nerdy – Page 12 – If we had a Marriage, These Would be its Thoughts

XBMC LIRC XBox DVD Dongle

July 25, 2012 8:53 pm

Just sticking this here for the rest of the Internet to benefit from.

I was rebuilding the HTPC with an updated OS and needed to get LIRC working again for my remote using the original XBox DVD dongle connected via USB.

I found this post which describes the process of building and using the lirc_xbox kernel module. Which worked fine in XBMCBuntu (based on Ubuntu 11.10). But when I tried to follow the instruction in Linux Mint 13 XFCE I got the error "E: Unable to find a source package for lirc" when attempting the line "sudo apt-get build-dep lirc".

I worked around the problem by running the same command on my laptop running Ubuntu 12.04 which gave me the correct list of dependencies:

autotools-dev build-essential debhelper devscripts dh-apparmor diffstat dpkg-dev g++ g++-4.6 gettext html2text intltool-debian libasound2-dev libdpkg-perl libftdi-dev libftdi1 libgettextpo0 libice-dev libirman-dev libjack-dev libjack0 libportaudiocpp0 libpthread-stubs0 libpthread-stubs0-dev libsm-dev libstdc++6-4.6-dev libunistring0 libusb-dev libx11-dev libxau-dev libxcb1-dev libxdmcp-dev libxt-dev patch po-debconf portaudio19-dev quilt x11proto-core-dev x11proto-input-dev x11proto-kb-dev xorg-sgml-doctools xtrans-dev

I used that list in a "sudo apt-get install [long list of package names]" and was able to successfully get the build dependencies installed and follow the rest of Mr. Plow's instructions and get my remote working properly.

Since I saw that a few other people were having the same issue, I thought I'd post this in case anyone else gets stuck on the problem.

Reducing power usage: SheevaPlug and Squeezebox Radio

July 7, 2012 12:36 pm

Since we've entered summer our electricity rate tiers have switched to the summer levels. This means that the lower (cheaper) tiers are smaller so you start running into the much more expensive tiers sooner. Tiers 1 and 2 are both pretty cheap, 13 and 15 cents per kWh, but tier 3 jumps to 30 cents per kWh. So whenever possible we try to avoid landing in tier 3.

A while back Mom gave me a Kill-A-Watt meter. Our electricity bill informs us that we're consistently using substantially more electricity than "similar homes in your area." Which seemed odd since we don't obviously waste energy. So I finally got around to checking on our electronics to find out what's guzzling our energy.

Jess has an old stereo thing that we use to listen to music when going to bed. I discovered that this stereo was drawing ~18 watts regardless of what it was doing, 24 hours a day, 7 days a week. So just having this stereo plugged in was costing us somewhere between $20 and $45 a year depending on the tier.

I have a desktop computer which I use as a file server and media center (via XBMC). It holds all of our DVDs so that the actual DVDs sit in a box somewhere out of the way. It also holds all of our music files and I use it to download various things via bittorrent (Linux ISOs, games purchased via the Humble Bundle, perfectly legal things, of course). Thus the computer was usually on 24/7 also.

So I was rather shocked and appalled to discover that it was drawing ~106 watts when running. Keeping that machine on was costing us ~$100-$200 a year! So the first thing I did was dig into configuration options and disconnect unused components. Via this route I was able to bring its energy usage down to about 80 watts. Better, but not great.

Enter the Logitech Squeezebox Radio and the SheevaPlug.

Logitech Squeezebox Radio

The Squeezebox Radio was something I've wanted for a little while. It's a nifty device and the very low power usage was just a nice bonus. It's, essentially, a music streaming device with built-in speaker and wireless network connection. So you just plug it in and you can listen to Internet radio stations, connect to a Pandora account (or most other music streaming services), and play music from a local server via the freely available Logitech Media Server. Something I like about it is that all the software is open source and they don't make any attempt at locking down the software or hardware.

Anyway, I received the Squeezebox Radio for my birthday this year. Part of its job was going to replace Jess' old stereo system. It's working great at that task and takes up less than a third of the space. It's small (smaller than I expected) and easy to move so Jess often moves it to the living room during the day, into the bathroom for Heather's bathtime, etc.

The Squeezebox Radio draws ~2.0 watts when running (~2.2 watts when the screen is on). So that's a big win over the stereo drawing 18 watts. But it also contributes in savings in other ways. Instead of running the full blown stereo system in the living room for music Jess uses the Squeezebox Radio, so that's going to count for something.

Overall I am very pleased with the Squeezebox Radio.

The Squeezebox Radio spends much of its time streaming music locally from the desktop (using the mentioned media server software). And the computer was the big power hog. So let's address that now.

SheevaPlug

To try and reduce the power usage of the computer I spent some time researching low-power computing options. I researched building an Intel Atom based machine, an AMD Fusion based machine, a dedicated NAS device, and a few other options. But based on those systems it looked like I was going to get a lot more power than I needed and still be pulling 20-30 watts.

I then turned my attention to the Plug Computer scene. Plug computers are designed to be cheap, low power, plugged in somewhere out of the way, and mostly forgotten. They have a vibrant community built around them.

There are several plug computers to choose from. I went with the SheevaPlug because it has a long history with many success stories and guides. Its age means it's a little less capable than some of the other offerings, but it looked like it would do what I needed just fine.

It's small, about the size of 3 decks of cards. It features a 1.2 GHz ARM processor, 512 MB DDR2 RAM, SD Card slot, USB 2.0 port, and a gigabit ethernet jack.

I set it up with a 4 GB SD card and a 64 GB USB flash drive. I had planned to use a 2 GB SD card, but it didn't like the one I had and a 1 GB card was too small.

I used the 4 GB card to install the operating system (Debian Linux) and other necessary software (like the Logitech Media Server, Transmission [a bittorrent client], etc.).

The 64 GB USB flash drive is holding all the data I need. It has our music library, backup files from the Board (the Board gets backed up nightly, previously to my desktop, now to the SheevaPlug), and any currently active torrent files.

Maybe some of the other plug computers are different, but setting up a SheevaPlug isn't exactly for the novice. I had to cobble together bits and pieces from various guides in order to get everything working correctly. It requires a working knowledge of Linux, a comfortable familiarity with command lines and a basic understanding of memory addressing (well, if you want to have any idea what the commands you're typing do, that is).

Here are the main resources I used:
http://www.cyrius.com/debian/kirkwood/sheevaplug/
http://www.cyrius.com/debian/kirkwood/sheevaplug/install.html
http://plug.noloop.net/sheevaplug-hacks/installing-debian/
http://wiki.slimdevices.com/index.php/SheevaPlug_Installation_guide
http://d-i.debian.org/daily-images/armel/20120705-08:35/kirkwood/netboot/marvell/sheevaplug/ (for the latest Debian installer images)
I also needed some decent Google skills to solve various issues along the way.

The SheevaPlug is up and running smoothly now. Running full tilt it draws about 3.5 watts. So the Squeezebox Radio and SheevaPlug together use about 6 watts compared to the ~125 watts previously needed for the desktop and stereo. So over the course of 1 year this set up will save us somewhere between $120 and $200 in electricity. The SheevaPlug costs $99, so it will pay for itself in a year and that's ignoring the reduced cooling costs.

Bayes' Theorem

May 12, 2012 3:38 pm

By request, here's my attempt to explain Bayes' theorem (cribbing heavily from Wikipedia).

Deriving Bayes' Theorem

We start with the standard definition of conditional probability (for events A and B):

Which reads: The probability of event A given the known occurrence of event B is equal to the joint probability of events A and B (e.g., the probability of both events occurring) divided by the probability of event B (assuming the probability of B is not zero). I'm not going to show the derivation of conditional probability.

The summation axiom helps us understand the joint probability of A and B:

It tells us that the joint probability of A and B is equal to the probability of A given B multiplied by the probability of B. All we're talking about is the likelihood of events A and B both occurring.

Keep the following equivalence in mind, we'll need it in a minute. It simply says that the order of variables when writing the joint probability is irrelevant. It should be fairly straightforward that the likelihood of events A and B both occurring is the same as the likelihood of events B and A both occurring.

Filling back in our definition of conditional probability, we have (with the understanding that P(B) is not 0):

The third equation is the simplest form of Bayes' theorem. It wasn't very hard to get to and the math, relatively speaking, is quite simple (we're not talking about deriving the Schrödinger equation or anything absurd). But its application, and understanding what it means, can be tricky.

P(A) is called the "prior," representing our prior belief in the occurrence of event A.

P(A|B) is called the "posterior," representing our belief in the occurrence of event A after (post) accounting for event B.

The remaining pieces, P(B|A) / P(B), are called the "support," representing the support event B provides for the likelihood of event A occurring.

Applying Bayes' Theorem

I'll re-use the medical testing scenario from the previous post.

Substituting T to represent a positive test result and D to indicate the presence of the disease our equation becomes:

We expand P(T) into P(T|D)P(D) + P(T|¬D)P(¬D) which is to say: The probability of getting a positive test, P(T), is equal to [the probability of getting a positive test when the disease is present, P(T|D), multiplied by the probability that the disease is present, P(D)] plus [the probability of getting a positive test when the disease is not present, P(T|¬D), multiplied by the probability that the disease is not present, P(¬D)].

So now we just need to map the numbers we have about the disease and the test to the variables in our equation:

P(D), our prior, is 1 in 200 million--the disease occurrence rate in the general population.

P(T|D), the probability of getting a positive test when the disease is present, is derived from the false negative rate. When the disease is present, our test will only incorrectly say it is not 1 out of 200,000 times; so P(T|D) is 199,999 out of 200,000.

P(T|¬D), the probability of getting a positive test when the disease is not present is the false-positive rate: 1 in 100,000.

P(¬D), the probability of the disease not being present is simply the other 199,999,999 out of 200 million.

So let's plug these numbers in step by step:

And we see that P(D|T), the probability that the disease is present given a positive test, is only 4.76%. (My previous post incorrectly reported this as 0.05%, I've corrected the error.)

The car example I presented didn't use hard numbers, but I'll frame the concept into Bayes' Theorem. S will represent a car being stolen and H will represent a car being a Honda Civic:

Which says, the probability that a car is stolen given that it's a Honda Civic is equal to [the probability that a car is a Honda Civic given that it's stolen] multiplied by [the probability of a car being stolen] divided by [the probability of a car being a Honda Civic].

The dealership was trying to push an insurance policy on me by only reporting P(H|S), but unless we account for the number of Honda Civics on the road in the first place, P(H), we aren't getting the full story.

(Notice that we don't have to actually care about P(S) if all we're doing is comparing car make/models. P(S) doesn't change for the different make/models so it doesn't affect the relative rankings.)

PaperTrust

October 5, 2011 7:10 pm

So, a while back I blogged an idea I had about cryptographically signing various documents. I specifically talked about checks, but you can apply the principle anytime you have a fairly small amount of data which is supposed to be issued from a trusted source: cashier's checks, money orders, driver's licenses, event tickets, passports, boarding passes, etc.

Well, I spent some time playing around and put together a working example. It's not fancy, but it does the job. It's been a few months, but it really didn't take that long. Especially since I had to do some reading about QR codes and using them, along with public-key cryptography, from Python. So I had a basic prototype done in about a week. Then back in August I decided to flesh things out a bit more and produce a nice demo application. I'm calling the system "PaperTrust" as it allows you to embed the trust element onto the paper item.

Here's a video demonstration:

Text description of the demo:
So, in my demo, we generate data for a cashier's check and then sign it using the demo private key. We stick the signed data (which includes a signing-organization ID) and the signature into a QR code and stick that onto the check and print it. Now the check is physical and can be carried around as usual.

Now say you're going to use this check to pay for something from a stranger. This stranger needs to know they can trust the check. So they use their verifier application to scan the QR code from your check. It reads the organization ID, looks up the correct public key for that organization, and verifies that the signature is valid. It also displays the signed data so the person can compare it to what's physically printed on the check. This is a cryptographically secure guarantee that the check is valid (or at worst an exact copy of a real check, which should make tracking down counterfeiters a lot easier). So you would use this in tandem with traditional anti-forgery measures like watermarks, micro-print, thermal ink, etc.

I've put the code up on GitHub: PaperTrust on GitHub.

A Rumination on Science and Education

September 7, 2011 8:21 pm

I'm currently reading a biography of the physicist Richard Feynman (by James Gleick). So far it's excellent. What I'm really fascinated with right now is (at least how Gleick portrays) the progression of science during Feynman's schooling years (the mid to late 1930s). The number of high caliber physicists at the time (and the time just leading up to it) is astounding: Einstein, Bohr, Rutherford, Heisenberg, Dirac, Lorentz, Schrödinger, De Broglie, Fermi, Oppenheimer, and I'm probably missing some still. Those guys are each incredible scientists in their own right and it's no wonder the understanding of physics changed so dramatically during the 1930s. The only comparison I can think of is the progression of art during the European Renaissance.

As I'm reading, I can't help but wonder about what set apart that time period in history from anything since in terms of scientific progression. Computer Science has a similar vein of tumultuous rapid progression during the era of Turing, von Neumann, Dijkstra, Gödel, Church, Cook, Levin, Kleene, Shannon...But as I'm looking at it, most of these pioneers (in fact, all but Dijsktra) were essentially contemporaries of the physics revolution being discussed. They all would have been products of the same time period of schooling (whether in the U.S. or Europe). Which further raises the question of what was so different about the education systems through which these incredible people went?

Sadly, I don't really have an answer. But if we're looking to reform our education system for better results, what better goal than to figure out what was happening in education from about 1910-1935?

But then, maybe it wasn't the education system at all. Maybe it was the societal mindset about learning and discovery. Maybe it was simply that the education system and society didn't inhibit the intense drive for understanding and innovation that these people felt. Quoting from page 63 of the book (Genius: The Life and Science of Richard Feynman):

At MIT in the thirties the nerd did not exist; a penholder worn in the shirt pocket represented no particular gaucherie; a boy could not become a figure of fun merely by studying....America's future scientists and engineers, many of them rising from the working class, valued studiousness without question.

If this is an accurate portrayal of the time period, it certainly helps explain to me why so many incredible scientists were produced during that era. Gleick describes in one passage of how Feynman and many of his contemporaries grew up reading the Encyclopedia Britannica eager to learn more about the world around them.

They tinkered with, broke, and repaired things--something I think is rarely encouraged these days. I know this is one of the ways I developed my own interests in science and computers. I wanted to learn how things worked, so I played with them, changed them, broke them, and attempted to repair them (sometimes successfully).

People are inquisitive by nature. I think we, as a society, are getting far too good at crushing that inquisitiveness with standardized lesson plans which allow no room for deviation to follow student interests, standardized pedagogy which insists all students learn in the same way, and standardized tests which demand that all students regurgitate their "knowledge" in one, simplified fashion.

If there's one thing I learned in the years I worked as a T.A. it's that students assimilate information in incredibly varied ways. Its hard to come up with new approaches to the material on-the-fly in order to try to help the student make the connection. But if you don't, and instead insist on "the one true approach" to the material, the student will fall behind, become discouraged, and lose interest in the subject matter.

We need to encourage the asking of questions and the seeking out of answers by research, experimentation, or otherwise. We need to foster the innate curiosity, creativity, and inquisitiveness that children have.

I'm not so concerned with the mindless consumption of media or playing of games because our minds need downtime to process and assimilate the world around us. However, I think the hours spent watching TV and browsing the Internet are more of a symptom than a cause; in that we still seek out "new" things, just in a manner that parents aren't worried about anyone getting hurt or anything getting broken. But situations where one might get hurt or something might get broken are, by far, the most likely situations where we might actually learn and remember a lesson.