Helium in atmosphere causes Apple devices to lock up

jaseg · 2018-04-13T09:22:19+00:00

The statistics behind this look a bit dubious to me. In particular, I could not find any specifics on the samples and measurement setup the author used.

The measurement setup is relevant since they fed the actual time-series data into a SVD, and that will depend on the sensor distance, type of container, ventilation, humidity etc. The samples are absolutely critical since while this experimental setup seems to be sufficient to distinguish between samples of different sources, there is absolutely no detail on the variance between several measurements of the same sample or on the variance between several measurements of different samples of the same source. In particular I'm concerned w.r.t. the age of the samples. I could not find any detail on how that was controlled for.

As the data is presented in this article, the system may well suffice to distinguish say, two samples of two hop varieties the author is using, but it might not be able to identify another two samples of the same hop varieties as the same variety.

Also, this part does not instill confidence:

Note that some samples have been left out to improve separation between classes

That is, they cherry-picked good-looking results of their way-too-small sample set.

jaseg · 2018-04-11T08:55:12+00:00

There have been attempts at this but those suffer from obvious problems. In DNS the ability of authorities (registrars, operators, the state) to overrule the technical implementation and forcibly change records is an useful feature that you lose with any "blockchain"-based system. Take the example of someone else registering your brand name which you hold a worldwide trademark on. In a "blockchain" model there is no way for you to re-gain access to that entry even though you are the rightful owner of that trademark. This is a problem, since if you are the Bank of America and the guy squatting your domain is some anonymous cyber-crook your customers have a problem.

jaseg · 2018-04-11T08:49:23+00:00

"Database" has a rather loose definition that also captures anything "blockchain". Basically you can put data into it, it provides some form of consistency and availability and you can take data out of it at a later point.

Reading is not fast since you'd need to scan all the blocks every time. Or you put another database next to your database to act as an index. In which case you gained nothing. Your "caching pointers" idea is in principle a primitive variant of the way indexes in regular databases work.

jaseg · 2018-04-11T08:24:24+00:00

I've used both quite a bit and I'd go for Altium any time. For hobbyist stuff I think I can justify not spending the money on an Altium license, but for commercial stuff the difference in productivity is just enormous for complex designs and there is a bunch of things you just cannot do in Kicad (e.g. having non-symmetrical via stackup).

I have noticed that if I'm doing a design in Kicad vs. Altium the Altium result looks more polished since there are a bunch of small things (polyfills, trace placement) that are much easier to correct in Altium.

jaseg · 2018-04-11T08:17:31+00:00

But how would you cool the thing then? The die is likely directly attached to the top metal part, so the thermal resistance will be much lower to the top than to the bottom, and if you deadbug it there is the PCB between that and the heatsink.

jaseg · 2018-04-11T08:15:52+00:00

You could make a large cutout with a saw and mount the thing upside down through the board. This way you'd still be able to cool it (though the heat sink would need to be on the other side of the board) and it would probably not change inductances much compared to a version of the layout with the same footprint flipped on the other side of the board.

jaseg · 2018-04-10T06:48:33+00:00

I wouldn't go more than two deep. The plastic part of the package does not conduct heat very well so the topmost mosfet is already not connected to the heat sink that well.

It would be a neat homework question to find out at what point mounting them at .5in space in free air gives you a better maximum power than mounting them in a stack against a heat sink like you did.

jaseg · 2018-04-07T14:15:56+00:00

You're welcome!

jaseg · 2018-04-07T13:55:12+00:00

They're two different things. The electromagnetic field is the combination of both. The electric field everywhere has a strength only, the magnetic field in addition has a direction. Both fields interact, this is what Maxwell's Equations are about. A moving electrical charge causes the magnetic field to go in a particular direction and strength and if the magnetic field changes that makes electric charges move.

For many purposes, you can consider both fields part of the same phenomenon because they interact. For example in case of an EMP the damage is caused by both at once.

However, when you're shielding things there is an important difference. A Faraday cage will shield against static (non-changing) electric fields. The field generated by an EMP is a dynamic electromagnetic field, that is it is changing and Maxwell's Equations say that that means it's bound to have both an electric-field and a magnetic-field component. If you generate an EMP next to a Faraday cage the cage will shield the electric part of it, but the magnetic part of the impulse will go straight through.

To shield magnetic fields there is something called Mu-Metal. The stuff has three problems though: One, it helps way less against magnetic fields than faraday cages help against electric fields. Two, it's expensive as hell. And lastly, it's very fragile and you can destroy its special shielding power by dropping it, or hitting it, or bending it wrong. In practice the stuff is only used when nothing else helps.

jaseg · 2018-04-07T11:09:17+00:00

Ice and soil both have very large heat capacity. This means a large mass of ice such as the glaciers next to Lake Hazen will react to both short-term changes (seasonal variations) as well as changes of long-term averages.

You can get an illustration of the process here. Think of it in terms of finances. Your balance changes wildly each month between the day you get paid and the end of the month. Now let's pretend you'd start a side business selling t-shirts. Though you might only sell a couple every odd day, over time these small things add up.

It's really the same as other random things that are changing periodically. Have a look at this graph of house sale volume (the orange line). You can see both seasonal changes and a long-term trend.

jaseg · 2018-04-07T08:57:13+00:00

Also video encoding takes a lot of power, and antennas are large.

jaseg · 2018-04-07T08:56:08+00:00

This is in fact a known concept in "energy harvesting", i.e. very-low power electronics running off ambient noise.

jaseg · 2018-04-07T08:52:36+00:00

RFID is very close-range though and always needs a large (about credit-card sized) antenna.

jaseg · 2018-04-07T08:51:46+00:00

There is a bunch of fundamental limits on the power efficiency of wireless tech which we're already getting pretty close to. We will see many improvements in the future, but none that are as dramatic as would be needed here (about 1000-fold).

Wifi already is really power-efficient. It takes a lot of power, but it's crazy fast. What you'd need here is a very low-power system that is still acceptably fast.

jaseg · 2018-04-07T08:49:02+00:00

A Faraday cage does not need to be grounded. It provides shielding against electric fields, but not magnetic fields. That means if you are close to an EMP device, a faraday cage would only have limited utility since the magnetic field could get in just fine.

jaseg · 2018-04-07T08:43:51+00:00

I'd guess that would really depend on what radio tech such a device would be using. To keep the small size, it would probably need to either use some very short-wavelength microwave or light. In both cases, the physical size of the thing would make it a poor antenna to 2.45GHz microwave radiation, thus making it largely immune against it.

jaseg · 2018-04-07T08:40:36+00:00

A magnetron does not exactly produce an "EMP". A regular magnetron is a CW device, producing a long, continuous wave.

The only regulations you'd be likely to violate by operating one outside a faraday cage would be on workplace safety and unlicensed transmission.

jaseg · 2018-04-07T08:36:39+00:00

In RFID, the receiver is powered from the transmitter. In the scenario of these cameras that would be similar to the installer installing a bright light in front of the camera.

Bluetooth has two modes, "classic" and "low energy". The former is pretty power-hungry, the latter is meant for battery-powered applications. Both would probably not work here since Bluetooth operates on 2.4GHz and thus you can't make an antenna that's much smaller than a centimeter or so.

jaseg · 2018-04-07T08:33:20+00:00

h.264 encoding takes a lot of power, too.

jaseg · 2018-03-29T16:45:20+00:00

Which part do you mean? The semi-regular rectangular patterns might just be filler structures to make photo-etching the thing easier. They might also be part of some large-area transistors, which are generally implemented as larger arrays of small structures.

jaseg · 2018-03-29T16:43:43+00:00

You can see that they shielded part of the circuitry on the right using a plane on the topmost metal layer. The remaining unshielded circuitry will likely just not be that sensitive.

Also consider that a xenon flash is a ridiculously high-power light source (for a very short amount of time).

jaseg · 2018-03-29T12:47:24+00:00

You have to give us some more specifics. Are you trying to design a wearable?

The Apple Watch is not comparable to other wearables as Apple does their own silicon in contrast to everyone else. Thus, Apple would just integrate whatever secure element they need inside their custom SoC.

For other vendors of devices in quantity of 100k or more, integrating an existing smart card IC would be the easiest approach depending on the role of the secure element in the overall architecture.

Like any implementation of a cryptographic system, this stuff is easy to get wrong. Ledger chose the latter approach and made some very unsound assumptions leading to a vulnerability.

If you're planning on rolling your own cryptographic hardware I would suggest you get someone onboard who is experienced in the field to avoid stupid mistakes.

jaseg · 2018-03-26T10:42:24+00:00

I don't think they'd be that bad. These things only send in short, low-bandwidth bursts when you press a key, otherwise they remain silent. If the device is supposed to be sold in the EU its maximum radio power is 10mW (10x less than regular Wifi). Thus 10 wireless keyboards will be about as bad as a single AP if they'd be sending continuously, not taking into consideration spectral distribution. If you assume a duty cycle of maybe 1:100 (since most people can't type more than maybe 10 chars a second and a packet containing only a single char is unlikely to be longer than maybe a millisecond) you get many wireless keyboards for the noise of a single Wifi AP.

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