21 Aug 2014
For those of you who have never been to a Rubik’s Cube competition, they are
controlled chaos. It goes something like this:
- A competitor is called to bring up his solved cube and place it on a blank
- A scrambler scrambles that cube, using the scorecard to determine which
scramble to give the competitor.
- A runner carries that scrambled cube to a timing station along with the scorecard.
- At the timing station, a judge performs the judging ritual
prescribed in the
- The judge writes the competitor’s solve time on the scorecard.
Both the judge and the competitor sign the sheet.
- A runner carries the competitor’s scorecard and solved cube back to the
- Repeat steps 2-6 until the competitor has done all of his solves.
- The completed scorecard is given to a staff member whose sole job is to
manually enter all the times into a computer.
Data entry is a mind-numbing, error-prone job.
C.A.L. Cube Timer (CCT) leveraged the
stackmat’s display port to plug stackmats into computers. Why not do the same
thing to automate data entry at competitions?
As a matter of fact, this has been done. Back in 2012, a group of programmers built a
custom piece of hardware to interface with the stackmat and send results to a
central server. They built enough infrastructure to actually run a few
competitions in Poland. There isn’t much information on the Facebook page for
it’s clear that the device they built is an impressive feat of
engineering. I’d love to know how they get data from their device to a central
server (Bluetooth? Wi-Fi? A custom wireless protocol?)
As cool as opencubeware is, I believe it’s not the right direction for us to go in.
Building a custom piece of hardware is expensive and inflexible.
Today, smartphones are ubiquitous and cheap, and they can be programmed to do
almost anything we want.
I believe we can build a more secure, reliable, easy to use results system on
commodity cell phones than we could ever achieve by building our own custom device.
It’s easier to create an android app than it is to write firmware for a
microcontroller. Furthermore, touch screens allow for a better, more flexible
user interface, and phones already have Wi-Fi, cameras, and some even support NFC.
Why hasn’t this already been done? Unfortunately, it turns out that plugging a
stackmat into a phone isn’t as straightforward as plugging a stackmat into a
computer (as CCT did).
wrote a stackmat interpreter in Objective-C in July 2010. He got it working in
the iPhone simulator on his computer, but when he loaded it onto his
phone, it just didn’t work. The signal he recorded was so distorted as to be unreadable.
Dan Cohen's distorted signal
In February 2012, I attended a hackathon at Berkeley with the intent of writing a
stackmat to phone interpreter. I was joined by
and Devin Corr-Robinett.
After a few hours, we ran into the distortion that had thwarted Dan Cohen.
Distorted signal on an iPhone 3Gs
I’m a software guy. This signal filled me with a combination of dread and
regret that I had not attended more of Professor Boser’s 8am EE42 lectures.
Unable to proceed without a better understanding of what was going on, we gave up.
It wasn’t until January 2014 that I revisited the problem. One day at work, my
Eithan Shavit caught me looking at pictures of
stackmat signals. As luck would have it, Eithan studied Electrical Engineering
before getting a job in software. He was interested in the distorted signal and
was able to shed some light on what was going on.
Amazingly enough, phones are designed to record human voice.
Human speech contains a large range of frequencies, approximately
300 Hz to 3400 Hz,
to understand what somebody is saying.
Phones can drop frequencies outside of that range if they want to, but they
must preserve all frequencies within that range.
Unfortunately, the digital data that comes out of a stackmat looks nothing like a speech signal.
To conceptualize what a phone does when it receives the square waves of the
stackmat signal, it is necessary to visualize the square wave in terms of its
component frequencies. This exercise is known as Fourier
Thank you, Wikipedia:
To approximate the square wave (in red), add together all the component sine
waves. The low frequency blue wave builds most of the shape, and the higher
frequency waves serve to square out the corners. It’s hard to imagine what
removing the blue wave from the signal would look like, but it should be
clear that without that low frequency component, our signal is going
to change dramatically.
To simulate the effect of sending a signal through a channel designed for
speech, we want to remove frequencies outside of the range 300 Hz to 3400
Hz. Audacity makes it easy to perform this experiment.
Unfiltered stackmat signal
Unfiltered stackmat signal frequency analysis. Note the bias towards low frequency signals.
Now we apply a band pass filter that attenuates all frequencies between 300 Hz and
3400 Hz. Note that we’re not harshly cutting off all frequencies outside of
this range, they are dampened instead. This is a reasonable approximation of
what happens in the real world: devices gradually lose sensitivity to
frequencies outside of their operating range, rather than dropping off entirely.
The resulting signal looks very similar to the distorted signal we saw when
plugging the stackmat into a phone!
300 Hz - 3400 Hz band pass filter applied to stackmat signal
300 Hz - 3400 Hz band pass filter frequency analysis. Note that a lot of the higher frequencies are gone, and the very lowest frequencies have dropped off.
If you’re interested in how different phones handle the stackmat signal, please
see all these signals I’ve collected.
This begs the question: how do you translate the stackmat signal into a range
of frequencies that phones do support? It turns out this problem (transmission of
digital bits over a link designed for human voice) has already been solved.
You use a modem!
The word modem is actually a portmanteau of the words modulator and
demodulator. The original modems used a modulation scheme called
(FSK) that is surprisingly simple. Pick two distinct frequencies. Call one your
mark frequency, it represents a binary 1. Call the other your space
frequency, it represents a binary 0. Whenever your digital signal is a 1, send
the mark tone, and whenever your digital signal is a 0, send the space tone.
If the mark and space frequencies you chose are within the range of frequencies
that phones support, you’re golden.
encoding of a digital signal
In the previous picture, the top signal represents the stackmat signal. The
“mo” part of a modem would take this signal and produce the wavy bottom signal
(FSK encoded). If we chose our mark and space frequencies correctly, then the
FSK encoded signal is safe to run into a smartphone, which
would then recover the original bits by doing frequency analysis in software.
Eithan did some research, and discovered the
DS8500 HART Modem.
It produces frequencies of 1200 Hz and 2200 Hz, perfect for a phone.
I was very skeptical that it would be able to speak the digital signal
stackmats produce, but measurements of the stackmat’s digital output voltage
were exactly the voltages the
DS8500 data sheet
asks for. The chip is only $12, why not give it a shot?
Unfortunately, using chips is not as simple as plug and play. Among other
things, the DS8500 requires an input signal of 3.6864 MHz.
Easy, run a current over a piece of quartz, and then filter the output of
that through some capacitors. Also resistors. Apparently you need resistors
everywhere (I’m convinced that resistors don’t actually do anything other than
cost a few pennies and look cool). This was quickly getting out of hand.
Suffice to say, reading the DS8500 data sheet
and distilling that information into a working board requires a degree in EE.
Eithan could do it, but he wasn’t sure he could get it right the first time.
Fortunately, you can purchase an
that has all the necessary components soldered to a DS8500 for you. For $46.86
after tax, the evaluation kit is a rip off (all the components on the board cost
pennies, and the chip costs ~$12), but it let us verify the chip without
the time and risk of designing and soldering our own board.
Unboxing the DS8500 evaluation kit
Wiring up the DS8500. This was tricky!
You can see a stackmat plugged into the modem above. I connected the output of
the modem into my desktop, Nexus 5, and iPhone 3Gs and recorded the incoming signal.
Output of the DS8500 as recorded by my desktop (gerty), a Nexus 5, and an iPhone 3Gs. The top row is the incoming digital signal, the output of a generation 2 stackmat stopped at 0.00.
Success!!! The signals recorded by the phones are slightly distorted (we’re
still not sure why), but it’s easy to tell where the 0s and 1s are. Having
proven that the DS8500 does what we want it to, it was time to design our own
board (Eagle CAD files available
We used OSH Park to print the board, which required us
to order a minimum of three boards. Eithan wisely insisted that we purchase at
least two of each component. Everything for one board cost $26 (see our bill of
If we move onto mass production, the price should go down significantly.
A board fresh from OSH park
It wasn’t until everything arrived that we realized just how small the DS8500 chip is. We had no idea how we were going to solder it to the board.
Could this be any tinier please?
Fortunately, we found a coworker on the hardware team who does board work. Part
of his job includes soldering these ridiculous
TQFN packages. We
gave him two of our boards and our two DS8500 chips. He attempted to solder the first
one using a hot air gun, but the board cracked (our two layer board is a lot
thinner than the boards he is used to working with). He
soldered the second chip by hand, which meant that he couldn’t solder the
“bellypad” (the large metal contact visible in the previous picture). Since the
bellypad is labelled as a ground, and many of the contacts on the outside of
the chip are also labelled as ground, we hoped that just soldering the outside
would be good enough.
Boards with the DS8500 soldered on. Note that the lower right board is charred and cracked from the hot air treatment.
After that, it was a simple matter of letting Eithan do all the work.
Eithan soldering the through hole components.
Neither board worked at first. The board design called for an LED to indicate
when the modem is turned on. The combination of modem and LED drew more power
than our battery could supply. After removing the LED, the burnt board still
didn’t work, presumably because the modem had been damaged by the heat.
This left us with one working board.
The finished board.
With our custom board in hand, I wrote code to demodulate the FSK signal
(see jfly/fskube). All that
Emscripten. This is what powers the
web demo at https://www.jflei.com/fskube/.
Since the code is written in C++, it will be easy to develop an iOS app if we
Darren Kwong, and I have already written an Android app using the
This is just the beginning. There is a lot of work to do before we can
run a competition with our phones. I aim to start running small practice
competitions with the Berkeley Cube Club in early 2015. I intend to see this
project though until it can be used at a large scale competition such as US
Nationals or Worlds.
- Test on more devices, especially iPhones.
- Design a new board for mass production. If you happen to know anything about
getting boards mass produced, please drop me a line! I’m very out of my element
- Integration with a live results system for WCA competitions.
- Eithan Shavit for teaching me everything I know about hardware and
electricity (that’s still not very much, but that’s not his fault).
- Patricia Li for encouragement and for proof reading. She also wrote the skeleton
of the Android demo app.
20 Aug 2014
It started with a coincidence: the
have the same 2.5mm communication port.
If you haven’t heard of a stackmat, it’s the timer used in Rubik’s Cube
competitions. I first played with one at Caltech Spring
After the competition, I convinced my mom to buy me one.
Before I started speedcubing, I was
very into calculator programming. Outside of the
time I spent using my beloved TI-83+ to do actual math, I spent hundreds of
hours and untold AAA batteries programming on that overpriced device with its
cramped keyboard. When I learned that some people had managed to
wire a PS/2 keyboard to their calculators,
it was something I had to try out. I bought all the components needed for the
project, but a general lack of EE knowledge
meant that I never actually got the project working, and my mom eventually made
me move the mess into our garage.
The upshot of all this is that when my stackmat arrived, I already
had the adapter and cables to plug my stackmat into my computer via the
My original stackmat and detritus from the abandoned TI-83+ PS/2 keyboard project
I wanted to practice with my new stackmat, but I also wanted to keep track of
my times without manually entering them into my computer. I scoured
the internet for a program to interpret the sound of my stackmat.
To my great surprise, I couldn’t find any! To this day, I believe I was the first
person to try this.
I graduated from high school in 2006, and spent some time that summer deciphering
the stackmat protocol. I’ll never forget the thrill of zooming into a recording
and seeing a repeated waveform.
A stackmat signal I recorded ages ago
After a long time staring at screenshots from Audacity, I was able to decode
stackmat signals by hand (many years later, I learned that I had reverse engineered
the RS-232 standard).
Friends and other projects kept me from writing a software signal
interpreter that summer. It wasn’t until I had settled into my freshman year of
college that I finally sat down and coded a truly awful, barely working
stackmat interpreter in Java.
I kept this project secret, confiding only in Darren
I decided to demonstrate my work at Berkeley’s first Rubik’s Cube competition,
Berkeley Fall 2006.
Of course, that didn’t happen. I had never staffed a
competition before, but there’s rarely a moment of free time. Berkeley’s first
competition was no exception. Fortunately, we
all met up at
apartment afterwards, and I was able to show off my hard work.
I like to think that people were impressed and excited about my work, but I honestly only remember the reactions of two people:
and Ryan Zheng.
Chris Hunt was the creator of the legendary JNetCube. At the time,
almost every cuber I knew used JNetCube. He proposed adding my stackmat support to
it, and I remember being honored by the opportunity to contribute to a real
piece of software that other people use.
I’m sure I would have become a contributor to JNetCube if not for Ryan Zheng.
Ryan wanted to see my awful code, and then felt compelled to improve it.
He eventually pulled out his own laptop, and we descended into a full night of hacking.
That night lengthened into a full weekend. At one
point, Ryan suggested the unthinkable: rather than just giving this feature to
JNetCube, we could build our own superior timer! That moment was the birth of
C.A.L. Cube Timer (CCT).
CCT received a lot of attention from the cubing community, and we worked for
years to keep up with the requests. Many features that were first introduced in
CCT live on in the the myriad cube timers we have today. Perhaps my proudest
moment was seeing this video
of a Chinese cuber using a translated version of CCT.
CCT eventually grew into TNoodle, the official WCA scrambling
program. I am very proud of TNoodle, and it gives me great joy to know that
it is used weekly as part of running official Rubik’s Cube competitions.