Servoelectric Guitar Is A Keytar With Strings


[Keith Baxter] has undertaken something of a ‘Mount Everest’ of guitar modifications. He’s developing a Servoelectric guitar that trades frets for a keypad. It is still a guitar in the sense that it has a body, strings, and pickups to sense the strings vibrations and pass them to an amplifier. The left hand, which traditionally would shorten the strings as needed by pressing them against a fret, now changes string pitch using a keypad. This is an interesting fusion between traditional guitar and 80’s phenomenon, the keytar.

Each string is connected to a different servo motor. When a key on the keypad is pressed, the corresponding servo adjusts the tension of that string, bringing it in tune at the new pitch. His original design involved a lot of custom circuitry but he’s evolved the project to include an Arduino controller. This second generation both simplifies the control circuitry and improves upon it.

We’ve embedded some video after the break. In the first example you can see the strings adjusting for each new pitch. In the second, take a look behind the guitarist… what do you think he’s got planned for those giant capacitors?

18 thoughts on “Servoelectric Guitar Is A Keytar With Strings

  1. Can you say “portamento”?

    There’s a country instrument that sounds like this as well. Steel Guitar? Slide Guitar? Makes for a really odd sound if you can’t turn that effect on and off like you would with the pedals on the instrument I’m thinking of. And yeah, he’s probably going to go through strings like mad, or at least be fine-tuning them forever.

  2. Similar sound to a Steel Guitar, but those are foot pedals and you need to know combinations of pedals and leg levers to make the thing work well. This you just have to push buttons and pluck a string. Sounds great! Look forward to seeing the keyboard along the neck of the guitar rather than on the side of the neck.

  3. Thanks for the link.

    So far I’m only on the first set of strings. I guess there is no reason the stretching should matter. Metallurgists?

    The tuning is okay because of the 1 inch steel pipe running along the backbone:) You have to do this so changing one string tension doesn’t detune the others.

    The keyboard location is a bit uncool. What’s a minigun/railgun? Maybe something to use those capacitors for!

  4. The Casio DG20 was a completely different beast – the strings were only note triggers and pitch selection was done by the frets which were touch sensitive. You could actually press the fret underneath the string and pluck it to get a note and I considered modding mine to move the ‘machine head’ anchor points down to the bottom of the neck but never got round to it. Horrible thing to play but ok for note entry into a sequencer.

    This is more like the Transperformance or the Gibson Robot however they use it only for tuning not for actual note selection. Great for switching quickly between different tunings thereby eliminating the need for multiple guitars on stage (or bad players who can’t tell when their instrument is drifting out of tune :) )

  5. Yup. Less time fiddeling with stuff that doesn’t matter (why did my clock stop? why can’t I read my ADC register? How do I calculate the stabilization caps? why am I doing this again?) and more time working on a fun idea like servo tuning or turning a waffle maker into a toner transfer oven.

  6. It seems to me you could do away with a lot of the mechanical nonsense by hacking into the pickup to separate it out into each of the 6 strings. The arduino could then do an A/D conversion on each string, detect the current pitch and adjust as necessary until the pitch requested by the user is what comes out. Then the system is continually self-tuning, and you maintain the closed-loop feedback necessary for stable operation.

  7. I just read deeper and saw this:
    “I spent some time investigating whether the frequency sensing could be done by monitoring the strings themselves but concluded to capture time was too slow both because of the inherent time window required for accurate frequency detection and the lags of the filtration needed to remove guitar string harmonics.”

    Maybe one could use something like a VCO, but the reverse function (convert the frequency into a voltage)? Then it’s just a quick A/D conversion away from a frequency measurement.

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