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Anyone tried 2017 5K with 2 displays: LG 5K and 1440p?

Xteec

macrumors regular
Original poster
Sep 21, 2012
146
71
Australia
Apple's specs say you can plug the LG 5K into the iMac and they should both be fine at 5K.

Has anyone tried adding a 1440p on top of this? e.g. via the other TB3 port (say, into a TB dock that outputs DisplayPort)?

EDIT:

So I did some quick research (prob not very accurate).

Assuming billions of colours means 10-bits per pixel, and I apply a colour depth factor of 1.2 and coefficient of 10

Then (according to Apple):

One 5120‑by‑2880 (5K) external display at 60Hz with support for 1 billion colours
per my calc would be:
5120 x 2880 × 60 × 1.2 × 10
10.62 Gbps per channel
31.85 Gbps total (RGB)

This is within the 40Gbps TB3 limit.

Checking the maths again:

Two 3840-by-2160 (4K UHD) external displays at 60Hz with support for 1 billion colours
per my calc would be:
3840 × 2160 × 60 × 1.2 × 10 (x2)
11.94 Gbps per channel
35.83 Gbps total (RGB)

This is within the 40Gbps TB3 limit.


So what I was proposing would be

((5120 x 2880 × 60) + (2560 × 1440 × 60)) × 1.2 × 10
13.27 Gbps per channel
39.81 Gbps total (RGB)

Very close....but probably over the 40Gbps of TB3 given the maths isn't at all accurate.
 
Last edited:

Brookzy

macrumors 601
May 30, 2010
4,874
5,110
UK
Your logic is sound however unfortunately there is a separate limitation.

While the 2017 iMac has two Thunderbolt 3 ports, these are driven by just a single Alpine Ridge controller (by comparison, the 15" MacBook Pro has two).

Each controller can drive up to two displays - and this is the limitation. The LG 5K counts as two displays because its picture is formed of two nearly 4K images.

There are therefore no DisplayPort lanes left over for any additional displays, despite the total TB3 bandwidth availability not being exceeded.

At least, that is my understanding, but I am not an expert and I would gladly be proven wrong. :)
 
Last edited:

Xteec

macrumors regular
Original poster
Sep 21, 2012
146
71
Australia
Yeah, that's a good point...thanks for the response.

I guess there's only one way to be sure... haha
 

Xteec

macrumors regular
Original poster
Sep 21, 2012
146
71
Australia
I can confirm now it does not work.

The 1440p display is a U2713H. It's connected to a TB2 dock (Elgato) which is connected to the other TB3 port of the iMac.

The iMac prioritises the LG 5K - which works fine.
 
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