## Pre-school mathematics

29.03.2012 20:02

Recently the following question has been circling the Internet and also ended up in my inbox a few times. It's yet another one of those silly exercises that ask you to continue a sequence based on a number of examples. They are a pet peeve of mine and I'll explain why I hate them here once and for all instead of replying to everyone that sent a copy my way.

The logic of this specimen is particularly broken, even putting aside the questionable statement about pre-school children being able to solve it in mere minutes. It's obvious that whoever wrote that is not familiar with the meaning of the equals sign, as 8809 isn't equal to 6. So strictly reading, this exercise gives you 21 false statements. From that it can be simply inferred that the author is looking for another false statement, so ??? can be replaced with any number not equal to 2581.

With that out of the way, let's look at what was probably meant with this notation. You are given N=21 pairs of values (xi, yi) such that:

f(x_i) = y_i \qquad i\in[0 \dots N-1]

And asked to find yN for a given xN:

f(x_N) = y_N

From a purely mathematical standpoint, this problem has an infinite number of solutions. To demonstrate, assume that f is a function that satisfies the condition set above. In that case:

f(x) + g(x)

where

g(x_i) = 0 \qquad i \in [0 \dots N-1]

also satisfies this condition. Note that g is a function that can have an arbitrary value at xN and hence this new solution will give a different value of yN.

Just to make this a little bit more practical, here's an example of a polynomial that goes through N points:

f(x) = \sum_{i=0}^{N-1} (x - a_i) \prod_{j=0; j \neq i}^{N-1} (x - x_j)
a_i = x_i - \frac{y_i}{\prod_{j=0;j \neq i}^{N-1}(x_i - x_j)}

And here is how its graph looks like if you plug in the given 21 values:

At x=2581 it gives the value of around 5.8·1073.

Note that using the formula above and N=22, you can get a polynomial that goes through the required 21 points plus the 22nd that you can choose to your liking.

This is just one example. You could construct functions that meet the requirements from trigonometric or exponential functions in a similar way.

Sure, so this is probably not the solution the author of this question was looking for. You could say that I'm overcomplicating things and that the rule that pre-school children would come up with, based on the shape of the digits the author arbitrary chose to represent these numbers in, is nicer. But there is no such thing as niceness in mathematics (see interesting number paradox).

This is the big difference between mathematics and physics. Our universe has physical laws that for some reason can often be represented in certain mathematical forms. If a liter of water has a mass of one kilogram and two liters a mass of two kilograms it's quite likely that three liters will have a mass of three kilograms and so on. But this is purely based on our experience and beliefs about our environment and not on any purely mathematical basis. Such a prediction, just like the one above and many like it, when stripped of any physical context, does not make any sense.

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## Cutting styrofoam the analog way

14.03.2012 23:47

My dad is an avid model airplane builder and he is currently building a new electrically-propelled glider of his own design. This particular airplane will have styrofoam-core wings and two weekends ago he was trying out a new way of making them.

Such wing profiles are traditionally cut out from blocks of styrofoam with a hot-wire cutter that is pulled by hand over plywood templates. However the width of the cut depends on the speed of the wire as it travels through the material and since you can't manually pull the wire at a perfectly constant speed the wings end up full of little pits and grooves. Of course, nowadays you can get computer controlled cutters that control the hot wire with servos and can cut out any shape with perfect steadiness and without the need for templates.

My dad however went for another approach: he made a purely mechanical device that pulls the wire over the templates with constant speed. The interesting bit here is that the two ends of the wire usually have to travel different lengths through the styrofoam in the same amount of time, depending on the wing taper ratio. He achieved this with an adjustable system of levers, pulleys and ropes that would make for a nice high-school mechanics class demonstration.

The whole thing is powered by gravity and an occasional nudge by hand. In fact, for perfect straight cuts just the weight of the hot-wire cutter is sufficient without additional mechanisms.

As you can see from the (long) video below, it takes one to two minutes to make one cut (and you need several per wing, depending on the number of segments), so it's not the fastest thing around. But it makes up for it with perfectly smooth cuts and if you only make a few wings per year it's perfectly sufficient.

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## Reverse engineering Energycount 3000

12.03.2012 20:52

A while ago Gašper gave me this Energycount 3000 kit from Voltcraft for logging household electrical energy usage, with a wish that he would like to access its measurements from a computer. All this time it's been mostly gathering dust on my desk, but last week I've found some time to give it a closer look and made a few discoveries that are worth reporting.

The box contains two sensors that can be connected between a wall socket and a plug and a tiny battery-powered remote control for reading out the data. The short instruction leaflet explains that the sensors broadcast their measurements through radio every 5 seconds. With a push of the SCAN button you can set the remote control into a 6 second listening mode which catches the transmission and displays it on the small LCD screen.

The remote also has some calculation functions, like predicting the next electricity bill, but is otherwise nothing more than a remote display for the sensors, which apparently do all the logging. This makes sense: the remote control is limited by its batteries and as much functionality as possible is pushed to the wall plug where there is abundant power. The radio link is also obviously unidirectional. The sensors transmit their periodic reports and the remote receives them. All user interaction with the sensor is done through a push button on the sensor itself.

To get the data from the sensor it appears that all I would have to do is to eavesdrop on the transmission. Unfortunately, the box says this device operates on the 868 MHz ISM band, meaning that my 433 MHz receiver was useless. I could get a 868 MHz receiver module for it, but I suspected that these devices use something more complicated than on-off keying, so I looked for other possibilities.

Here is how sensors and the remote look from inside:

As you can see each device has two integrated circuits under a blob of epoxy. I'm guessing one is a microcontroller and the other obviously some kind of integrated ISM band transmitter or receiver. In both the sensor and the remote control they are connected with 5 copper traces. Having recently encountered and worked with chips like the CC1101, my first guess was something like that. I figured the five traces would carry a digital bus like SPI or I2C, so I soldered some wires to the tiny traces on the remote (destroying one of the termination capacitors in the process) and hooked it up to a logic analyzer.

Unfortunately, what I saw there didn't look at all like a synchronous transmission. Out of 5 lines, 2 seem not to carry any useful signals (all I saw there were some transients that looked like glitches). The remaining 3 might act as a digital bus immediately after the microcontroller wakes up the radio chip, but otherwise look like some pulse-width modulated signals for the majority of the 6 seconds when the radio is turned on.

This was a sort of a dead-end until I came across chips like the TH81112. These are ISM band receivers that can be used to receive ASK or FSK transmissions, but are much simpler that system-on-chip products like CC1101. They merely contain a tuner, intermediate frequency and a phase detector and therefore rely on the microcontroller or some other logic to do full FSK demodulation. In hindsight, it makes sense for Energycount to use something like this. It's a relatively cheap product and an expensive general-purpose transceiver like the CC1101 would probably be far too expensive for it.

Update: Gašper found a thread that has some interesting insights, particularly regarding the communication between the radio chip and the CPU. It's possible I was looking at the signals at a completely wrong time scale.

But at this point I didn't bother to mess further with the original receiver. I started up GNU Radio and Fun Cube Dongle Pro and tried to catch the transmissions with that. It turns out fishing out the correct channel in the 868 MHz ISM band is a challenge in itself if you're only limited to the 90 kHz bandwidth of the Fun Cube Dongle. But luckily having the transmitter and receiver close by meant that I was able to see the transmission burn through even when the receiver wasn't tuned exactly to the correct frequency. So after some bisection I found the transmission at 868.388 MHz.

The double peaks in the spectrogram gave me more confidence that this is indeed frequency-keying with 20 kHz deviation and with GNU Radio's FM demodulator block the bits in the packets became clearly visible.

Actually, it's impressive how easy software defined radio makes tasks like this. Throwing blocks around in the GNU Radio Companion in a few minutes is something that would otherwise take you weeks with a soldering iron. Next time I'm doing something similar I'll most likely skip the whole logic analyzer part and just skip right to sniffing the radio waves.

To conclude, this demodulated signal is now something that can be piped to the capture process from my AM433 project and it will hopefully produce binary data. But of course, getting useful information from a binary blob is a whole new matter and that will come in a follow-up post.

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## z80dasm 1.1.3

06.03.2012 19:42

Last week Eric Smith found a bug in z80dasm, the disassembler for the Zilog Z80 microprocessor I put together a few years ago when I was researching Galaksija's ROM. It turns out a corner case in relative addressing where the offset would wrap around the 16-bit address space boundary of the CPU wasn't handled correctly. For such cases the code would create labels with excessively long names which overflowed some internal fixed-length string buffers, leading to stack corruption.

Hence a new release of z80dasm after almost 4 years. You can download the source tarball from http://www.tablix.org/~avian/z80dasm/.

I also used this opportunity to move the code from my old CVS repository to git, so you can now also clone the repository with:

\$ git clone http://www.tablix.org/~avian/git/z80dasm.git


As for binary packages, Eric is packaging z80dasm for Fedora. I've put together updated packages for Debian and will do my best to get them into the Debian Unstable as soon as possible (as Debian is now three releases of z80dasm behind).

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## Equestrian encounter

03.03.2012 17:38

One of the more unusual sights at the 28C3 last year in Berlin was a circle of hackers of all ages and genders sitting on the floor around a large flat panel monitor. They were watching a cartoon show with colorful ponies, while behind the screen you could see the dark basement with flags of various groups and hackerspaces hanging from the ceiling and a disarray of electronic devices strewn on tables. Maybe not surprisingly, this gathering also happened to be almost immediately below the no photography sign, which means that I was unable to find any evidence of it.

I have heard about this show now and then in the usual background noise of the web even before the congress. However this encounter made me somewhat curious and, having never actually overgrown watching an occasional cartoon, I decided to see what all this is about. For scientific reasons, of course.

Image by Chromamancer

I am talking about the latest remake of My Little Pony. It's an animated TV series that was created to advertise a line of Hasbro toys for girls, but in a funny turn of events actually got a sizable following of the opposite gender and in another age group. I guess something that would be unlikely to happen without the internet and the pseudo-anonymous discussions it enables.

After watching some of the show, I can say that I can understand to some degree why it has attracted such a broad audience. They say that a sign of good content for children is that it is also worth watching for adults. And in this particular example the latter certainly appears to be true.

You can often read that the show is well drawn and animated. For sure that is one part of the attraction. Remember my thoughts about the Avatar movie? I guess modern artists have become increasingly good at triggering emotional responses in their audience. The combination of human and animal visual traits in a character allows you to trigger more image recognition circuits than what is possible with only a human face. There's actually a word for that: superstimulus and it's quite amusing to see the parallels between songbirds falling for a red stick with white bands and people looking at what would be pretty deformed body shapes in nature.

Another thing that is often mentioned is the lack of cynicism in the show. Not surprisingly for a show that also carries an educational mark, the characters always find out that it's better to work for the greater good than for only your personal benefit. I can certainly see the appeal in this. It's nice to loose yourself in such fantasy after spending day after day in society that is increasingly focused on grabbing as much of the pie for yourself as possible and there isn't much space for laughter, truth, generosity and loyalty.

However, if this is one of the causes for the popularity among grown-ups, it's somewhat ironic if you consider why the show has been created in the first place: to increase profits of a multinational company that turns around billions of dollars each year. And in fact Hasbro has controlled the show from the start, making sure that it features things they can sell as toys.

But if you put these arguments aside, the creators of the show managed to paint a pretty consistent picture of a world where things are operated with hooves and mouths instead of fingers and three races divide their work to cover for each others shortcomings. This has made it possible for fans of the show to build upon it with their own original stories and, not having to stick to the original constraints, some of them are just as, if not more amusing than the original itself. So it's not all in the visual appearance either.

To sum it up, once you survive through the first view of the pink sugary overkill of the brand logo, it's an enjoyable show with memorable songs and a surprising lack of elements that could be described as fitting for a stereotypical little girl. If it makes more people think about their actions from the perspective of broader society, so much better. And if marketing departments can miss their target audience that much, I guess that's also a sign to be optimistic and might mean we have some time left before they figure out how to control our every thought.

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