rMBP13 Late 2013 with Dell P2415Q, only 40 Hz on 10.9.5

Discussion in 'MacBook Pro' started by Amethyst1, Oct 28, 2015.

  1. Amethyst1 macrumors newbie

    Oct 28, 2015

    this is my first post here. I'm using a 13-inch Retina MacBook Pro (Late 2013) with Intel Iris 5100 graphics running Mavericks (10.9.5) with all available updates and have connected a Dell P2415Q monitor, configured to SST mode, using the supplied mini-DP-to-DP cable. As expected, the system defaults to 3840x2160 at 30 Hz.

    I have attempted to increase the refresh rate by using SwitchResX 4.5.2. However, I can only get up to 40 Hz using CVT-RB timings; aiming for a higher refresh rate causes all screens to go black and I have to hold the power button to turn off the MacBook. 40 Hz is marginally better than 30, e.g. I can see the mouse cursor is a little less laggy/jumpy than before.

    When using Linux, I can get 3840x2160 at 52 Hz or 3520x1980 at 60 Hz using the exact same system, cable and monitor without any problems. These two modes take full advantage of the pixel clock limit which is 450 MHz for Haswell-ULT according to Intel. The monitor's OSD confirms it is operating at these modes when I set them and the mouse cursor movement etc. is much smoother than at 30 Hz (in fact, I see little difference between 52 Hz on the P2415Q and 60 on my other monitors).

    These tests have been performed with the MacBook's internal display being enabled and the P2415Q connected to the Thunderbolt port closest to the MagSafe port, nothing was plugged into the second TB port or HDMI (so two displays in total).

    As I've not tried Yosemite or El Capitan yet, this problem may be due to Mavericks (my experiments have proven it's not a hardware problem) but has someone else come across this oddity?
  2. Amethyst1, Oct 28, 2015
    Last edited: Oct 28, 2015

    Amethyst1 thread starter macrumors newbie

    Oct 28, 2015
    I apologise for replying to my own thread, but I think I've gained an important insight.

    On Linux, the following modes work fine (pixel clock as reported by the utility I used to generate the modes):

    Resolution Refresh Pixel Clock
    3840x2160..52 Hz...460.25 MHz (?)
    3520x1980..60 Hz...449.75 MHz

    Those are the expected results given the pixel clock limit imposed by the hardware's DisplayPort implementation.

    Whereas on Mac OS X 10.9.5, the limit I can reach is substantially lower, as indicated by the following modes I am able to get to work (pixel clock as reported by SwitchResX):

    Resolution Refresh Pixel Clock

    3840x2160..40 Hz...352.06 MHz
    3408x1917..50 Hz...349.81 MHz
    3104x1746..60 Hz...351.56 MHz

    3840x2160 at 41 Hz (resulting in 361 MHz) doesn't work. And while the increased refresh rates are seriously nice, reducing the resolution (but having the screen upscale it) causes text to look blurry. The positive thing is that I can get by with 3104x1746 on 23.8" without turning on HiDPI :)

    So, the question can be rephrased as: why is the pixel clock limit on OS X lower than the hardware's constraints?

  3. RoboWarriorSr macrumors 6502a

    Feb 23, 2013
    If I remember correctly, OS X has an artificial limit, I believe windows should be able to do at least 52 hertz. Since you are on Mavericks, there was a tweak here that increased the clock speed on display port allowing 52 hertz. As for why, it's probably another Apple thing they may take out later, I mean this is not the first time they have done this before.
  4. Amethyst1 thread starter macrumors newbie

    Oct 28, 2015
    Thanks for your reply, I'm aware of the pixel clock patch but thought it only removes the limitation concerning HDMI. However, it seems it may also allow higher clocks via DisplayPort - going to give this a try :)

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