Microsoft's HoloLens secret sauce: A 28nm customized 24-core DSP engine built by TSMC Microsoft today revealed a first look at the inside of its Holographic Processing Unit (HPU) chip used in its virtual reality HoloLens specs. The secretive HPU is a custom-designed TSMC-fabricated 28nm coprocessor that has 24 Tensilica DSP cores arranged in 12 clusters. It has about 65 million logic gates, 8MB of SRAM, and a …
So it's "up to" 10W for the DSP, plus something for the CPU, sensors, and the display. Let's say 15W altogether. This is not an insubstantial amount of heat to dissipate for a fan-less, head-mounted device. My headlamp at full power has roughly the same heat output (8-10W), and it comes with a rather conspicuous heat sink.
I am curious how the hololens dumps the heat? Or am I supposed to use it for a few minutes, then stand very still for half an hour while it cools down?
But your headlamp concentrates that heat at one spot on your head. With a wraparound design like Hololens, they can spread out the heat generators so instead of one 10W hotspot, it's several 2-3W spots spread around your head; you're less likely to feel several smaller hotspots. Plus with smaller hotspots, it's easier to find ways to dissipate the heat through thermal conduits and so on.
There's probably a few reasons why the CPU isn't merged with the DSP as you suggest. Firstly, it's developer kit, so it isn't quite product optimised for mass production - an all-in-one SoC would be more expensive and take a little longer to develop (though cheaper down the line when mass production occurs). A separate CPU and DSP means if their maths were wrong, they could change the CPU for something meatier without too much redesign. The marketing drones also have the opportunity to offer more powerful models for those who want to splash even more cash. Though dissipating the heat from a 24-core Xeon might be an issue...
Passive cooling means it'll be drawing cool air in through vents at the bottom, as warmed air rises through vents at the top.
Your skin does offer a better thermal sink than air, but only where the incoming air is warmer than your skin temperature, and only if the heat-producing elements are not be placed directly in contact with your skin. In reality, what's against your head will be the plastic shell, then an air gap, then heat-source.
> Maybe they should have just gone for an ARM-based SOC instead of a custom thing with an Atom on the side, Windows 10 is supposed to be cross-platform after all.
Quite a few of the software partners are used to developing for x86:
http://www.winbeta.org/news/microsoft-announces-long-list-hololens-partners-new-use-case-scenarios
Not sure why you suggest an ARM-based chip in place of this custom DSP - even phones use GPUs that aren't ARM-based.
The DSP will be doing more work when its 'view' changes - i.e when the wearer moves or rotates their head. When the wearer moves or rotates their head, there will be more airflow.
I'm assuming that Hololens production won't ramp up until MK II or III or whatever - so there's some scope to fab at process sizes smaller than 28nm.
<i>Since your skin offers a far better thermal path than open air, almost all of its heat will go to your head.</i>
Not necessarily. This would be true if the chips were pressed directly against your skin, but all Microsoft has to do is use something that's thermally insulating between you and the device and the problem is solved. Since most VR devices have some sort of padded interior, it's not difficult to solve the problem.
"but all Microsoft has to do is use something that's thermally insulating between you and the device "
Suggest a tinfoil layer with 50 mm of good insulation. Great Balaclava, fetish mask and means of cooking your head even without the electronics. What could be nicer !
Whether you use an insulating layer (like an air gap) between it and your head, or use metal to try to concentrate the heat elsewhere to somehow "radiate" it out, heat still follows paths that are more thermally conductive over those that are less. Air is a very poor conductor of heat.
Heat simply doesn't radiate well to the air unless it is moving fairly quickly (hence fans) Passive cooling with vents on top and bottom won't do much, nor will your head movements. If the heat can't get to your head due to an insulating air gap, and can't escape the device very well due to lack of moving air, then it will get hotter and hotter until it is so hot that even passive cooling, slow moving air and yes an insulating air gap between it and your (much cooler by comparison) head can be more easily crossed because of the larger temperature difference.
If it was wired instead of wireless they could run coolant through it, which could be radiated to the air (with a fan) elsewhere. It could also be a lot lighter since it wouldn't need an onboard battery. The problem is, of course, who the hell wants something like that if it has wires? I wonder if a compromise device would work better - you wear a little backpack or hip bag that contains the battery, CPU/DSP, etc. that is wired to your much smaller/lighter headgear which would only have a display so would output very little heat.
"If the heat can't get to your head due to an insulating air gap, and can't escape the device very well due to lack of moving air, then it will get hotter and hotter until it is so hot that even passive cooling, slow moving air and yes an insulating air gap between it and your (much cooler by comparison) head can be more easily crossed because of the larger temperature difference."
You forget the thermal insulating layer between the air, the CPU, and the housing, which will be even more difficult to penetrate since it will be designed for the purpose. And you're talking as if 10W, spread all around your head like a halo, is a big thing. It's already been mentioned the brain generates 20W on average already. Plus even if air is a poor thermal conductor, it's at least greater than zero.
For a point of comparison, the average human brain is supposed to consume about 20W of power, about 20% of the body's 100W. So it's not that unreasonable an amount of heat for the Hololens to dissipate.
Unless your brain stops working while you're playing, it means doubling the amount of heat your head must dissipate. I suppose couch potatoes playing Hololens games will feel like they are getting a workout if their head gets sweaty, and ignore the fact that's the only part of their body sweating :)
"Unless your brain stops working while you're playing, it means doubling the amount of heat your head must dissipate."
How is +10W double 20W (this adds up to 30W, a 50% increrase, vs. 40W)? Plus this is exposed to the outside so at least has other avenues to vent heat.
The design has 2 "rings" - an inner and an outer ring. The inner ring is lined with soft foam and is adjustable to your head. The outer ring (small gap in the back to allow access to inner ring tightener) contains the electronics - where the heat will be generated.
It is quite obvious from the images: https://www.microsoft.com/microsoft-hololens/en-us
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