A New Ribbon Based Gestural Controller

For the longest time I've wanted a ribbon controller. I mean, what red-blooded adult has never yearned for the ability to place one's finger on a strip of plastic and produce a tone dependent upon the relative position of the chosen spot of contact? Alas, I'm too cheap to just buy a ribbon controller (Doepfer makes a pretty swell one, for example). Besides, it's just so against my grain to buy a piece of gear I could potentially build.

"Potentially" is the key word here. What my DIY ambition lacked was a nice piece of resistive wire that I could use to make the ribbon. Even that piece of wire would probably languish in my Bin of Broken Dreams, because my mechanical skills fall short in such a way as to prevent me from actually aligning said resistive wire with a plate of metal without it falling apart during some passionate ribbon controller solo. And forget those video tape creations...ppft. I'm sure one could be built sturdily enough, but not by me. I even thought about raiding 3M for some heel-strap material - that conductive stuff that doesn't let you zappify electronic comoponents (providing you're standing on a conductive floor of course). But what would I do with it once I got it? No, what I needed was someone to actually put together a generic ribbon thingie that I could buy off the shelf and mold into my own personal Controller Of Doom.

And someone did.

A company by the name of Spectra Soft manufactures just such a material, which they call the "SoftPot". Better yet, Spark Fun sells them. I'd heard of SoftPot before, but never really encountered it in my usual Surplus/Hobby store trolling. And there it floated before my eyes, transfixed upon the computer monitor daring me to just....buy it. It was, of course, a simple matter of reflex to plunk down the credit card and order it without a second's hesitation. Clicking that "Confirm Order" button sent my brain down the path of (to quote David Byrne) "How do I work this?", from which my system has yet to recover.

As I awaited the arrival of the Sacred SoftPot, I began to scheme on how to make it do what I wanted it to do. The SoftPot really is just a potentiometer that's long and flat; sort of like a slider pot, I suppose. Instead of turning a the knob on a round pot or moving a slider on a slider pot to determine where on the resistive strip the tap of the pot is situated, the SoftPot uses pressure to "press" the tap of the pot against some point on the resistive strip. Unlike a round pot or a slider pot, the tap won't stay at that point once you release the pressure - the resistance at the tap from either end of the SoftPot goes, for all practical purposes, to infinity.

Now, one can easily put some voltage on one end of the SoftPot, ground the other end of the SoftPot, and take the voltage off of the wiper to control whatever it is you plan on controlling. Obviously, the catch is that once you release the pressure, the voltage drops to 0V or to max voltage, however you have it wired. So, that would work fine if you start at one end of the SoftPot, slide your finger to the point(s) you want, then (if you don't want a sudden jarring transition when you release pressure), slide the finger back down to where you started (likely zero volts). This would work. This would also be lame.

No - when I think Ribbon Controller, I think pressing the ribbon at any point, releasing it, and having the voltage *stay at that point*. In other words, it has to "remember" where you last touched it. And, in addition to that, the ribbon must generate a positive gate signal when it is pressed, and a zero gate signal when it is released. And, due to my affection for triggers, it has to generate a trigger pulse when first pressed (but that's easy if there's a gate generated).

My mind also dug up some faint memory of watching somebody play a synth that had a ribbon controller on it - this person was using it as a pitch bend for the keyboard. The thing that impressed me about it was that the player didn't seem to care *where* on the ribbon controller he pressed it - it bent up and down the same amount wherever he pressed it. I wanted that, too, but I had bigger fish to fry just getting the bare minimum I wanted (Memory, Gate, Trigger).

I knew the Moog Ribbon Controller produced a gate signal and had memory, so I had a look at those schematics. Hmmm....to generate the gate, there were a pair of metal plates that produced the gate signal as long as they were pressed. Cool, but not what I had in mind. My fantasy ribbon controller must only require the single touch of a finger to do anything. I just didn't want to futz with aligning plates or metallic tape along the ribbon. I'd be lucky to apply the SoftPot straight, much less anything else.

The Moog uses a sample and hold for memory (something I'd already realized would be required). However, this sample and hold "remembers" the voltage on a 1 uF cap when the resistive wire breaks contact with the conductive portion of the ribbon. Yikes! No help there - if I'm not using a plate to generate the gate, it has to be generated some other way - and it has to fall after the memory kicks in.

This was a conundrum - I planned on using a comparator to go high when contact was sensed on the tap of the SoftPot. That meant the gate would happen before the voltage had risen to the full level of the pressure point - I didn't know, but that just didn't sound good. Then, when the pressure was released, the comparator would fall low - but the voltage would have had to fall low to make the gate fall low. So, the voltage must be somehow triggered to hold *before* it fell far enough to cause the gate to go low. How in the world could you make the ribbon "predict" when to sample (or stop sampling) before the voltage dropped low, without some sort of additional external control? This would go against my "one finger only" dream.

I figured some kind of lag would help out - delay the sample and hold the same time constant as the lag, so by the time the lag had risen to voltage, it could be delayed to gate on then. The same lag would be used to sample a lagged version of the voltage after the original voltage had dropped low enough to drop the comparator. The comparator would also be the source of the gate and trigger signals. Easy as pie!

Heh.

To make the start of a long story short, the SoftPot did arrive. And I've built a ribbon controller that exceeds my wildest ribbon controller fantasies - it's more than a ribbon controller, in fact. But developing it was not the bed of sweet smelling petunias I'd imagined. The experience was more like trying to sleep on a hot, sticky, poison ivy covered cot. A big plus derived from the experience, other than having a kick-ass ribbon controller, is that I discovered combinations of certain words that had never occurred to me before. For a while there, on and off, the air was fragrant with them.....

You'll have to read on to find out why.

TO BE CONTINUED......

This Page Copyright (C) 2008, Scott Stites