MIDI, the Graph Tech ‘Ghost’ system, and How it all Works
 We owe a lot to Robert Moog, for the first synthesizers which gained wide public acceptance. Keys on his keyboard generated different voltages, which attached sound modules interpreted to mean 'play middle C' or some other note.
Later on, folks decided to simplify this voltage signaling system with a digital system -- less calibration, easier signaling. Thus was born MIDI (the Music Instrument Digital Interface). By sending digital info out a serial port, the keyboard could tell the sound module which note to play, but also other information such as: how loud.
In the world of synthesizers, one uses a keyboard which translates musician key-presses into a digital message. This is sent over a five-pin 'MIDI cable' to the sound module, which obeys orders. The sound module has audio outputs connected to an amplifier, and we hear the resulting note.
In the world of midi-guitar, there's a problem. How will a wiggling string be translated into a digital message that the sound module will understand?
The best solution evolved so far is like this:
Your guitar (or Megatar) is fitted with an unusual pickup, which actually provides a separate tiny pickup for every string. Since there are now six different signals, a regular guitar-cable cannot carry the signal, so we use a special 13-pin cable. (There are some control signals in addition to the six signals from strings.)
From the guitar or Megatar, the 13-pin cable carries the string signals to a 'brain', which analyses the string movement and figures out that the note is, for example, middle-C. Then, the brain sends out a digital message over a five-pin midi cable to the sound module: 'play middle C'.
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Two 13-Pin Outjacks
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Folks at Roland and elsewhere developed several versions of this 'brain', and some work better than others. Roland now provides several MIDI brains. Some are combined with the synth, and lie on the floor like a fat guitar stomp box. Other Roland units provide only the ‘brain’ and you need some other synthesizer to actually play the note.
For example, to use a Roland GR33 Guitar Synthesizer, an all-in-one unit, one just connects a 13-pin cable between the Megatar and the GR33 unit, and you start playing.
Is that all there is to it? Not quite. Each musician plays a bit differently, and so there are some sensitivity settings inside the MIDI brain that you'll need to adjust. Also, signals from the different strings are not equal, so there are some MIDI brain settings to adjust these. You should expect some tweaking to get smooth and even response, but it's not difficult. You will also find that certain sounds work better than others. For example, bass and string sounds are generally easy and seem to track perfectly. Some pianos and bells are more sensitive and require you to play very cleanly to avoid accidental notes.
Generally, newer MIDI brains work better than older models. But all of them require you to play cleanly.
Bass Strings and MIDI
What about bass strings? Many people have discovered that many MIDI brains do not track bass strings very well, probably because the MIDI brains are generally optimized for guitar and not the slower-moving bass strings. Some people have reported modestly good success with the ‘neural network’ MIDI brains made by the AXON company, but generally Roland MIDI brains have not, so far, been able to track bass strings very well.
Roland’s V-Guitar and V-Bass Modellers
But consider the Roland 'V-Guitar' system, and the very similar Roland ‘V-Bass’ system. First, it is not a sound module that is driven by a digital midi message. Think of it as a very elaborate effects modeler which works separately on each string's signal, as delivered from the instrument's hex pickup via the 13-pin cable.
A normal synthesizer sound module usually has an oscillator, which generates the basic signal, then some circuits which modify the basic signal to produce all the sounds in the world of synthesizers. One way to think of the Roland V-Guitar system is that it is like a synthesizer, but it uses each string's actual signal as the oscillator, then a bunch of circuits to modify that basic signal.
The V-Guitar System is especially good for modeling different guitar and bass sounds, as well as different amplifiers and cabinets, but it can also do unusual synth-like sounds. It does not have all synth sounds. For example, there are no flutes, bells, nor violins. But it remains unsurpassed for providing wonderful guitar and bass tones. And there is no tracking issue at all, because the string does not have to be interpreted. The string itself is the oscillator.
Magnetic ‘Hex’ pickups versus Piezo pickups
Roland does provides the old ‘GK’ series of magnetic pickups. These are usually called ‘hex’ pickups, because there are six tiny pickups on a little bar that you stick beneath the strings. However, positioning of this little bar and the tiny pickups is sometimes a problem, and exact positioning is needed to get the unit to work well. For one thing, the tiny pickup must be positioned at the very end of the string so that it does not pick up the higher-frequency wiggling of the harmonics in the vibrating string.
A better solution is the Graph Tech ‘Ghost’ system, that we provide on Mobius Megatar instruments. Here, we will install string saddles with piezo elements inside each one. Automatically we now have the piezo pickup positioned at the very end of the string. Furthermore, the piezo pickup inside the saddle is durable and sturdy and works very well.
The Ghost MIDI system requires a small circuit board which we’ll mount inside the pickups cavity with the rest of the wiring. You also get a MIDI volume knob and a patch-select switch mounted on your pickguard, so you can control the MIDI brain from the instrument.
We can install the Graph Tech ‘Ghost’ MIDI system on any Mobius Megatar instrument. It is a factory modification. The MIDI system can be installed on the melody side only, or we can install a system on the melody and another on the bass. In that case, you’d have two 13-pin MIDI ready outjacks, and you’d need two MIDI brains or synth units.
Mobius ‘MidiTapper’ Models with Ghost MIDI
We offer two specific ‘MidiTapper’ models (the ‘solo’ is melody side only, and the ‘duo’ is both melody and bass), but remember, the MIDI system can be added to *any* Mobius Megatar instrument.
With a Mobius Midi-enabled instrument, you can have the best of both worlds. You can play the Megatar, with the standard magnetic pickups, or you can connect the 13-pin cable to the Roland products to play bass and guitar. (Or you can play flutes and bells, french horns and piccolos, bugles and drums.)
When thinking about the MidiTapper and other custom MIDI-enabled Mobius instruments, we like to say 'All the Sounds of the Rainbow', because a rainbow actually has no sound of its own, yet it suggests all the colorful sounds in the world!
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