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Rorguitars claim to have the best MIDI* guitar system on the market. Their website says:

Our guitars do not have MIDI pickups. There is NO pitch detection to send the MIDI signals. Our instruments use an electronic system that senses the frets you press on the fretboard and sends the corresponding notes. [Thanks to Steve G for pointing this out.]

It's not possible to get polyphonic guitar to MIDI conversion based on electrical string/fret contact scanning alone (segmented frets pose wear problems, and it doesn't look like they use them in their product), so some additional sensing must be involved. Multipress keyboards are possible but they can use diodes to prevent crossfeed between rows and columns. This isn't possible on the point of contact between string and fret.

Apparently developed at University of Limerick, Ireland, there is no further info on the web. Probably they don't want to disclose it.

What techniques could be employed to determine the highest fret pressed on each string in such an instrument while handling the short circuits produced by multiple string presses on multiple frets? (Each string press contacts the fret above and below the point of pressure.)

To illustrate:

enter image description here

Frets and strings form a matrix which can be scanned.

Single notes and notes apart more than two frets pose no problem, but ambiguity arises with close spaced chords, here a simple doublestop.

Both the G and D string read frets 2,3 and 4, as the conducting path in blue crosses strings.

Determining the shortest path (in green) provides the additional information required to discern chord shapes.

Out of the possibilities

  • Capacitance
  • Resistance
  • Reflectometry

for string sensing, lets pick the last option - not least for the purposes of being specific. Also the first two might be influenced by hand contact.

It is not necessary to detect exact lengths, being able to compare whether one fret is closer than the other is sufficient.

So the question is whether it's feasible, given the required rise times, to implement this in a suitable form factor.


Edit:

I found an interesting patent from 1984 (US4630520) by Turtle Beach cofounder Carmine Bonanno.

Wonder why it never took off.

It's the combination approach I had in mind, only with resistance sensing, as suggested by Transistor.

Frets and strings are scanned separately by multiplexers. As reliability issue he stressed the fret/string contact resistance, requiring low current, therefore high input impedance.

I wonder however if an (initial) sufficiently high wetting current (snubber capacitor?) might help alleviate noisy junctions.


  • MIDI (/ˈmɪdi/; short for Musical Instrument Digital Interface) is a technical standard that describes a protocol, digital interface and connectors and allows a wide variety of electronic musical instruments, computers and other related devices to connect and communicate with one another.

MIDI carries event messages that specify notation, pitch and velocity, control signals for parameters such as volume, vibrato, audio panning, cues, and clock signals that set and synchronize tempo between multiple devices.

Generating MIDI signals from non-keyboard instruments such as, in this case, a guitar poses some interesting electronic design challenges. While keyboard instruments are relatively straight-forward to digitise string and wind instruments have many more nuances that are difficult to detect and measure and, therefore, difficult to convert to MIDI.

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  • \$\begingroup\$ On the FAQ page they state Our guitars do not have MIDI pickups. There is NO pitch detection to send the MIDI signals. Our instruments use an electronic system that senses the frets you press on the fretboard and sends the corresponding notes. So they do use fret sensing. \$\endgroup\$ – Steve G Sep 22 '16 at 21:07
  • \$\begingroup\$ @Steve G Exactly. If you do the math, you'll discover that you can't do polyphonic detection this way. Hence the mystery additional sensing tech. \$\endgroup\$ – LogicBreaker Sep 22 '16 at 21:10
  • \$\begingroup\$ @Transistor Thanks for the edit. I was thinking along the lines of Reflectometry. There's various methods (FWR, SWR, PD-FDR), all probably lacking the required resolution. However, all we need is sufficient relative resolution to compare fret distances. \$\endgroup\$ – LogicBreaker Sep 23 '16 at 2:10
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It may be a lot simpler than you think. Joe Brown has hacked a cheap guitar and is using resistance measurement to read the first point of contact on each string.

enter image description here enter image description here

Figures 1 & 2. A constant current is fed from the bridge to the top nut on each string. All the frets are grounded.

schematic

simulate this circuit – Schematic created using CircuitLab

Figure 3. Basic layout.

My understanding is:

  • A constant current is driven through each string from the bridge to the top-nut. The strings are grounded at the nut (a zero-fret or metal nut would be simpler than the arrangement of Figure 1, I suspect) and all the frets are grounded.
  • A 4-wire measurement is performed to eliminate voltage drop along the current carrying wires. It's not clear how this is done at the nut.
  • A calibration scan is performed on power-up to measure the bridge voltage on each string.
  • The fretted position can now be read from the bridge voltage reading and a lookup or calculation performed to get the fret position.
  • I assume that bends can be accommodated as the resistance and length (and cross-section?) of the string will change with the stretch.

The amazing thing is that it appears to work. I think he has published the design and there is a lot of interest on the DIY stomp boxes forum.

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  • \$\begingroup\$ Yes, I've come across his site before. Unfortunately he didn't post any video, I've contacted him. Should be possible to combine the two methods, the only problem that comes to mind immediately would be resting your right hand on the strings, e.g. palm muting. \$\endgroup\$ – LogicBreaker Sep 25 '16 at 17:29
  • \$\begingroup\$ I considered that but the system is low impedance due to the low resistivity of the strings. Sweaty palms would be relatively high. \$\endgroup\$ – Transistor Sep 26 '16 at 9:10
  • \$\begingroup\$ Right. I measured about 1.6 mΩ (top) to 0.8 mΩ (lower strings), end to end. With amplification this shouldn't be a problem even for 10 bit Arduino ADC - not speaking of bending. The biggest contender is probably contact noise. \$\endgroup\$ – LogicBreaker Sep 26 '16 at 23:17
  • \$\begingroup\$ Interesting, as I proto-typed such a guitar using this principal back in the 1990s. All frets were connected to a low resistance bus with a very low regulated voltage. At the bridge position, separate elements sensed the current to ground, converted it to a voltage, and then to a proportional frequency. In my case I used that frequency to serve as a sample rate to play back stored guitar string tones, and I sensed picking the string to start the sample playback. The premise worked and could be done more easily these days, converting the voltages and pick strokes to midi data. \$\endgroup\$ – Randy Dec 22 '18 at 22:48
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Pablo Schramm emailed me regarding my current work with my MIDi Guitar implementation. I've proved the principle in practise, implementing the system on the two smallest diameter strings. The software is (mostly) finished, but practical implementation on various instruments as a kit was being examined when work was halted last year because of other, more pressing demands on my time. The Expressiv MIDI pro looks good - I also play a Telecaster, as well as acoustics, and would like to trial Rorguitars product, but my impression is that there are still none available.

My personal preference is for a kit/kits - rather than being 'stuck' with an instrument ill-suited to my needs. Hopefully, at some point soon, I will resume my work and publish it.

A couple of additional points:

WRT to 'palm-muting', the constant-current method I employ would not be affected by the relatively high resistance of someone's palm - even a sweaty one like mine. String bending, as a means of tone-bending MIDI, is however doubtful using my current note-determination implementation, but with a faster high-resolution ADC, and a powerful microprocessor, this should be possible.

Regards, Joe Brown.

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  • \$\begingroup\$ @LogicBreaker: Well that's blown your cover! ;^) \$\endgroup\$ – Transistor Sep 26 '16 at 9:07
  • \$\begingroup\$ omg, I have to behave now. \$\endgroup\$ – LogicBreaker Sep 26 '16 at 9:48

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