Mac Studio M4 Max and BenQ PD3226G Display: HDR and Refresh Rate Limitations

<< 1) the monitor is 144Hz, so why can I only set it to 100Hz in Settings>Display? >>

Because there is not enough bandwidth in a typical Thunderbolt cable to run any faster than what you are achieving.

<< 2) HDR is not available unless I lower the frequency from 100Hz to 60Hz. Is this correct? >>

Yes, because there is not enough bandwidth in a typical ThunderBolt cable to do that.

If you want the data rates you are asking about, the Thunderbolt cable has to 'gear-shift' from its current HBR3, about 26 G bits/secs up to the next step, UHBR10, which runs just shy of maximum outbound speed of 40 G bits/sec.

This is ONLY possible with Thunderbolt cables shorter than 0.5 meters, or with tuned ACTIVE cables that cost a lot more.

imacken wrote:

Thanks for that.

Could you explain why 3360x1890 goes to 6720x3780 and the 2160p doesn’t go higher?

If your display has a native resolution of 3840x2160 pixels, and you select a Displays Settings "resolution" of 3360x1890 pixels, the computer has two choices:

• It can draw on a 3360x1890 pixel canvas. There will only be 3360x1890 pixel's worth of information in the image. Either the Mac or the monitor will have to stretch out that image to fill the 3840x2160 pixel display. Stretching the image does not add resolution any more than converting a 128 Kbps MP3 file into a WAV file restores the part of the music that was thrown away by the lossy compression.
• It can draw on a 6720x3780 pixel canvas, and then downscale the image it sends to the display to fit on the 3840x2160 pixel screen. Although this also involves a form of "digital zoom", you're starting with an image which has much more detail. So the downscaling to 3840x2160 reduces effective resolution to 3840x2160. Which is better than 3360x1890, even if the GPU and display generators have to do more work to get there.

If your display has a native resolution of 3840x2160 pixels, and you select a Displays Settings "resolution" of 3840x2160 pixels, then a NON-Retina 3840x2160 pixel canvas already has as much detail as your display is capable of displaying. Drawing on a 7680x4320 (8K) canvas, and then downscaling that back to 3840x2160 pixels (to fit the 3840x2160 pixel display) would essentially be a lot of "make work" for no gain.

I believe this is why, if you tell Displays Settings to show all resolutions, as a list, you'll see both Retina choices, and "(low resolution)" choices, for many resolutions – but only one choice for your display's native resolution.

For that resolution, only non-Retina mode makes sense.

imacken wrote:

So, I think from your reply, this is where we have the confusion. I am getting nominal 4K (3840x2160) and HDR at 144Hz, and I am not getting it at lower resolutions like 3360x1890, where it is restricted to 100Hz and no HDR. This is the reason for my question from the start.

I bet the "lower resolution" you selected was Retina "looks like 3360x1890" – not "3360x1890 (low resolution)". In Retina "looks like 3360x1890" mode, the Mac would be drawing things on a 6720x3780 pixel canvas, before downscaling the contents of the 6720x3780 pixel canvas to fit onto the 3840x2160 pixel screen.

6720x3780 pixels is a lot of pixels. It is higher than the native panel resolution of a 32" Apple Pro Display XDR, a display that has "only" 6016x3384 pixels.

Data rates @ 100 Hz:

• 3840x2160 pixels => 0.83 billion pixels per second
• 6720x3780 pixels (Retina "like 3360x1890") => 2.54 billion pixels per second

Data rates @ 144 Hz:

• 3840x2160 pixels => 1.19 billion pixels per second
• 6720x3780 pixels (Retina "like 3360x1890") => 3.66 billion pixels per second

Now the 2.54 billion pixels per second and 3.66 billion pixels per second "just" refer to data rates that are internal to the Mac. But maybe – just maybe – the bandwidth available for sending the final 4K signal to the monitor over USB-C (DisplayPort Alt Mode) is not the only consideration here.

Maybe the chip can handle a 6720x3780 pixel canvas @ 100 Hz, but 144 Hz is "a bridge too far."

4k 8 bits color 120 Hz

(just under) 4K, 10 bits color 100 Hz

that does not seem puzzling to me.

the table for 10 bits color are the next-to-last HUGE green and pink large tables near the end of the Wikipedia article

the 8 bits table are up a page or so.

https://en.wikipedia.org/wiki/DisplayPort

that BenQ PD3225U display appears to be a 4K display with HDR

Connectivity:

HDMI (v2.0). <-- this is unfortunate, because it severely limits the highest resolutions to

50 Hz refresh rates without compromising number of colors.

HDMI 2.1 would be FAR better, allowing up to 144 Hz. but the display does not support that

2

DisplayPort (v1.4)

1

Thunderbolt 3 ( PowerDelivery 85W, DisplayPort Alt Mode, Data )

a USB-C connection allow the use of 4K at 60 Hz. if you drop to 8 bits/color, you can attain 75 Hz.

For higher refresh rates, you MUST connect this display with a genuine ThunderBolt cable, marked with the Thunderbolt trademark, or you will never see high refresh rates at HDR 10 bits/color.

ThunderBolt cables for this use should generally be 0.5 meters or shorter, unless you are using (expensive [over US$100]) tuned ACTIVE ThunderBolt cables.

-------

you also have the option to connect with TWO data cables, and send the left or right half-display over each cable. It sounds much more complex than it actually is, and readers report it works great.

I am having no problem reading your posts -- I have my Forums settings for chronological order -- perhaps yours is set to something different?

I read the resolutions you posted. Each and every once conforms with what is expected with the setup you have.

Resolution, refresh rate, and number of bits/color all conspire to drive up the overall data rate required. With the computer, the cables, and the display you have, you have now successfully enumerated what is possible.

I have also stated what it would take to get the rates you desire.

It is not just s simple bit/sec calculation. There are 'brackets' similar to tax brackets, that are supported. But if you calculated precisely, rather than intuitively, I expect the results would be the same.

3560x2160 at HDR yields a refresh rate just shy of the absolute maximum data rate that is possible. Others miss by a wide margin, but the next combination step up in any dimension may push you beyond the limit.

There are some limits tables on wikipedia I could point you to, and you could look up the major settings yourself if you like.

you should be able to get 120 Hz at full 4K with 8 bits/color, but there may not be a settings for that.

your 3560x2160 'gets by' at just shy of 4K. The tables shows full 4K with HDR can attain 98 Hz. Since you dropped a few rows and columns, you get 100, but it is AT the limit.