How Apple Could Make a Great 5K Thunderbolt Display

Ever since Apple introduced the 27 inch 5K iMac in 2014, I have been wishing for a standalone version of its display. It’s been 16 months and Apple has yet to update its non-retina Thunderbolt Display. What are they waiting for?

Well, as many have pointed out, there are two difficult technical challenges that Apple must solve in order to allow Macs to connect to such a display. I have an idea as to how Apple could go about solving them but before I tell you about it, let me describe the challenges.

First, a 5K display contains a whopping 14,745,600 pixels. Animating so many pixels smoothly requires a GPU more powerful than can currently be fitted on most notebooks without compromising portability or battery life. In other words, the graphics on a 5K display would likely be very choppy when connected to a MacBook, MacBook Air or MacBook Pro 13". Even a 5K iMac might suffer from middling to bad graphics performance if you connected a second or third 5K display to it.

The second challenge is that even if your Mac had a GPU powerful enough to drive a 5K monitor, you would still need a way to connect one to the other. The only connection standard powerful enough to support screens of that size is DisplayPort 1.3 and Intel does not plan to support it until late 2017. Apple could use Multi-Stream Transport, a technique that uses two DisplayPort 1.2 connections to achieve the desired bandwidth, but it’s a rather ugly hack that comes with its own set of issues.

So? What’s the solution?

First, let’s start with the connection. Thunderbolt 3 is the standard everyone speculates Apple will use for a 5K standalone display, and for good reasons. Thunderbolt 3 manages to pack into one thin cable: two DisplayPort 1.2 connections, up to 4 PCI Express 3.0 lanes, USB 3.1 gen. 2 and up to 100W of power to charge your MacBook without having to connect your power brick. It even has a thin reversible USB-C connector. You should expect Thunderbolt 3 ports on most if not all new Macs released in 2016. Many assume that Apple will use the two DisplayPort 1.2 connections included in Thunderbolt 3 to drive a future 5K display.

I think Apple could do better. What if the display, instead of being driven by the Mac’s graphics hardware, was driven by its own GPU? One that would be located inside the 5K display’s enclosure, like the current 5K iMac. That GPU could be made appropriately powerful for one 5K Thunderbolt Display, would suffer from none of the power constraints that notebooks face and could conceivably be driven, as we’ll see below, by the most portable of laptops.

What about the connection between the Mac and the GPU inside the display? As I’ve stated earlier, Thunderbolt 3 is able to carry multiple PCI Express 3.0 lanes which is exactly how regular video cards connect to the system bus. There wouldn’t be much for Apple to do except to connect the relevant bits.

Now, how many PCI Express lanes are available to the GPU will depend on the particular Thunderbolt 3 chipset. So far, Intel has released two Thunderbolt 3 chipsets: the Alpine Ridge 4C which connects to 4 PCI Express lanes and the Alpine Ridge LP, a low-powered version, that only connects to 2 lanes. Although we can’t know for sure, it’s a good bet that desktops and MacBook Pros will get the 4C chipset while the 12" MacBook will get the LP.

Now, it is true that regular desktop video cards are typically connected by 16 PCI Express lanes while Thunderbolt 3 supports 2 to 4. However, those 16 lanes are mostly there because they were needed in previous iterations of PCI Express. Consider that four PCI Express 3.0 lanes have a combined bandwidth of 31.5 Gbit/s which is more than enough to transfer 5K 60 fps uncompressed video in real time to the video card. In other words, there is enough bandwidth in those four lanes to have most everything calculated by the system and the GPU to act as nothing but a display driver. But none of that is necessary. The whole point of a GPU is to offload all graphics processing from the rest of the system. In normal times, the only thing transiting between the system and the GPU are vector graphics instructions, font specs and the occasional image. There is absolutely no reason that 4 or even 2 lanes can’t provide enough bandwidth for anything but the most demanding video games.

While embedding a GPU in a standalone display is novel, connecting external video cards via Thunderbolt is not. There are plenty of PCI Express Thunderbolt expansion chassis out there and a number of people have successfully used them to drive video cards from their laptops. In fact, last summer, Intel demoed such a Thunderbolt 3 connected external video card. Their setup allowed a laptop to drive two external 4K displays with ease and post reasonably good 3D graphics benchmarks.

What about the cost? I don’t think it would make much of a difference. 5K displays are expensive to manufacture and while GPUs can get pricey too, the Radeon R9 M380 that powers the base 5K iMac is much less expensive than the screen itself.

So there it is, a recipe for a 5K Thunderbolt Display that could be produced today, using a connection standard available today and that could work smoothly with any Thunderbolt 3 equipped Mac, even the ultraportables. As an added bonus, because all of this is done using existing standards, even Wintel computers could connect to this display and join in on the fun given the proper drivers.