1. liu
  2. MIDI Hardware
  3. Wednesday, 01 November 2017
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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:

http://www.kynix.com/BlogImage/11.2.3.jpg

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 such as:http://www.kynix.com/Detail/3673/944U700K142ACM.html ?) 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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Geoff Accepted Answer Pending Moderation
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Hello Liu,

Just to get past the SPAM item above (now removed).

There is actually quite a lot of information on the RORguitars site, including various videos, and quite a bit of comment from other folks. The technical specs also give some hints of other things.

It seems that the design will depend on the creation/use of some existing midi controllers, and some 'new' ones which I assume will use various user or manufacturer controllers which are available within the spec, but not normally defined. How this guitar system uses such controllers will be up to them. Their equipment may know about this, no-one else may take any notice at all.

Some of the comments are raising questions, basically why?

The 'expressiveness' of a guitar (in the right hands) is a specific feature. If you want to play a guitar, then play a guitar. If you want to play a synthesiser/keyboard, then play a synth. Do the two really mix, and what is the point?

Yes, there are guitar controllers, these are really to allow a guitarist (non keyboard player) to play a synth via the guitar. Using a guitar, but really playing a synth.

Midi does not really have enough scope to do the job properly, although with a lot of work it can do fairly well. Using a lot of special 'tricks'. The main point of midi is to enable wide compatibility, this would be lost if you could play the instrument ONLY via specific software, and specific hardware, even more so if that was all fairly expensive.

Many instruments have similar problems. There is no ONE specific sound. The sound varies, quite a lot, depending on how the instrument is played. Even a piano has this problem. Most string instruments even more so. MUCH more so?

For a guitar, you could collect a variety of different guitar sounds and 'tweak' a midi file to swap between various sounds to create a simulation of the expressiveness you seek, but it would be hard work?

Geoff
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