Can a 2019 iMac with a 580X support a 5k+4k or 2 x 5k external displays @60Hz?

[theorist9]

I have a 27" 2019 Core-i9 iMac with a Radeon Pro 580X GPU.

Apple's general specs for all 27" 2019 iMacs say it can support up to 2 x 4k @60Hz:

iMac (Retina 5K, 27-inch, 2019) - Technical Specifications - Apple Support

support.apple.com

However, those specs cover all the video cards, from the 4 GB 570X up. But what if you have an 8 GB 580X—could that support a 4k and a 5k, or two 5k's?

We can see Apple's specs for the 580X specifically here:

Use the Radeon Pro 580X MPX Module with your Mac Pro (2019) - Apple Support

Learn about the many different display setups you can use with your MPX Module on your Mac Pro (2019).

support.apple.com

In order to support 2 external 5k's, the 580X would need to be able to drive 3 5k's total, which is 4.4 x 10^7 pixels. That's less than what's listed in both the 2nd bullet (2 x 5k + 2 x 4k = 4.6 x 10^7) and the 3rd (6 x 4k = 5.0 x 10^7). So based on the above, it at least tentatively qualifies. Of course, a 580X in an MPX module in a Mac Pro will not be the same as a 580X in an iMac (certainly the Mac Pro offers better cooling; and Apple may have configured the GPU in the MPX module for a higher clock speed).

So: Has anyone connected a 4k + 5k, or 2 x 5k (all at 60 Hz) to a 2019 580X iMac and, if so, how well does it work? In particular, even if does work, I don't want to be pushing the GPU to its limit, i.e., I don't want there to be performance lag or high fan noise.

 

[joevt]

I have a 27" 2019 Core-i9 iMac with a Radeon Pro 580X GPU.

Apple's general specs for all 27" 2019 iMacs say it can support up to 2 x 4k @60Hz:

iMac (Retina 5K, 27-inch, 2019) - Technical Specifications - Apple Support

support.apple.com

View attachment 2010304

However, those specs cover all the video cards, from the 4 GB 570X up. But what if you have an 8 GB 580X—could that support a 4k and a 5k, or two 5k's?

Yes, the specs cover all the video cards, including your 8 GB 580X.

We can see Apple's specs for the 580X specifically here:

Use the Radeon Pro 580X MPX Module with your Mac Pro (2019) - Apple Support

Learn about the many different display setups you can use with your MPX Module on your Mac Pro (2019).

support.apple.com

View attachment 2010305

In order to support 2 external 5k's, the 580X would need to be able to drive 3 5k's total, which is 4.4 x 10^7 pixels. That's less than what's listed in both the 2nd bullet (2 x 5k + 2 x 4k = 4.6 x 10^7) and the 3rd (6 x 4k = 5.0 x 10^7). So based on the above, it at least tentatively qualifies. Of course, a 580X in an MPX module in a Mac Pro will not be the same as a 580X in an iMac (certainly the Mac Pro offers better cooling; and Apple may have configured the GPU in the MPX module for a higher clock speed).

That's for a 580X MPX module for the Mac Pro 2019.
The difference is that the MPX module has connections to the two Thunderbolt controllers of the Mac Pro 2019 which allows 4 displays from them and 2 HDMI displays from the HDMI ports of the MPX module.
Your iMac only has one Thunderbolt controller so it can only connect two single link SST displays or one dual link SST display.

A dual link SST display uses two DisplayPort connections, one for each half of the display. Examples are: an LG UltraFine 5K display (two DisplayPort connections over a single Thunderbolt cable) or a Dell UP2715K (two DisplayPort cables).

A single link SST display uses only one DisplayPort connection.

The Apple Studio Display and Apple Pro Display XDR have a dual link SST mode and a single link SST mode.

The single link SST modes are limited to HBR2 link rate which requires DSC to get the full resolution/refresh rate. Your GPU doesn't support DSC which means you can do 4K60 10bpc. Maybe you can do 5K30 10bpc. 5K60 6bpc is possible in Windows (Boot Camp) but not macOS without some patches. An LG UltraFine 5K may be able to do 5K39. I don't think I've seen anyone try reduced refresh rate for the Apple displays.

There are single link SST 5K displays that accept HBR3 link rate to get 5K60 8bpc. DSC is required to get 5K60 10bpc but that doesn't matter in your case.

You can get 5K2K displays that support both dual link SST (Thunderbolt) or single link SST.

 

[theorist9]

A dual link SST display uses two DisplayPort connections, one for each half of the display. Examples are: an LG UltraFine 5K display (two DisplayPort connections over a single Thunderbolt cable) or a Dell UP2715K (two DisplayPort cables).

Thanks for your detailed reply! It sounds like you're saying that both the LG UltraFine 5k and Dell UP2715K are MST, except that the LG 5k can accept both SST streams using a single cable. Is that right?

What would happen if I tried to connect both an LG UltraFine 5k and, say, a 2k display? Would only one of them run?

There are single link SST 5K displays that accept HBR3 link rate to get 5K60 8bpc. DSC is required to get 5K60 10bpc but that doesn't matter in your case.

What would those be? I'm not aware of any stand-alone 5k displays other than the three you mentioned (Dell UP2715K, LG UltraFine 5k, and Apple Studio Display). Would my iMac be able to drive two single-link SST 5k's, and would the 8bpc limitation matter if I'm just using them for text work?

OT, but just out of curiosity: For current Apple products (e.g., the Apple Studio Max and Ultra), is the inability to drive the Dell 8k purely a limitation in MacOS, as opposed to a hardware issue?

 

[joevt]

It sounds like you're saying that both the LG UltraFine 5k and Dell UP2715K are MST,

No, I said dual link SST.

MST is two or more streams in a single DisplayPort connection. Some old 4K displays used two streams over an MST connection - one stream for each 1920x2160 half of the display.
https://web.archive.org/web/20150227163311/https://support.apple.com/en-us/HT202856

Dual link SST is two separate DisplayPort connections with no MST involved. The LG UltraFine 5K and Dell UP2715K displays use two SST DisplayPort connections, one connection for each 2560x2880 half of the display.
https://support.apple.com/en-us/HT210205
https://web.archive.org/web/20160304021707/https://support.apple.com/en-us/HT202856

except that the LG 5k can accept both SST streams using a single cable. Is that right?

The Dell can also accept an SST signal for 4K60 or 5K30.

What would happen if I tried to connect both an LG UltraFine 5k and, say, a 2k display? Would only one of them run?

If you connect the LG first, it will run at 5K and the 2K won't connect.
If you connect the 2K first then the LG may do a single SST connection for 4K60 or 5K39.

What would those be? I'm not aware of any stand-alone 5k displays other than the three you mentioned (Dell UP2715K, LG UltraFine 5k, and Apple Studio Display). Would my iMac be able to drive two single-link SST 5k's, and would the 8bpc limitation matter if I'm just using them for text work?

The iiyama PROLITE XB2779QQS-S1 is one example. It's EOL like the Dell UP2715K.
https://iiyama.com/gb_en/products/prolite-xb2779qqs-s1/
There might be some DIY display driver boards that can also do single link SST HBR3 x4 to an iMac 5K display.
https://forums.macrumors.com/threads/diy-5k-monitor-success.2253100/
Maybe you can google to find a 5K display but they're rare. I think there were some other examples. Search MacRumors for iiyama 5K.

OT, but just out of curiosity: For current Apple products (e.g., the Apple Studio Max and Ultra), is the inability to drive the Dell 8k purely a limitation in MacOS, as opposed to a hardware issue?

A limitation in macOS probably. Apple could fix it but they don't. Or they recently tried to fix it but I haven't seen anyone test the result or report success (one person tested on M1 Mac without success but I don't trust M1 to work with anything greater than 6K)
https://forums.macrumors.com/thread...-on-mac-pro-yes-you-can.2309750/post-30961553

 

[theorist9]

A dual link SST display uses two DisplayPort connections, one for each half of the display. Examples are: an LG UltraFine 5K display (two DisplayPort connections over a single Thunderbolt cable) or a Dell UP2715K (two DisplayPort cables).

Of academic interest only, but found another one:

27" HP Z27q - Display

Information about the diagonal size of the display of HP Z27q and its resolution. Comparison with the size and resolution of other HP models and models by other brands. HP Z27q is also known as HP J3G14A4, HP J3G14A8.

www.displayspecifications.com

HP Business Z27q 27" Class 5K UHD LCD Monitor, 16:9, Black - Walmart.com

Buy HP Business Z27q 27" Class 5K UHD LCD Monitor, 16:9, Black at Walmart.com

www.walmart.com

Requires 2 DP cables, like the Dell UP 2715K (I suppose it might be a rebranded Dell...).

 

[joevt]

Of academic interest only, but found another one:

27" HP Z27q - Display

Information about the diagonal size of the display of HP Z27q and its resolution. Comparison with the size and resolution of other HP models and models by other brands. HP Z27q is also known as HP J3G14A4, HP J3G14A8.

www.displayspecifications.com

HP Business Z27q 27" Class 5K UHD LCD Monitor, 16:9, Black - Walmart.com

Buy HP Business Z27q 27" Class 5K UHD LCD Monitor, 16:9, Black at Walmart.com

www.walmart.com

Requires 2 DP cables, like the Dell UP 2715K (I suppose it might be a rebranded Dell...).

I should have linked HT206587 which does mention the Dell and HP displays by name.
https://web.archive.org/web/20191123020413/https://support.apple.com/en-us/HT206587

 

[theorist9]

I should have linked HT206587 which does mention the Dell and HP displays by name.
https://web.archive.org/web/20191123020413/https://support.apple.com/en-us/HT206587

OK, I've abandoned the idea of driving two 5k's with my iMac.

But could I instead drive a total of three external displays (one more than the specs allow) by driving a single 4k (3840 x 2160) with one TB port, and daisy-chaining, say 2 x WUXGA (2 x 1920 x 1080) with the other?

More generally, if I could daisy-chain, what's the maximum number of pixels I could drive @10 bpc, 60 Hz with the 2nd, daisy-chained port? Given that HBR2 has a maximum video data rate of 17.26 Gb/s, I calculate:
(17.26 x 10^9 bits/s) /(60 frames/s)/(30 bits/pixel) =9,588,889 pixels/frame.

Is that correct? If so, it means I could do 2 x 2560 x 1600, but couldn't do 4k + 1920 x 1080.

 

[joevt]

OK, I've abandoned the idea of driving two 5k's with my iMac.

You could run two LG UltraFine 5K at 39Hz each.

But could I instead drive a total of three external displays (one more than the specs allow) by driving a single 4k (3840 x 2160) with one TB port, and daisy-chaining, say 2 x WUXGA (2 x 1920 x 1080) with the other?

Daisy chain means MST. macOS doesn't support MST for multiple displays.

There are adapters that can make two displays appear as one display.
https://www.matrox.com/en/video/products/other#th2go

Or you can try DisplayLink adapters which use USB instead of a GPU connection.
https://www.synaptics.com/products/...field_displaylink_category_value=usb_adapters

More generally, if I could daisy-chain, what's the maximum number of pixels I could drive @10 bpc, 60 Hz with the 2nd, daisy-chained port? Given that HBR2 has a maximum video data rate of 17.26 Gb/s, I calculate:
(17.26 x 10^9 bits/s) /(60 frames/s)/(30 bits/pixel) =9,588,889 pixels/frame.

Is that correct? If so, it means I could do 2 x 2560 x 1600, but couldn't do 4k + 1920 x 1080.

MST will work only in Windows. In Windows, you can select different bpc and encoding options using the Radeon software.
I like to think of the bandwidth limit in terms of pixel clock because you need to add pixels for the horizontal and vertical blanking time of the display. You can allow more pixels by reducing bpc, or using chroma sub sampling, or lowering the refresh rate.

For HBR2 link rate, the limits are:
30bpp 10bpc: 576MHz
24bpp 8bpc: 720MHz
20bpp 4:2:2 10bpc: 864MHz
18bpp 6bpc or 4:2:0 12bpc: 960MHz
16bpp 4:2:2 8bpc: 1080MHz
15bpp 4:2:0 10bpc: 1152MHz
12bpp 4:2:0 8bpc: 1440MHz.

But the 580X should be able to support HBR3:
30bpp 10bpc: 864MHz
24bpp 8bpc: 1080MHz
20bpp 4:2:2 10bpc: 1296MHz
18bpp 6bpc or 4:2:0 12bpc: 1440MHz
16bpp 4:2:2 8bpc: 1620MHz
15bpp 4:2:0 10bpc: 1728MHz
12bpp 4:2:0 8bpc: 2160MHz

Unless the 2019 has an Alpine Ridge Thunderbolt controller instead of a Titan Ridge controller which would be strange since the Mac mini 2018 has Titan Ridge? Or maybe Apple limited the link rate of the 580X? I'm not sure because the tech specs says 4096x2304 8bpc which is a HBR2 limit but maybe this refers to known 4096x2304 displays such as the original LG UltraFine 4K (USB-C only version).


CVT-RB2 and HDMI timings:
1080p60 = 133MHz (148.5MHz HDMI)
1440p60 = 235MHz
4K30 = 258MHz (297MHz HDMI)
5K30 = 456MHz
4K60 = 522MHz (594MHz HDMI)
6K30 = 628MHz
5K60 = 924MHz
8K30 = 1020MHz (1188MHz HDMI)
6K60 = 1273MHz (XDR is 1286MHz)
8K60 = 2069MHz (2376MHz HDMI)

OT, but just out of curiosity: For current Apple products (e.g., the Apple Studio Max and Ultra), is the inability to drive the Dell 8k purely a limitation in MacOS, as opposed to a hardware issue?

Apple added Dell UP3218K 8K60 support to Ventura beta: #287 but you need a Mac Pro 2019 or a Intel Mac with a Lilu/WhateverGreen patch. I don't know about Apple Silicon Macs.

 

[theorist9]

There are adapters that can make two displays appear as one display.
https://www.matrox.com/en/video/products/other#th2go

Or you can try DisplayLink adapters which use USB instead of a GPU connection.
https://www.synaptics.com/products/...field_displaylink_category_value=usb_adapters

Thanks for the tip!

Daisy chain means MST. macOS doesn't support MST for multiple displays.

MST will work only in Windows.

What about this, from LG?:

Computer Monitors: Gaming, UltraWide, 4K, OLED | LG USA

Shop Computer Monitors from LG Electronics. Make the Most Out of Your Work or Gaming with the Right LG Monitor. Explore 4k, Curved, Gaming, UltraWide Monitors & More!

www.lg.com

Also, I vaguely recall an Apple Support article about connecting MST displays (but I can't find it anymore). So does MacOS support MST for non-daisy-chained displays?

Apple added Dell UP3218K 8K60 support to Ventura beta: #287 but you need a Mac Pro 2019 or a Intel Mac with a Lilu/WhateverGreen patch. I don't know about Apple Silicon Macs.

Nice. I'm thinking that its 280 ppi could make for a small UI. Do you know if Apple will offer exact 3x integer scaling for that monitor (7680 x 4320 => 2560 x 1440), which would give the same UI size as 187 ppi at 2x? It would be a shame if they didn't since, with non-integer scaling, you'd be losing a lot of the beautiful sharpness you're paying for.

Unless the 2019 has an Alpine Ridge Thunderbolt controller instead of a Titan Ridge controller which would be strange since the Mac mini 2018 has Titan Ridge? Or maybe Apple limited the link rate of the 580X? I'm not sure because the tech specs says 4096x2304 8bpc which is a HBR2 limit but maybe this refers to known 4096x2304 displays such as the original LG UltraFine 4K (USB-C only version).

Not sure...

https://forums.macrumors.com/threads/imac-2019-titan-ridge-tb3.2196728/

 

[joevt]

What about this, from LG?:

Computer Monitors: Gaming, UltraWide, 4K, OLED | LG USA

Shop Computer Monitors from LG Electronics. Make the Most Out of Your Work or Gaming with the Right LG Monitor. Explore 4k, Curved, Gaming, UltraWide Monitors & More!

www.lg.com

Thunderbolt daisy chain is different than MST. You can connect two displays to a Thunderbolt port if each display uses only one of the two DisplayPort connections that a Thunderbolt port can provide.

Also, I vaguely recall an Apple Support article about connecting MST displays (but I can't find it anymore). So does MacOS support MST for non-daisy-chained displays?

Yes, macOS supports MST for old 4K displays that used a separate stream for each 1920x2160 half of the display. Those displays were discussed above #6
Support for those old 4K MST displays might be broken in Apple Silicon.

Nice. I'm thinking that its 280 ppi could make for a small UI. Do you know if Apple will offer exact 3x integer scaling for that monitor (7680 x 4320 => 2560 x 1440), which would give the same UI size as 187 ppi at 2x? It would be a shame if they didn't since, with non-integer scaling, you'd be losing a lot of the beautiful sharpness you're paying for.

I don't know what the 3840x2160 HiDPI mode looks like on the Dell. Maybe since it's so sharp, the small UI might be tolerable? Plus it's 32 inches so it shouldn't be too bad. Just move your face closer to the display
I wonder if the arbitrary integer scaling code still exists in macOS like it did back in Mac OS X 10.5 Leopard. Then a patch might be able to add x3 modes such as 2560x1440 HiDPIx3 mode. Maybe 1920x1080 HiDPI x4 would be fun.

Not sure...

https://forums.macrumors.com/threads/imac-2019-titan-ridge-tb3.2196728/

Looks like Titan Ridge. Do you have any DisplayPort 1.4 displays or dongles or MST hubs?
Then you could run AllRez to dump DPCD registers which will show the DisplayPort link rate and lanes (which Apple should put in System Information.app along with MST topology, dongle capabilities, and DSC/FEC support).

 

[theorist9]

I wonder if the arbitrary integer scaling code still exists in macOS like it did back in Mac OS X 10.5 Leopard. Then a patch might be able to add x3 modes such as 2560x1440 HiDPIx3 mode. Maybe 1920x1080 HiDPI x4 would be fun.

I looked into this further, and the developer of one of the Mac dispaly scaling apps told me that 3x scaling would do no good, because of limitations in how MacOS works. Specifically, he said that, because HiDPI on MacOS is limited to a 2x framebuffer, 3:1 scaling would result in an undersampled desktop, meaning that beautiful 8k clarity would be wasted. Are you saying that, with arbitrary integer scaling, it would be straightforward for MacOS to offer a 3x framebuffer?

 

[joevt]

I looked into this further, and the developer of one of the Mac dispaly scaling apps told me that 3x scaling would do no good, because of limitations in how MacOS works. Specifically, he said that, because HiDPI on MacOS is limited to a 2x framebuffer, 3:1 scaling would result in an undersampled desktop, meaning that beautiful 8k clarity would be wasted. Are you saying that, with arbitrary integer scaling, it would be straightforward for MacOS to offer a 3x framebuffer?

Yes. Text and vector graphics would be perfect. Only bitmap graphics would have issues since a developer would likely only provide 2x scaled bitmap images in their apps.
https://www.tonymacx86.com/threads/adding-using-hidpi-custom-resolutions.133254/post-2175982
https://forums.macrumors.com/thread...an-early-intel-recently.2285317/post-29870126