[huy-nguyen]

The article states the wrong resolution for the Apple display and it’s an interesting mistake because these days there are actually 2 versions of 6K in consumer-marketed computer monitors: the one used by the Apple display (6016x3384) and the slightly larger one used by the Dell U3224KB 6K that came out earlier this year (6144 x 3456). In fact, an interesting thing people found out when they use the Dell 6K display on Intel MacBook Pros running Mac OS between 10.15 and 13.6 is that the Mac cannot do Display Stream Compression at the Dell's native 6144 x 3456, hence the Mac can only drive the monitor at 30hz instead of 60hz. However, if they can fool the Mac into thinking the display is 6016x3384 (same as the Apple display), DSC magically works and they get 60hz on the Dell (at the expense of sacrificing some screen real estate). Apple must probably hardcode the 6016x3384 resolution somewhere in their OS code. Thankfully people report that this problem has been fixed as of Mac OS 14.1 but that bug existed for 4 years.

Edit: this problem only seems to happen on Intel, not Apple silicon machines.

[fabiensanglard]

Thank you for pointing the mistake. I had no idea, this is super interesting.

I have fixed the article and added a footnote to this comment.

[sinfulprogeny]

> that the Mac cannot do Display Stream Compression at the Dell's native 6144 x 3456,

Can't, or won't? M1 MacBook pros for some reason can't do 4k120 over hdmi unless you buy a specific usbc-hdmi adapter and fool it into thinking it's displayport (or something like that, I'm paraphrasing. You can find info if you search for cablematters DDC 4k120 m1.)

[wtallis]

There's no "fool it into thinking it's displayport". What you're describing is having the Mac actually literally emit a DisplayPort signal, and a separate device converting that to an HDMI signal. The USB-C HDMI Alt mode standard was never implemented by any real products, and all USB-C to HDMI converters are active adapters that consume DisplayPort signals and emit HDMI signals. Not all of those support HDMI 2.1, which introduced a drastically different signalling mode for HDMI in order to support much higher data rates (and also added display stream compression, further increasing the maximum resolution and refresh rate capabilities).

[thejazzman]

You're missing the point -- you have to use custom firmware on those adapters or Apple still only puts out 4k60

I went deep on this last night shopping for a cable

[wtallis]

The custom firmware isn't actually all that interesting of a point, because slightly broken display behavior is extremely common if you look closely at anything other than normal everyday TV resolutions and refresh rates.

USB-C/DP to HDMI adapters often need to do some amount of rewriting EDID information because they need to be transparent to the host computer and the display, so it's the adapter that's responsible for ensuring that modes that cannot be handled on both sides of the adapter are not advertised to the PC. When you layer that complexity on top of the existing minefield of ill-conceived EDID tables widespread in monitors, on top of the limitations of macOS (limited special-case EDID handling, little to no manual overrides/custom mode settings), it would be more surprising if there weren't some common use cases that theoretically ought to work but are simply broken. Applying the necessary EDID patch via adapter firmware is simply the easiest option where macOS is involved.

Even on Windows with a DP cable directly from GPU to display it's not all that rare to need a software override for EDID in order to use modes that ought to work out of the box (eg. I have a recent Dell monitor that cannot simultaneously do HDR and variable refresh rate out of the box).

[brigade]

That "some reason" is that a standard DP-to-HDMI 2.1 protocol converter can't negotiate beyond HDMI 2.0 link rates without the host computer knowing about and doing FRL training on the HDMI side. Completely unrelated to any limitations related to 6144 x 3456.

[lxgr]

Couldn’t the adapter do that?

[brigade]

As I understand it, automatic fallback/limitation to HDMI 2.0 speeds was desired by VESA in the event of using an 18gbps cable or other signal integrity issue, so ultimately they chose to require the host to be more aware of HDMI for the converter to enable HDMI 2.1 speeds rather than requiring the converter to be smart.

[lxgr]

Displayport to HDMI 2.1 adapters already seem quite complex: https://www.paradetech.com/products/ps196/

Is the problem expressing the maximum supported bandwidth end-to-end, i.e. not as a function of the DP channel quality that the source sees?

[brigade]

Yes, as a specific example, if the HDMI sink wants DSC, maximizing the quality (minimizing the compression ratio) fundamentally cannot be done without knowing the end-to-end bandwidth.

[eludwig]

I can verify that an M1-M3 Mac running Sonoma (14.1.1) the U3224KB supports 6144 x 3456 at 30bit (60Hz). Under Ventura this did not work. Seems fixed now.

[stephenr]

I got 6144x3456@60 on my m2 studio ultra from the day it (studio) arrived, back in... I want to say July (so not running sonoma)

[eludwig]

Yes, 6144x3456@60 was working, but not at 30bit, only 24bit. Sonoma fixes that part (for me)

[trogdor]

What is an “M1-M3 Mac”?

[pests]

A Mac which has the M1, M2, or M3 chips.