4k60Hz on 2016 Macbook - the issue is not USB 3

Discussion in 'MacBook' started by Vladie, Apr 20, 2016.

  1. Vladie macrumors newbie

    May 5, 2011
    I am starting this thread in the hopes that those with more correct information than what has been shared to date will chime in and educate us.

    I have seen a number of posters allege that the reason the new 2016 Macbook cannot do more than 4k30Hz is because it has USB 3.1 Gen 1 (as opposed to Gen 2) and that, because Gen 1 is limited to 5Gbps, it cannot carry anything more than 4k30Hz.

    That is wrong.

    USB 3.1 Gen 1 is a protocol for transferring data. But when you connect a monitor to the rMB through the USB C port, you are not transferring data using the USB protocol. You are transferring data using the DisplayPort protocol using what is known as USB Alternate Mode.

    In fact, I think 4k30Hz is already 8+Gbps.

    Somebody please correct me if I've messed all this up.

    But if I haven't, then I think the question of whether future software can enable 4k60Hz output on the new 2016 rMB is a valid question that cannot be easily dismissed by simply pointing at USB bandwidth limitations.
  2. lightup macrumors newbie

    Nov 11, 2014
    Wow if this is right I'll buy one right away.

    I also agree with you. It seems there are non thunerbolt3 just usb c laptops there like chromebook pixel..and more. And it supports 4k@60hz output.
  3. PTLove macrumors 6502

    Sep 12, 2014
    Hmm, myself have been parroting the bandwidth issue myself, but you appear correct. Its definitely using the Displayport alternate mode. Im very curious what the technical reason is as well now for the 30fps limitation.

    Per this Anandtech article, it may still be a hardware issue (though I know not what the hardware reason for having only 2 lanes is):


    At this point I have not had a chance to try a 4K@60Hz SST monitor, so I’m unsure whether this is a specific MST incompatibility, cable incompatibility, or if the MacBook doesn’t support 4K@60Hz (Apple for their part does not specify). Given the lack of detection I’m wondering whether Apple only supports up to 2 lanes of DisplayPort (4 are needed for 4K@60Hz), but without further confirmation that’s just a guess. Regardless a low-power device like the MacBook is not a great candidate for driving a 4K@60Hz display, but for the moment we can only confirm the MacBook works with displays up to 4K@30Hz.
  4. boast macrumors 65816


    Nov 12, 2007
    Phoenix, USA
    My guess would be that if there was a separate displayport that connected directly to the processor, you would get the 4k60. But since the displayport goes through USB-C, which is serial and doesn't have dedicated transmission pairs for display, the controller for switching between data and display is not fast enough to handle the 4k60.

    I can't seem to find a block diagram for the Intel core m processors to get an idea of how things could be hooked up.
  5. MrX8503 macrumors 68020

    Sep 19, 2010
    The only thing that's holding me back is the MacBook's inability to drive a SST 4K 60hz display.

    Most likely will get a rMBP instead.
  6. SR71 macrumors 68000

    Jan 12, 2011
    Boston, MA
    So then it should be able to do 4K 60Hz, right? It has DisplayPort 1.2 which supports 4K 60Hz, so what else is holding it back?
  7. violet ink macrumors newbie

    violet ink

    Apr 21, 2016
    The CPUs support 4K@60Hz this time, unlike the 2015 CPUs. I've got an order placed for the m7 and I have a USB Type-C to DisplayPort adaptor ready to try it.

    I have high hopes.
  8. NICKXXXXXX macrumors regular

    Oct 9, 2014
    One thing i'm wondering though... Lets say the Apple TV gets 4k support. Would you be able to do 4k through airplay?
  9. zhenya macrumors 603


    Jan 6, 2005
    It is a little bit difficult to suss out exactly what is going on - this is the nature of a highly complex evolving standard that crosses a couple of generations of usb specifications, power delivery, display capabilities, and Thunderbolt capabilities, among others. I have been thinking of hitting up Anandtech on Twitter because I think this is an issue that deserves a full article by them.

    This is my understanding as it sits today.

    The Core-M CPU is capable of driving a display at 4k/60hz. However outputting that display over the USB port present in the MacBook is the limitation. This is indirectly due to the 5Gbps speed limit of the USB 3.1 Gen 1 port present on the MacBook (more on this later). For now, let's look at the pinout of a USB-C receptacle.

    (image courtesy anandtech - http://www.anandtech.com/show/8558/displayport-alternate-mode-for-usb-typec-announced)

    What we can see here is that four lanes are provided for USB Superspeed (5Gbps in this implementation, 10Gbps in Gen 2 implementation due to improved encoding) in Green and Blue in the above diagram. DisplayPort alternate mode is simply an alternate use of some or all of these pins. Here is the key. DisplayPort 1.2 requires 4 lanes to support 4k/60hz. That would leave you with only a single USB 2.0 lane for all of your USB connections. It's my opinion that Apple has made the not-unreasonable decision that they will only allow DisplayPort alternate mode to occupy two of these lanes, giving us 4k/30hz while retaining full simultaneous usb 5Gbps capability.

    USB 3.1 Gen 2 will solve this issue, indirectly, by allowing more efficient use of these lanes, as they will be capable of 10Gbps, meaning that 4k/60hz will occupy only two of the four pairs, leaving two full pairs for 10Gbps simultaneous data transfer.
  10. zhenya macrumors 603


    Jan 6, 2005
    I've thought about this a bit more, delved a little further into the specs, and I think I'm wrong about my last point. I don't see the change to USB 3.1 Gen 2 (10Gbps) as being enough in and of itself to allow for 4k/60hz video over a single USB-C cable. Part of the technical specification of DisplayPort states 'Forward link channel with 1 to 4 lanes.' Bandwidth per channel is dependent on the DisplayPort standard utilized. For v1.2 that's 4.32Gbps per channel or 17.28Gbps for a 4-lane link. 4k/60hz requires ~14Gbps of bandwidth (depending on timing and color data specifics). Thus, we need chipset support for DisplayPort 1.3 in order to get 4k/60hz 4:4:4 over just two lanes of the four available in a single usb-c connection. v1.3 specifies a bandwidth of 8.1Gbps per lane, providing enough bandwidth for that 4k/60hz display over 2 lanes, leaving 2 more lanes for USB 3.1 (either Gen 1 5Gbps or Gen 2 10Gbps).

    Since there are few if any DisplayPort 1.3 laptop controllers in the wild, the only option we have at the moment to get 4k/60hz over a single usb-c connection is via Thunderbolt 3. The way this is done is that the Thunderbolt controller encapsulates the DisplayPort signal in the Thunderbolt datastream, effectively moving the DisplayPort signal nominally requiring 4 lanes over just 2 lanes utilized by Thunderbolt at a higher bandwidth.

    In the end, we are right back where we started; if you want 4k/60hz over a single usb-c connector, at the moment you need Thunderbolt 3, which means the Alpine Ridge controller.
  11. Serban Suspended

    Jan 8, 2013
    for 4k@ 60 hz you need current mac pro or next gen macbook pro, or next gen macbook with kaby since it has by default thunderbolt 3
  12. curmudgeonette macrumors 6502

    Jan 28, 2016
    Or you need a cable that uses all four lanes for DisplayPort 1.2 signaling. That means no USB3 in the cable - but USB2 would still be available. Thus there could be a USBC 4K Cinema Display that powers the MacBook, has a hub to attach a keyboard and mouse, and provides audio out over the USB2 connection.
  13. izzyfanto macrumors regular


    Nov 22, 2011
    Very thorough and helpful, makes sense to me. Thanks!
  14. zhenya macrumors 603


    Jan 6, 2005
    Yes, exactly. So in a device with two usb-c ports, this would be possible - one for display, one for usb 3.
  15. toddzrx macrumors 6502a


    Nov 20, 2012
    Interesting discussion. Question though: where would you see the difference between 30 and 60 Hz? I would think this matters for gaming or if you're watching action sequences of a movie that's shot in 4K (rare)? Why does 60 Hz matter so much?
  16. NICKXXXXXX macrumors regular

    Oct 9, 2014
    My advice would be too look at the difference on youtube. It's quite noticeable...
  17. zhenya macrumors 603


    Jan 6, 2005
    It actually matters a lot just for regular computer use. The smooth scrolling we are used to, smooth mouse cursor movements, even fast typing starts to lag behind on 30hz. All of those things start to become noticeably jittery below about 50hz.
  18. PatriotInvasion macrumors 68000


    Jul 18, 2010
    Boston, MA
    Thorough and helpful, thanks. In the end, people need to realize that Apple is targeting this MacBook at students, and business travelers who need a super thin and light Mac on the go. These buyers are highly unlikely to know what 4K is, let alone plop down $500+ on a 4K display for desk use. I think us 4K display watchers are just going to have sit back and wait for the MacBook Pros at WWDC (hopefully) which should have Thunderbolt 3 and the graphics horsepower to handle high res displays with ease.

    Unfortunately, no machines released in 2016 will be Apple to power a mythical 5K Thunderbolt Display using SST over a single USB-C/Thunderbolt 3 cable. That dream setup is going to have to wait until 2017 when Thunderbolt 3 and Intel chips support DisplayPort 1.3. A MacBook Pro powering a Retina 5K Thunderbolt Display over 1 port that also charges the Mac is the perfect setup but we're just not there yet.
  19. glindon macrumors regular


    Jun 9, 2014
  20. PatriotInvasion macrumors 68000


    Jul 18, 2010
    Boston, MA
    That'll probably be a nice 4K display for a new MacBook Pro that can run it at 60Hz. Pretty sure hooking a new 12" MacBook up to that will lock you in to 30Hz at 3840x2160.

    That said, that monitor is one of the first with USB-C, but I think what many really want is a Thunderbolt 3/USB-C display that also sports an additional Thunderbolt 3/USB-C out port for connecting additional Thunderbolt 3 devices. This is probably the display Apple will release once it's possible to have a DisplayPort 1.3 implementation that allows for 5K over SST at 60hz.

    The reason they haven't done and probably won't do a 4K Thunderbolt Display is because HiDPI mode of 1920x1080 on a 27" display would result in less than idea real estate and only 163 PPI. That PPI isn't worthy of the "Retina display" marketing label. A 5120x2880 5K display like the iMac maintains the same 2560x1440 real estate of the 27" Thunderbolt Display while providing 217 PPI.
  21. Narcaz, Apr 24, 2016
    Last edited: Apr 24, 2016

    Narcaz macrumors 6502

    Jul 18, 2013
    You don't need a next gen macbook pro for this. From https://support.apple.com/en-us/HT202856 :

    With OS X Yosemite v10.10.3 and later, most single-stream 4K (3840x2160) displays are supported at 60Hz operation on the following Mac computers:
    MacBook Pro (Retina, 13-inch, Early 2015)
    MacBook Pro (Retina, 15-inch, Mid 2014)
    Mac Pro (Late 2013)
    iMac (27-inch, Late 2013) and later
    Mac mini (Late 2014)
    MacBook Air (Early 2015)

    And MST displays are supported by the
    MacBook Pro (Retina, 15-inch, Late 2013) and later
    Mac Pro (Late 2013)

    You are right, a 27'' 4K display isn't probably enough for the retina label. But a 24" with 3840×2160 has 183 PPI (some oem variants at 23,6'' provide even 187PPI). Viewed from around 30 inches, this should be enough to classify it as a "retina", where most people cannot make out individual pixels. And Apple could have offered a 24'' 4K Retina display since late 2013 (see above). I wonder what was holding them back, because the Thunderbolt Display wasn't really top notch back in days (TB1/USB 2.0?!). In 2015 it could have been easily replaced with this imaginary 24'' TB Display, because most of the Macbook lineup (except the 12'') would have supported it.

    Your mythical 5K Thunderbolt display with DP 1.3, that allows 5K over SST at 60HZ is probably still 1-2 years away. Canonlake could make it possible in 2017 (or later with Intel delays). Afaik Skylake and (the "refined" Kaby Lake) processors will not be able to drive a 5K Thunderbolt Display with SST. They only support MST, which is prone to occasional performance issues. The underlying issue is that both processor lineups sadly lack support for DisplayPort 1.3.

    Edit: I just saw your reply to my post on news thread about the HD 515 GPU and the theoretical 4K 60Hz (via DisplayPort) capabilities. Fanless enclosure isn't a problem. The fanless Surface Pro 4 with Skylake Core M supports 4K/60hz with one display.
  22. InfiniteLoopy macrumors 6502

    Dec 14, 2010
  23. PatriotInvasion macrumors 68000


    Jul 18, 2010
    Boston, MA
    I had been beating that drum a year or so ago, but if you think about it, Apple has always marketed it's displays to the high end of the market, and in 2016, there wouldn't be many high end resolution-conscious pros interested in a 23.6" or 24" display. Hence, they declined to go this route (although I wish Apple would release a consumer level display in that size with a palatable price point - I think they'd sell a bunch of them). If anything, the existing Thunderbolt Display should have been updated in early-2013 with USB 3.0, updated MagSafe 2, iMac-like thinness, and laminated display. They could have at least sold that at $999 with somewhat of a straight face versus what they sell now.

    Is Kaby Lake officially out of the running for DisplayPort 1.3? I haven't been able to find anything definitive about that. If it did support it, there is a very very slim hope we could see Kaby Lake MacBook Pros at WWDC shipping during the mid-summer months along with a 5K Thunderbolt Display, but as you said, the more likely scenario is mid-to-late 2017 for that setup.

    Hmmm, so if the fanless enclosure is not the problem, and the Intel HD 515 GPU/Skylake combo supports 4K at 60Hz, and the MacBook's USB-C port has DisplayPort 1.2, than why on earth can it not do 4K at 60Hz? I'm truly surprised that port on the 2016 MacBook isn't Thunderbolt 3 considering Apple has been Thunderbolt's biggest backer since 2011. Shame. I can only theorize that they think buyers of this machine will have very little interest in external displays and super high speed data transfers so why stuff those in there when they are good upsell traits of the MacBook Pros.
    --- Post Merged, Apr 24, 2016 ---
    I thought this was a very well written review as well. Worth the read for those interested in a fair assessment of the 2016 MacBook.
  24. zhenya macrumors 603


    Jan 6, 2005
    ??? I explained in great detail why the MacBook doesn't do 4K/60hz above. To do so over the single existing usb 3.1 port would limit anything else connected to that port to USB 2 speeds, and Thunderbolt would require an extra daughter-card which is not fitting in the minimalist vision of this notebook.
  25. joscejrod macrumors 6502


    May 16, 2015
    Has someone try to connect new macbook with a 3840x2160 sst display using an usb-c to minidisplay port cable? It should run at 60hz.

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