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.

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.

<< Why do I get 144Hz HDR at native res, yet I can’t at lower values? >>

you don't get nominal 4K at HDR at 144 Hz. You only get that when you drop the screen size lower than 3840 wide -- to 3560 is what you wrote.

You are able to achieve that because these display protocols do not increase their speeds by a few GHz/second when you increase the resolution by a few pixels. There are several fixed overall transmission speeds {HBR, HBR2, HBR3, UHBR10}. If your data fits inside one of the envelopes, it runs. if not, you have to reduce resolution or refresh rate.

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."

HBR2 is the speed you can can get on a USB connection when the cables are under a meter. The basic rate is 10 G bits/sec, and then it 'turns around' and runs the inbound lanes as outbound lanes part of the time, to get a speed of 17.28 (less than the nominal 20 max).

HBR3 requires a Thunderbolt cable, which effectively runs twice as fast as USB. When you try to run a one meter ThunderBolt cable as high as UHBR10 data rate, it makes too many errors and falls back to the slower HBR3 speed. You need a 0.5 meter Thunderbolt cable, or an ACTIVE cable to get the top data rate.

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.

imacken wrote:

Latest response from BenQ support says:

‘Regarding why 1920 x1080 at 144Hz shows up while the monitor only supports up to 120Hz at this resolution, we have confirmed that this is a MacOS issue. When set to 1920 x 1080, the signal out put from Mac is still at 3840 x 2160, and the refresh rate is supported up to 144Hz at that resolution. In other words, due to the way MacOS works, the monitor is still detecting 3840 x 2160 from the source computer.

I would definitely expect things to work that way if you were running a 3840 x 2160 pixel monitor in Retina "like 1920 x 1080" mode. In that mode, things are being sized "as if" the monitor had 1920 x 1080 "workspace", but things are actually being drawn at a 3840 x 2160 pixel level of detail.

If you go into Displays Settings, and display all resolutions as a list, and choose "1920 x 1080 (low resolution)", I am not sure whether the Mac would send a 1920 x 1080 signal to the monitor – or whether it would still send the monitor a 3840 x 2160 signal, albeit in this case, one with only 1920 x 1080 pixels' worth of actual detail.

I am surprised that the monitor supports a higher refresh rate for 3840 x 2160 than for 1920 x 1080!

it appears that the PD3226G display you mentioned DOES support HDMI 2.1.

It would provide higher resolutions over HDMI. This REQUIRES certified ULTRA HDMI cables, which are backward-compatible.

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.

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

you are NOT running 4K, you dropped your row width much lower. 3840-3560 is 280 columns of data, times 2160 * [10 bit Bytes] each! That's 6,048,000 bits PER FRAME * whatever refers rate -- not trivial -- and maybe that's what lets you get in under the wire.

when you are running "ThunderBolt" the protocol you are actually running is DisplayPort-over-Thunderbolt.

ThunderBolt 5 only adds speed when the Port, the cable, and the external device ALL support Thunderbolt-5. Otherwise, it is running at ThunderBolt 3-4 speeds.