| iPhone time |
[Dec. 5th, 2009|04:45 am] |
So after a long saga with Sprint became the final straw haycart (including someone hijacking my account, me notifying Sprint within an hour of that, hours on the phone that Friday instead of a nice dinner out, a bill for over $5,000 of calls to Cuba, all but a few dollars of which happened after my first call to them, them disconnecting my phone without telling me (they told me by leaving a voicemail I could have listened to if they hadn't disconnected my phone), calling them and manually resetting the phone's unique ID, then actually getting the bill for over $5,000, then asking them if I had to pay it, then being told that my account has been credited, but they just wait on their lazy asses and send out $5,500+ bills for the fun of it and wait for the customer to get in touch with them, then realizing that they credited the long distance, but not the US airtime, leaving $45 more unaccounted for, then my sprint.com account got disconnected from my phone account, then when I fixed that I couldn't download the bill PDF 'cause I couldn't find the link, then finding a help entry for downloading the PDF of the bill, but then the instructions didn't agree with the actual page, then having the dude on the phone not be able to help with the PDF download, then having him not understand that me adding up my valid daytime minutes that aren't to Cuba gives me 200-something which is a lot less than my 450-minute plan and also a lot less than the 700 they wanted me to pay for, him getting confused, then throwing up his hands and just crediting me the $45 I was asking for because I was a loyal customer), it's time to switch.
Two questions:
- 16GB at $200 or 32GB at $300?
- What's the process for transfering a phone number? Does it Just Work or is it one of those pain-in-the-ass media-company interactions?
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| warning: comparison between signed and unsigned integer expressions |
[Nov. 17th, 2009|12:43 pm] |
Dear Lazyweb,
Philosophically, why is it a warning to test equality between signed and unsigned integer expressions in C++? Is it a performance issue (i.e., u==i would become bitwise_equal(u, i) && i >= 0)? Is it just that it thinks mixing signedness is a bad idea? |
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| References for Statistics on Manifolds and such? |
[Oct. 2nd, 2009|02:01 am] |
For work and for fun, I've started thinking hard about statistics in non-Euclidean spaces. For example, how do you average angles or find the MLE of a point in SE(3)n given a collection of relative rigid-body transformations or a generalized median of several homographies?
I've found some interesting papers, but no great references... do you have any recommendations? I'd like to know more about Remannian geometry, to understand affine connections, and statistics in these spaces. Any book references would be appreciated.
Yay math! |
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| Total derivatives and spring systems |
[Jul. 29th, 2009|12:06 am] |
My wife is at a conference, so my mind turns to... linear algebra.
Suppose you have a spring system... like a horizontal yardstick bolted to the table at one end, flat side facing the table (not standing on edge). You can move the tip up and down and it will resist, springing back into place. You can also bend it horizontally, but it's much stiffer in that direction due to its shape. (For the sake of simplicity, let's give each element two degrees of freedom—no crazy axial motion or buckling or twisting modes; this is a toy example.)
Suppose I'm curious what happens to the shape of the yardstick when I move the end in either of those directions. This winds up being a differential equation and you can solve for it numerically or using FEA.
I'm interested in better understanding the FEA part of it. Considering just the tip of the yardstick, and assuming small deflection (so assuming no geometric nonlinearity), I expect the restoring force to be proportional to the displacement of the tip, and so
f=−Kx
where f is the restoring force, K is the spring constant (a 2×2 matrix), and x is a 2×1 vector describing the horizontal and vertical displacement of that tip. Similarly the energy would be
u=½ xTKx
Now, FEA can give me a system stiffness matrix, a matrix relating the forces to the displacements of every little piece of the yardstick. That's all well and good, but how do we go from that to the 2×2 black box system matrix describing the displacement of the tip only?
My feeling is that it works like this: Intuitively, if you push the end of the yardstick up, it'll go up, and the rest of the yardstick will go along with it in what is clearly the first bending mode (the eigenmode of K corresponding to the smallest nonzero eigenvalue). Similarly if you push it horizontally, it'll move that way (and not deflect up). This is the second eigenmode. So if we want to know what happens to the shape when we push the end around, it looks like we just want to find enough low-eigenvalue eigenmodes to span the space we want to explore (tip goes up or down, tip goes side-to-side), then fit those to the prescribed deflection.
If that's right, I'm not sure exactly what basis make sense. If we eigendecompose the K for the full system so you have
VDVT=K
with the eigenvalues in ascending order, then can we could just say we'll use the first n eigenvectors with nonzero eigenvalues. But what if those don't span the space in which we are interested? Unless we have repeated eigenvalues (let's assume we don't), we could just keep adding eigenvectors until we do span the space, but if we wind up with more than n eigenvectors then we don't have a unique solution. On the other hand, the only other thing I can think of is to build a basis by adding eigenvectors in increasing order, but skipping those that aren't linearly independent within the space of prescribed degrees of freedom. That seems most likely, but also seems totally kludgy.
So...
- What's the right way to do it?
- Is there an elegant way to solve this that I'm just missing
- Can you describe this
black boxing as finding a total derivative of the system energy with respect to the degrees of freedom you want to prescribe, constrained by minimum system energy?
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| Communal storage and backup |
[Jul. 6th, 2009|08:53 pm] |
![[info]](http://l-stat.livejournal.com/img/userinfo.gif) mew0422 and I are trying to sort out our data-storage needs. She has a laptop, I have a desktop. We want some NAS and we want a good backup solution.
It looks like Apple's Time Machine is entirely client-based and so can use any mountable storage, be it a Time Capsule, a USB HDD, or something else.
With that in mind, it seems like we'd be best to get non-Apple NAS—something upgradable and it shouldn't need to be a Wi-Fi server itself. (Recommendations?)
Here's what I'm thinking: I am inclined to use the JWZ Backup Solution for my computer, possibly rsyncing to NAS in addition to swappable drives. mew0422 would use Time Capsule to backup to the NAS box.
The only thing left is how to back up the NAS drive (financial files, shared media, etc.) Any thoughts?
I hate IT. |
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| MPI & Python |
[Apr. 13th, 2009|09:46 pm] |
Dear Lazyweb,
There appear to be more than a few MPI bindings for Python. Which should I use?
In particular, which should I use if I want to use Python to call C++ code that needs to take an MPI_Comm object?
Do any of you have any other experience with using Python with MPI? As one new to Python, I am particularly curious how garbage collection might interact with MPI if you have an object shared across computers. |
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| DTV |
[Feb. 11th, 2009|10:41 pm] |
We don't have cable; we watch a few shows on the Providence, RI CBS affiliate (heading: 212°, distance: 11.8 miles). With analog, we get OK signal. It's sometimes a bit snowy and occasionally black and white, but it's there and there is always good audio.
A month or two ago, the broadcasters all teamed up to put a DTV infomercial in the middle of Criminal Minds on all channels. We had been planning on getting a converter box and turning it on by February 17, but noooo, they had to go and make it seem like we couldn't trust analog TV to actually carry the last quarter of our crime dramas, making it worse than useless.
After filing for a $40 government discount (why are my tax dollars paying to help people watch TV?) I waited. And waited. And finally caved and went to Best Buy where (as usual) I got overcharged by $20, but walked out the proud owner of a DTV tuner.
I plugged it in and (after realising that the battery to the remote was wrapped in clear shrink wrap making it not provide any power) it worked crystal clear color picture and great sound!
Except for when it doesn't. When the signal fades, the entire screen crumples out and worse, the audio seems to go first. It might be watchable if the video dropped out first, but with the audio jumping in and out, it's worthless.
I just got a Philips MANT-410 Amplified Indoor dipole antenna, but it seems to be next to useless.
The infomercial said something about never needing rabbit ears; as it is we are this close: ⇥⇤ to giving up on broadcast TV and trying to just watch TV online.
All that said, antennaweb.org mysteriously lists a bunch of "Live Dates", including WPRI-DT channel 12.1, live 17 February. It thinks that will come in better than the otherwise identical entry without that live date. This suggests to me that they may be going to boost their signal strength on the 17th, but the station hasn't mentioned anything of the sort. I'll be really pissed off if they are making me jump through these hoops only to discover that I shouldn't actually expect DTV to work properly until the 17th.
For reference, I posted to this WPRI discussion board as well.
PS
How do antennas work? It's magic. Why is a tiny little spindle of metal the best way to pick up radiation from a long way away. One pair of bunny ears doesn't exactly subtend much of the sphere around the tower 11.8 miles away.
Tune in tomorrow for a rant about old-fashioned Fortran-based large-scale sparse eigensolvers that turn their inner loop inside out, making you do all the dirty work. Grrr. |
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| Ubuntu keyboard issue |
[Feb. 1st, 2009|12:33 pm] |
For a while, my keyboard has been misbehaving in Ubuntu. In particular, some of the keys just don't work or are mapped strangely. In particular, my arrow keys don't work at all, forward delete (as opposed to backspace) doesn't work, and page down is mapped to the context-menu key and page up is mapped to forward slash. With NumLock turned off, the number-pad arrow keys work.
The keys appear to work normally in the console (Ctrl+Alt+F1).
How would I debug this? I figure some keyboard mapping must be set somewhere since this is a regular keyboard that is just not working right...
Any ideas? This seems to be the same with my PS/2 keyboard plugged in via a USB adapter as with my Apple USB keyboard. |
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| Home Theater Lazyweb |
[Jan. 18th, 2009|10:46 am] |
Dear Lazyweb,
I got a long-term loan of a projector from my brother-in-law elect. Our current A/V situation is as follows:
We have an office with speakers shared between our two computers. We have a Living/dining/kitchen area that includes one CRT TV with built-in sound. We have a bedroom with no sound. We both have iPods, of course.
In the big room, we would like to be able to plug in an iPod, to watch movies/TV with big sound, and to listen to the radio. I suppose digital radio would be nice while we are at it, but it's not necessary.
What does a basic home-theater audio system look like? I don't need to go all-out, but don't want chincy crap.
Any suggestions?
Maybe I can take advantage of Circuit City's liquidation... |
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| Linearity of 2D spring system? |
[Nov. 22nd, 2008|02:05 pm] |
Somewhere along the line I convinced myself that finite deformation of a 2D (and 3D and higher) spring system was nonlinear. Now, I still think it is if there are rotational springs involved, but if you just have a system of particles each of which has a desired distance from some of the other particles, it looks like it is linear:
Let x be the system state, a column vector of the x and y positions of each of the particles, x=[x1 y1 ... xn yn]'.
Define a connectivity matrix, C so that Cx is the vector of distances between points. So in the simplest case of two points connected, you would have
C = [ -1 0 1 0;
0 -1 0 1 ];
You then apply your boundary conditions, so if the first point is fixed, you drop that column from C and those rows from x and move those columns times those rows to the right-hand side of the equation.
That leaves you with Cx=y where y is the vector of lengths of the springs when the exert no force.
Now here's the question
If I solve that system as
x=C\b
which I believes solves the overdetermined linear system in the least-squares sense, is that the same as minimising the energy in the springs?
That is, if we solve it that way, we should be minimising
||Cx−y||2
where
s=Cx−y
is the vector of stretches in the springs in x and y
so
||s||2 = ⟨s,s⟩ = ∑i (sxi2+syi2)
which is proportional to the sum of the energy in all the springs, so minimising that should be the same as solving the spring problem.
Is that right? Is an n-dimensional spring-particle system with finite deformation really linear?
Update
I was confused.
The above is right, but it isn't exactly a spring–particle system. In an ideal spring–particle system, the springs can rotate about the particles so the constraints are distances between particles, not distances in x and y separately. If you had a physical realisation of such a system, one particle would torque its neighbour in the network when you push on it in a transverse direction. If you include that torque, you get a nonlinear system.
Alternately, if you only have a distance constraint, you also have a nonlinear system. Consider the simplest case of two particles connected with a unit-length spring. The energy function is proportional to (r−1)2 so it's proportional to (√(x2+y2)−1)2 which is clearly nonlinear. |
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| Skis |
[Nov. 22nd, 2008|12:41 pm] |
I am looking into getting some new skis. I have classic K2s – black with neon K2 logos running down them. I got 'em used in probably '95, so they are probably from the late '80s; either way, they are literally falling apart.
At the same time, I've been becoming a better skier. I particularly want something that will do well in bumps and tight quarters and that won't get bogged down in soft (by New England standards) snow.
Any suggestions?
Also, what about bindings? They aren't free; are they a commodity, or are some appreciably better than others? |
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| Stupid Computers |
[Jun. 3rd, 2008|09:34 pm] |
I posted recently about what I thought was a hard-drive issue. I replaced it and thought things were OK, but that didn't last. It started acting flaky (flickering video card, random HDD crapout, etc.) and now it won't boot. As in it won't start the memory test.
My suspicion is that some component on the motherboard is shot (although I'm open to other hypotheses). It's been always on since April 2004, I think something just gave up the ghost.
That said, I don't feel like investing in or setting up a new computer right now. I'd rather just replace the MoBo and keep rolling with the AMD Athalon XP2500 it has. I have a perfectly good case and a stack of (probably good) SATA drives. Do I just need any old MoBo with SATA and a Socket A CPU socket?
Or, does it make more sense to buy a new CPU and MoBo and upgrade to an Athlon64 ala Ars's 2008 budget box? Would that Just Work in my old box? The only things that I'd plug into it are my Radeon 9600, My SATA drives, my IDE DVD drive, and my DDR400 ram and of course my old Antec Sonata I case. |
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| Backup |
[Apr. 6th, 2008|06:36 pm] |
Since my previous post on the subject, I went and got a 500GB SATA hard drive, threw Ubuntu on it (which was amazingly easy), and cp -rp'd my data over. It is now much much happier, with no clicking and with disk-intensive apps such as f-spot being quite responsive.
With that scare over, it is time to set up a good backup solution. One option would be to get another disk and periodically rsynch to it. On the other hand, I am enamored with the Apple Time Capsule approach of stand-alone storage. It looks like it is possible to use it with Ubuntu Linux. Plus it would Just Work for mew0422's MacBook.
Do any of you have a Time Capsule? Have you tried using it with Linux? Do you have other backup suggestions?
For reference: jwz's backup recommendations. |
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| Hard drive clicking? |
[Apr. 1st, 2008|09:52 pm] |
If dmesg says the following, accompanied by a click and a pegged CPU, does it mean it's time to buy a new hard drive?
[ 1350.808000] res 40/00:00:00:00:00/00:00:00:00:00/00 Emask 0x4 (timeout)
[ 1350.808000] ata2: soft resetting port
[ 1351.000000] ata2.00: configured for UDMA/100
[ 1351.000000] ata2: EH complete
[ 1351.008000] sd 1:0:0:0: [sda] 488397168 512-byte hardware sectors (250059 MB)
[ 1351.008000] sd 1:0:0:0: [sda] Write Protect is off
[ 1351.008000] sd 1:0:0:0: [sda] Mode Sense: 00 3a 00 00
[ 1351.020000] sd 1:0:0:0: [sda] Write cache: enabled, read cache: enabled, doesn't support DPO or FUA
[ 1419.700000] ata2.00: exception Emask 0x0 SAct 0x0 SErr 0x0 action 0x2 frozen
[ 1419.700000] ata2.00: cmd 35/00:08:78:a7:64/00:00:12:00:00/e0 tag 0 cdb 0x0 data 4096 out
[ 1419.700000] res 40/00:00:00:00:00/00:00:00:00:00/00 Emask 0x4 (timeout)
[ 1419.700000] ata2: soft resetting port
[ 1419.892000] ata2.00: configured for UDMA/100
[ 1419.892000] ata2: EH complete
[ 1419.892000] sd 1:0:0:0: [sda] 488397168 512-byte hardware sectors (250059 MB)
[ 1419.892000] sd 1:0:0:0: [sda] Write Protect is off
[ 1419.892000] sd 1:0:0:0: [sda] Mode Sense: 00 3a 00 00
[ 1419.892000] sd 1:0:0:0: [sda] Write cache: enabled, read cache: enabled, doesn't support DPO or FUA
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| 802.11 audio |
[Mar. 1st, 2008|03:14 pm] |
Dear Lazyweb,
Suppose you live in a household with a Linux box connected to decent speakers and a MacBook that gets used all over the place. Is there a way to send the Mac's audio out to the Linux box via wireless?
I know Apple's AirPort supports AirTunes, which uses Remote Audio Output Protocol, a proprietary variant of Real Time Streaming Protocol. But apparently nobody has figured out how to get iTunes to talk to a non-Apple sound sink.
Is there a way around this? Can we get the Mac to send all of its audio out over the network, perhaps?
Update: A related discussion thread. |
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| Off-camera flash |
[Jan. 20th, 2008|09:02 pm] |
Dear Lazyweb,
danielmarley did me the disservice of exposing me to the time suck that is Strobist. Now I am eager to get my Canon 420EX Speedlite off of my camera so I can use a softbox or an umbrella.
So the question is: what's a good way to get the flash free of the camera given that I have a Canon flash and a Canon camera. I could get an expensive master unit (an ST-E2). I could pay even more for a master flash (580EX). Or I could go for third-party wired solutions that apparently don't support all of Canon's E-TTL goodness.
I am inclined to avoid funding a closed standard (granted I bought into the closed EF lens system). On the other hand, if E-TTL really is all that, I don't want to miss out on the goodness.
Any thoughts? Have any of you Canon folks done anything with off-camera flash? |
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| Cameras |
[Nov. 24th, 2007|06:12 pm] |
Dear Lazyweb,
The danger of learning about optics is that you start to see the shortcomings of your otherwise great 4-year-old camera with a point-and-shoot form factor. (The camera was released December 2002, almost a full 5 years ago.) I'm looking for a DSLR. At the moment, I lean toward Canon, which is the brand I have, including having an external flash. With that in mind, the choice seems to be the 40D or, for half the price, the 400D. Looking at your Flickr metadata, I see a few of you with a DigitalRebel (like the 400D), I also see a Nikon D40, similar (in price at least) to the 400D, and the FinePix S5 Pro, more like the Cannon 40D in price.
Any thoughts?
Any thoughts on lenses? |
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| Belts |
[Nov. 4th, 2007|11:13 am] |
Dear Lazyweb,
I usually wear a full-grain black leather belt with a silvery buckle. Unfortunately, the Gap belt I wear daily is getting old after only a few years and my dress belt is having similar problems. You might wonder: how does something made out of full-grain leather and metal get old? I'll tell you how: the buckle is nickel-plated brass, and the plating is coming off. Given that it's the top half of the buckle, I assume this is due to salt, etc. from my body; eew. For me, part of the appeal of materials such as leather and metal (and glass and wood) is that with minimal attention they can last for a long long time, and continue to look good.
So my question for you: can you buy a plain black belt with a stainless-steel buckle? I was somewhat surprised to find that every simple belt buckle I found in several stores and on Amazon either say solid brass or say nothing. I can find buckle-less belts, but the only stainless-steel stand-alone buckles I find are Texas-sized buckles, which isn't the look I'm going for.
Any ideas, or do I have to go with this?:
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| What's a Router? What's a Gateway? |
[Sep. 13th, 2007|05:22 pm] |
I posted this on Wikipedia, but didn't get any responses.
I'm confused about exactly what a router and gateway do; I want technical specifics not watered-down confusing explanations. I have a standard Linksys wireless router . It has two modes, Router and Gateway ; Gateway is the default. It has five Ethernet jacks on the back, 1, 2, 3, 4, and Internet. I don't understand what the Router or Gateway behavior is.
The router has a local IP address, which I can set (default: 192.168.1.1) and a subnet mask, which I can set from a dropdown (default: 255.255.255.0).
My sense of how it works is as follows (assuming the defaults above):
- In Gateway mode...
- If a packet is received on port 1–4 or via wireless...
- If that packet is addressed to 192.168.1.x (based on the local IP and the subnet mask), it will be retransmitted out the appropriate numbered Ethernet port or via wireless (determined via ARP).
- If that packet is addressed to something other than 192.168.1.x, it will be retransmitted out the port labeled Internet.
- If a packet is received on the port labeled Internet...
- It should be addressed to 192.168.1.x; retransmit it out the appropriate numbered Ethernet port or via wireless.
- If it's not addressed to 192.168.1.x, the sender did something wrong.
- In Router mode...
- I'm not sure how it's different. Care to explain?
Is the above accurate? If not, how is it wrong?
Update
I got it working with the topology I wanted: DSL modem (192.168.1.1) to Internet port of Wireless router (internally: 192.168.2.1) wireless to game adapter (192.168.2.250, gateway: 192.168.2.1) which then connects via wires to the Uplink port of a hub, which then connects to everything else. Sweet!
The only reasons I can guess that it didn't work before are (1) that the router was previously on the same subnet as the DSL modem and (2) that these devices would get confused as they got re-plugged together. When I powercycled them after plugging them together, they seemed happier. I think it must be their ARP or routing tables that needed to get rebuilt. |
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