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

Discussion in 'iMac' started by Xteec, Aug 17, 2017.

  1. Xteec, Aug 17, 2017
    Last edited: Aug 17, 2017

    Xteec macrumors regular

    Joined:
    Sep 21, 2012
    Location:
    Australia
    #1
    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.
     
  2. Brookzy, Aug 18, 2017
    Last edited: Aug 18, 2017

    Brookzy macrumors 601

    Brookzy

    Joined:
    May 30, 2010
    Location:
    UK
    #2
    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. :)
     
  3. Xteec thread starter macrumors regular

    Joined:
    Sep 21, 2012
    Location:
    Australia
    #3
    Yeah, that's a good point...thanks for the response.

    I guess there's only one way to be sure... haha
     
  4. Xteec thread starter macrumors regular

    Joined:
    Sep 21, 2012
    Location:
    Australia
    #4
    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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