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Negative "eeny meeny miny moe"

As a kid, I learned the rhyme as:
Eeny, meeny, miny, moe,
Catch a tiger by the toe.
If he hollers, let him go,
Out goes Y, O, U!
Since kids can't predict where it will end, and adults are not supposed to try, it's a reasonably fair way of drawing lots.

At times I've heard versions where the selected person wins instead of loses, and while with two kids it doesn't matter, with three or more it matters a lot!

Let's model each kid having a choice at each stage between "accept" and "protest". While protesting probably doesn't work, if enough of you protest it might. If you do the positive version, where the selected kid wins, the winner accepts but the others may choose to protest. This isn't good: everyone has reason to protest except the single winner.

On the other hand, with the negative version, where one kid is eliminated at once, it's the other way around. When the first kid is eliminated they may protest, but the other kids all accept because then they retain their chance to win. With each successive round the dynamic is the same, plus the already-eliminated kids all choose accept out of a desire for fairness. Even with the last elimination there's still only one person choosing protest.

The iterative process is O(n) instead of O(1), but it also works much better because it keeps a majority for "accept" at each stage.

(If you have a very large group of kids, then I could imagine a O(log(n)) version being worth the added complexity. Divide the kids into three groups, and do negative eeny meeny miny moe on the groups. A third of the kids may protest, but you've still got two thirds accepting. Then redivide those remaining two thirds into three groups, and keep going.)

full post...

Battery Powered Contra II

A few weeks ago I was getting ready to run sound for an outdoor contra dance in a place without electricity available. I brought my battery system but I also borrowed a generator, which was good because the battery system didn't work and we needed to use the generator the whole night!

The way the system failed was that when I would play especially loud bass notes on the piano the inverter would briefly switch off. This makes sense: amplifying bass requires a lot of power. If we hadn't had the generator we could have used EQ to take most of the bass out of things and probably done ok, but what would I need to run a full dance with good sound on just battery?

I had three guesses about what might be wrong:

  • The inverter was not able to handle its full rated power.

  • I was using the cables that came with the inverter. They weren't marked for AWG, but they did say 4mm2. That corresponds to 12 AWG, which is way too low for an inverter with a surge rating of 1000W.

  • While I had two batteries, I didn't have them wired together. This meant I was only running the system off of one battery at a time. The more current you draw from a battery the more its internal resistance starts to matter.

Since the second two issues seemed substantial, I decided to fix them and figure that the inverter was probably ok.

In my earlier testing I was seeing a draw of ~200W, and because sound can have some very sharp peaks I want to be using wires that can handle the full 1000W surge current the inverter is rated for. At 12V, 1000W is 83A, which is a lot. There was probably 3ft of wiring, round trip, so my 4mm2 wires were probably seeing a peak voltage drop of maybe ~7%. Using 4 AWG wire would bring us to ~1% loss, but to be on the safe side I decided to go with 2 AWG since the cost increase for such short wires was small.

I also wanted to wire the batteries in parallel, so they could both be supplying power at once. With this many things to connect, I also wanted to be using proper connectors instead of clip-on jaws. It looks like Anderson SB connectors are very standard for this sort of thing, so I decided to wire this up as:

   +-------------+
+--| 12v battery |--+
|  +-------------+  |
+--------+ +--------+
         SBC
         SBC
+--------+ +--------+
|  +-------------+  |
+--| 12v battery |--+
|  +-------------+  |
+--------+ +--------+
         SBC
         SBC
+--------+ +--------+
|  +--------------+ |
+--| 12v inverter |-+
   +--------------+
         [ac]
This is four SB connectors, each with their lugs bolted to something: one on the first battery, two on the second battery, and one on the inverter. It's possible to buy the parts yourself and assemble them, but after looking into the tools I would need to solder or crimp them, and the chance that I would screw it up and need to re-order, I decided to just buy them already assembled.

Here it is, all set up for testing with two QSC K10.2s:

I tested with a relatively loud section of a recent Kingfisher Amherst gig (mp3). If I turned it up loud enough it would start cutting out, but I had to make it very loud. I turned it down until it reliably wouldn't cut out, which was still very loud. I don't have a way to quantify how loud it was, but:

  • Wearing my 28dB hearing protection it was unpleasantly loud.
  • This was 15dB louder (according to the mixer) than the maximum volume I could tolerate with no hearing protection at 10ft.
I think this is substantially louder than we had it for the outdoor dance, but it's hard to say because my basement and a windy plaza are very different places.

Running at this volume, the power meter I had on the battery varied with the music's transients and it's hard to know the peak power usage because I don't know how short a period it's averaging over. I'm guessing the peak was just below 1000W, though, because that's what we'd expect from the inverter's surge rating. The most the meter read was ~300W. When I took measurements before I only got up to 75W, and when I was trying it at the dance I saw the power cutting out when the meter hit ~200W, so it does sound like it's doing better.

The meter also has Wh measurement, and I measured it as using 10 Wh in 266s and then again 13 Wh in 309s. That comes to 135W and 151W on average for very high energy parts of music.

I also had a kill-a-watt style meter hooked up on the AC side of the inverter. It measured a maximum of just under 2A (240W) with a power factor between 0.60 and 0.75.

Overall this does now look like the problems from before are resolved and this should work, but it's hard to say for sure without trying it outside in an environment more like a dance.

(We should also be fine for a capacity perspective. At high draw we were talking about 2Wh per minute, so a 3hr dance would be 360Wh. I have two 100Ah batteries at 12V, so this is a 15% discharge. Double this for everything else on stage that uses power, and we're still only at 30%.)

full post...

Rent Map v2

A bit over a year ago I made a set of one-off maps with some improvements, and I've now ported those improvements over to the main map:

The changes are:

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Pronouns for preverbal children

How should parents decide what pronouns to use for their babies? In my part of the world there's strong agreement that you should use someone's indicated pronouns, but what does this mean for children too young to communicate? I think in today's world this is a close call, and how you go depends on how you weigh potential harm through misgendering against other more likely but less severe harms.

The main reason to use "they" for preverbal children is that you don't know their internal experience of gender (if they even have one) until they're able to start expressing it. Using "they" allows the child to continue without an assigned gender until they start feeling like they have one and request otherwise, or they may choose to continue with "they" indefinitely. The experience of being assigned a gender that is contrary to your internal experience is one that trans folks have written a lot about. It's clearly a strong negative, and something I don't want to subject my children to.

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Driveway Car Lifts?

Here in Somerville it's common for houses to have a driveway with tandem parking for two or three cars, often shared among multiple housemates or even multiple units in the same building. Tandem parking is generally annoying, since it requires people to coordinate around driveway usage. Why don't we use car lifts?

The idea is you put a lift on the outer parking space, sufficient to clear access to the inner parking space. Now you can park two cars with almost as little hassle as with side-by-side parking, with room for an occasional extra car.

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Larks/Robins?

Contra dance has two roles, which traditionally have been divided by gender. Like many other social dances, the contra dance community is in the process of moving to gender-free terms, but unlike other social dances we've had a lot of trouble figuring out what those terms should be. After about five years of experimentation, the community has been settling on Larks/Ravens.

Unfortunately, there's a conflict with 'Ravens' that we weren't aware of when choosing the term. The Tlingit people of the Pacific Northwest divide their society into two groups (moieties), one of which is 'Ravens'. Traditionally, marriages could only be between a Raven and a non-Raven.

This use of 'Raven' as part of a binary identity is uncomfortably close to the contradance usage, and there's recently been discussion on the Gender Free Contra group about how to handle this.

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