 limit of 48.
Running that would require Display Stream Compression or DSC, which is a visually lossless compression algorithm that enables higher resolutions and refresh rates to be used than the port’s bandwidth would otherwise allow. Both DisplayPort 1.4 and HDMI 2.1 can support DSC, as do the M1 generation of Macs, so naturally it should work on the M2 Macs as well.
So here’s the coincidence and where I get into conjecture mode a little bit. 5K resolution is what scaled 1440p renders at. That means the macOS 13.2 limitation that doesn’t allow you to set scaled 1440p @ 120Hz with HDR on seems to be because of the required bandwidth going above 48 Gbps. However, the fact that you COULD set this combination with macOS 13.0 suggests a couple possibilities.
First, that Display Stream Compression is necessary for how Apple does resolution scaling. Maybe Apple was trying to support DSC to enable the higher resolutions previously, but there were issues (like the Green Screen of Death), and so they disabled it in macOS 13.2 (hopefully as a stopgap measure).
Alternatively, it may just be a bug with Apple’s resolution scaling. When using a scaled resolution, the GPU renders at 4 times the pixel density in a virtual buffer, and then downscales it. But if that’s the case, then even if scaled 1440p is rendering at 5K, it should be downscaling to UHD before sending it to my UHD display, which means bandwidth issues shouldn’t even come into play at all!
Either way, it seems like it could be just software bugs. And it’s certainly not the only one I’ve run into. The HDR setting sporadically switches off for me sometimes in between computing sessions, and I have to toggle it back on again.
Working on DisplayPort
Lending more credibility to this idea is a comment on my previous video that says a 4K monitor hooked up with a USB-C to DisplayPort connector is able to run scaled 1440p resolution at 144Hz with HDR on. Keep in mind that DisplayPort 1.4 has a max bandwidth of 32.4 Gbps, which is less than the HDMI 2.1 port bandwidth of 48 Gbps. If that’s also on macOS 13.2, then it seems either DSC or the virtual buffer downscaling is still working with DisplayPort Alt mode, just not HDMI 2.1.
Unfortunately I can’t test this out myself to show you as I don’t have a high refresh rate HDR DisplayPort monitor, and I’m using an LG C1 TV that only has HDMI 2.1 ports. I do wonder, however, if a USB-C to HDMI 2.1 adapter might yield different results.
Save Us, Apple!
In any case, at the end of the day, it’s hard to say if Apple will bring back the ability to select those higher resolutions over HDMI in a way that works and without crashes. However, my impression is that Apple places priority on supporting its own end-to-end experience first, which means the hardware that it sells, such as the Apple Studio Display. Other external display support is likely low on the list of things they prioritize, so we can only hope that they get around to this issue sooner rather than later.