How Do You Measure Position on a Fingerboard?

17 06 2012

My first idea was just to put long ribbon controllers (like laptop touchpads, but long and skinny and sensitive only to one dimension) under the strings.  But there are two problems with that.  First, nobody seems to make ribbon controllers any longer than about 100mm.  Second, my bass strings can, over time, wear grooves in even ultra-hard ebony fingerboards.  I’d expect plastic ribbon controllers to last maybe ten minutes.

My second idea was to put a little piezoelectric transducer on the bridge end of the string and have them fire periodic compression pulses down the string, listen for the reflection from the point where the string is held down, and compute position from the delay.  But there three problems with that.  First, it’s going to be difficult to damp those pulses out at the bridge end; most likely, they’ll reflect off each end a couple of times before they disappear into the noise floor.  How am I going to tell the reflection of the pulse I just sent from the continuing echoes of its predecessors?  Second, holding a string against a fingerboard definitely stops transverse waves; but I have no reason for believing that it would cause a clean echo of a compression wave.  My instinct is that there’d be a very weak reflection from the fingered position followed by a nice sharp one from the other end of the string, so I’d have to find the weak, fuzzy one among all the sharp, clear ones.  Third, a compression pulse travels at about 20,000 feet per second down a steel string.  If I want, say, 1mm resolution in positioning, then even if the other two problems evaporate I’m going to have to be able to measure the delay with a granularity of 164 nanoseconds, which isn’t particularly realistic for the sort of microcontroller hardware one expects to find in a MIDI controller.

My third idea was to make the fingerboard of some hard, conductive metal and the strings of resistance wire–that is, wire that has a significant resistance per unit length, unlike copper or aluminum.  Then you could use an analog-to-digital converter to measure the resistance between the fingerboard and the bridge end of each string, and you’d get either infinity–if the string wasn’t being fingered at all–or a value roughly proportional to the distance of the fingered point from the bridge.

There are a whole host of problems with this third idea.

Resistance is affected by more than where the string is fingered.  For example, a small contact patch (light pressure) will have more resistance than a large contact patch (heavy pressure).  A place on the string or fingerboard with more oxidation will have more resistance than a place with less oxidation.  But…stainless steel doesn’t oxidize (much), and neither does nichrome resistance wire, if you’re not using it for a heating element.

Nichrome resistance wire doesn’t have as much resistance as one might imagine.  In one diameter that’s roughly the size of a middling guitar string, its resistance is about one third of an ohm per foot.  I can crank my ADC’s reference voltage down as far as 1.0V, but if I run 1.0V through about eight inches of wire (what I expect to be left over at the end of the fingerboard once a string is fingered in the highest position), Ohm’s Law says I’ll get 4.5 amps of current, which will burn my poor little ADC to a smoking crisp, as well as frying the battery and even heating up the strings significantly.  But…I could limit the current to one milliamp and use an operational amplifier with a gain of 1000 or so to get a low-current variable voltage between 0 and 1V for the ADC.

A/D converters don’t measure resistance, they measure voltage.  Sure, it’s easy to convert resistance into voltage; but it’s easy to convert all sorts of things into voltage, and much harder than you would think to avoid converting them–things like electromagnetic interference from elevators, power lines, air conditioners, and so on.  The signal arriving at the ADC is liable to be pretty noisy, especially given the required amplification.  But…I could add another string on the neck and put it somewhere where it couldn’t be played.  That string would be subject to all the same noise and interference as the one being played.  The only difference would be that it wouldn’t be being played; so you could use the differential mode of the op amp to subtract one from the other and cancel out the common stuff.

One idea I had to get rid of the amplifier, and all the noise it would amplify, would be to ditch the 16ga nichrome wire (about the size of a guitar string) and use 40ga instead: very skinny, with a significantly higher resistance per foot.  Then coat it with insulating enamel, like magnet wire, and wrap it around a central support wire made of steel or bronze, so that it looked very much like a real guitar string.  Then sand off the enamel from the outside of the string so that the nichrome can contact the steel fingerboard.  In this case, our nichrome wire is much thinner, with more resistance per foot, and much longer, because it’s wound in a helix instead of being straight.  I ran the numbers and discovered that I could expect about 10,000 ohms per foot for a string like that: almost perfect for the ADC.

Then I remembered that the resistance of the human body is about 10,000 ohms: the resistance of the system would be significantly modified if anyone touched it without insulating gloves.  So…ditch that idea.  (A 10,000-ohm parallel resistance will have no perceptible influence on the 1-ohm resistor you get from a three-foot 16ga nichrome wire.)

So there are problems with this third approach too, but all of them at least appear to be surmountable.  It might be worth a try.



One response

28 12 2013

Glad I found your post – I’ve been thinking along similar lines. I was thinking there are three approaches: 1) resistance, as you describe; 2) voltage; 3) capacitance including aerial behaviour. The last one is interesting because it’s how a Theremin operates – as your hand moves near different points on its aerial, the circuit is altered. There are touch-sensitive switches that work this way. So could the guitar strings be low-powered aerials and we detect position of the fingers where they touch? However, perhaps a fourth approach is to consider the strings and frets as a switch matrix: when you press a note, a contact is made between string and the fret. Can a small electrical current pass along the string and emerge at the fret? Seems some ribbon cable would be needed to wire up the neck. Not sure whether the strings are senders or receivers, or what diodes are needed to manage unhelpful shorts. Interested? pls PM me.

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