February 28, 2010 Comments Off

Bongo Machine v. 2

The bongo machine was the first automaton that I built, which perhaps makes it appropriate to sacrifice on the altar of technological progress. The picture at left shows the new version under construction, and you can see that three legs from the original have been cannibalized for the new machine. I hope they carry some residue of rhythmic wisdom with them.

The new machine has a quasi art deco look (perhaps hard to see at this stage and in this pic) and will allow me to adjust the vertical and horizontal position of the solenoids over the drum head, just as with the djembe machine. This is very important. If you’re building your own percussion automata, I can’t emphasis how useful flexible positioning of the beaters is. When you’re designing the machine, you don’t necessarily know what will make the best beater, how big it will be, or where to position it. If you give yourself the flexibility to change the position of the solenoid later, you can experiment with different beaters and positions over the drum. This is a lesson I learned the hard way with the bongo machine and incorporated into the djembe machine.

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February 27, 2010 7 Comments

Optos and Switching Time

I need the help of the EE people on this. I’ll have a more general interest post about microtonal harmony soon.

I mentioned in an earlier post that I think the optos that I’m using in my solenoid driver circuit are adulterating the PWM signal by acting as a low pass filter. The first indication that there was a problem occurred during testing, when I noticed that the voltage across the solenoid did not vary linearly with the PWM duty cycle. Instead, the voltage crept up slowly for low duty cycles and then shot up rapidly as I approached the mid duty cycle range. This is what one would expect if the PWM signal had its edges smoothed out by a low pass filter. (Of course, if I had a scope, I verify this theory). I managed the problem in software by using only a narrow range of duty cycles, but even then, the non-linear relationship between the voltage over the solenoid the PWM signal has caused a lot of annoyance.

[Update: thanks to peabody for correcting my unit conversion below, but now I'm just more confused]. Over the past few days, I’ve gotten two comments on the topic from people more expert than me; one supported me and one disagreed, saying that the switching time for optos is in the tens of nano-seconds and therefore shouldn’t matter. That may be true for some optos, but I looked up the datasheet for the 4N29 optos that I’ve used, and it looks like the switching time is on the order of 150 to 300 nanoseconds, which in the context of a 1000 hz PWM signal, might make a difference for lower duty cycles. A 1000 hz PWM signal has a period of 1 ms = 1,000,000 nanoseconds. Even at a 10% duty cycle, which has an on-time of 100,000 nanoseconds, smoothing out the first 300 nanoseconds shouldn’t make a difference.

My second question is, if I wanted to get rid of the optos, how would I redesign the circuit? I see no reason that I couldn’t eliminate the opto and connect the PWM chip, which pulses a connection to ground, to the base of a PNP Darlington. What I’d like to do for my next machine is use Darlington arrays, but Darlington arrays seem to come only in the NPN flavor, which means I’d have to invert the signal from the PWM chip from active low to active high, which requires more components. Suggestions?

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February 24, 2010 1 Comment

Rhythm Science

For years I’ve been interested in what makes some rhythmic patterns more compelling than others. Certain beats and certain clave patterns have the power to entrance listeners or at least sustain a rhythmic texture for minutes. These rhythms tend share a number of mathematical characteristics, and the computer scientist Godfried Toussaint has done fascinating work in investigating these characteristics. His work becomes particularly interesting when read alongside the work of musicologists and percussionists like Simha Arom and David Locke. Locke argues persuasively in his book Drum Gahu that one of the defining characteristics of the Gahu bell pattern is metrical ambiguity–specifically, the property that almost any strike in the Gahu bell pattern could function as the downbeat. The ambiguous metrical foundation laid by the bell allows other instruments to influence subtly how the listener perceives the meter. My speculative thinking is that the ambiguity of the pattern is what keeps it interesting. Players in the ensemble can flip the listener’s metrical orientation with subtle accents, and the attentive listener can even perform a Gestalt flip on his own by deliberately trying to hear a particular bell stroke as the downbeat.

A couple years ago I tried to pivot off of Toussaint’s research to quantify metrical ambiguity. The resulting paper is here. This research, believe it or not, has actually influenced how I improvise. With Jazari, I usually try to have at least one instrument play a metrically ambiguous pattern–which isn’t to say that I’m doing the equations in my head. Rather, once you’ve researched metrically ambiguous rhythms, you get a sense for how they feel, and it’s this feel that I go after.

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February 22, 2010 2 Comments

By Popular Request, You Can Support Jazari

Who am I to argue with the people? And rest assured, your donations will not fund the extravagant roboticist lifestyle we’ve seen depicted in Popular Mechanics in recent years. No Lear jets, no strippers, no coke-snorting automata. Every cent will help build (physical) drum machines and a bass machine. Oh yes, there will be a bass machine.

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February 22, 2010 4 Comments

Circuits, Solenoids, Microtiming

I’ve had some requests for a diagram of the circuit that I use, and I’m going to post it with the proviso that no one should actually imitate this design. Partly due to my own ignorance at the time, and partly due to a failed attempt to isolate the power electronics from the Arduino, I used a combination of an opto-coupler and an NPN transistor where a single PNP transistor could have been used. But for the curious, here it is:
When I have nothing else to do, which will be approximately never, I’ll rebuild the circuits with PNPs and eliminate the opto-couplers, which I suspect damage the switching speed of the PWM signal. I don’t have a scope to verify my hunch, but it would make sense; optos are used in compressors because they smooth out a signal, which is the opposite of what I want. Right now, everything works, so this is a lower priority. (more…)

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February 20, 2010 8 Comments

What’s the Point?

I’ve funded Jazari without grants, which means I’ve never had to put together a proposal in which I justify my project with references to obscure aesthetic philosophers, giving back to the community, or helping children overcome disabilities (I could see some potential in the last, actually). Being freed from the constraints of the grant process has saved me a lot of time and the world a lot of bad prose, but it’s left me under-prepared to answer challenges like “So, what’s the point?” Here, I’ll try to sketch a few thoughts on the subject.

The original impetus came from the failure of my electronic style-modeling music to connect with audiences. I had implemented algorithms that built statistical models of a song or style that allowed me to generate original music in that style. These algorithms worked on a symbolic (note-based) level, so the results of the algorithm had to be manifested as audio with synths or samplers. So, I was a guy on stage with a computer, and the music came out of speakers. No one had any idea what I was doing to create the sounds they were hearing, and for all the audience knew, I pushed play and checked my email. Continuing in this vein was a dead end. (more…)

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February 9, 2010 6 Comments

For the Gearheads

When I played guitar in high school, conversations with other guitarists inevitably turned to gear: makes and models of guitars, amps, pedals, and so on, all the way to preferred pick thickness. Maybe these types of dialog are the musician’s conversational safety net; failing any meaningful social bond, guitarists can always weigh the merits of the Flying-V. At the time, these conversations annoyed me because I was more interested in music theory than esoteric fuzz boxes. I thought, What’s the point in effects if you don’t know what notes to play? Today I could see other sides of the argument, but I’m still suspicious of conflating interesting gear with interesting ideas.

That said, people are naturally curious and want to know how things work. Here I’ll present some of the details that I skipped over in my How It Works youtube video, which you will be able to view in the video page sometime around mid February. I’ll start at the beginning of the signal chain.

The WiiMotes transmit a Bluetooth signal that is received by my MacBook Pro. This signal contains raw information about the state of the Wii: which buttons are depressed, its orientation in space, and its acceleration in three axes. On my MacBook, I use a small, clever program called OSCulator to translate that data into OSC messages and make them available on a local port. In MAX from Cycling ’74, I parse OSC messages with a custom Java external. Once parsed, state information from the OSC messages is sent to other homebrew Java externals that use the state of the buttons and the orientation of the Wii to determine if a note should be played, when it should be played, and how loud. How that process works is much easier to demonstrate than describe, so I would encourage you to watch the video if you haven’t done so already. (more…)

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