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Page created December 17th, 2007
Many people ask about this stuff, so I figured I might write a bit about it to explain what it is, and what it isn't.
First of all, the pickup, which many call "MIDI pickup", it isn't a MIDI pickup. It does not produce MIDI signals at all. The correct term would be GK pickup. GK is the standard for hexaphonic guitar signal transfer. This is what the GK pickup is: it has one tiny pickup for each string - six small humbucking pickups packed in the long slim plastic housing that sits under the strings by the bridge. These six individual signals are sent out through the 13-pin DIN connector, along with some other signals: there is one pin for the regular guitar signal, two pins for power supply (7 volts bipolar), one for each of the two switches (called S1 and S2), one for the "synth volume" potentiometer, and a common ground in the shield.
This GK signal can be fed to any of the processing units designed for GK. The most common units are probably the Roland GR-30, GR-33, GR-20 and GI-20 guitar synthesizers and also the VG-8, VG-88 and VG-99 V-Guitar units and the V-Bass unit, and also the Axon AX-100 guitar synth and the Yamaha G-50 guitar synth. Smaller units from Boss have also been available: the OC-20G PolyOctave and the WP-20G Wave Processor. These last two are rather simple one-trick-ponys, their name pretty much says it all.
The Roland V-Guitar and V-Bass units are rather special: they use the GK signal as a basis for digital modeling of the instrument, in principle much like the Line6 Variax system. With these units you can place virtual pickups on your instrument, emulate acoustic instruments and do things that are not physically possible but which may sound excellent. On my V-Bass unit I can, by the press of a button, change between Jazz bass, Precision bass, Music Man Stingray, Thunderbird, Hofner, upright, synth bass, and other models; and they all sound very good and respond well tomy playing. These are not MIDI instruments, but digitally modeled instruments, and they will react realistically to all the playing techniques that I employ while playing the bass - unlike a MIDI synth which will play the same sound no matter if I pluck, pick, slap, pop, tap or whack the string.
The Roland GR and GI units, and the Axon and Yamaha rack units, are usually called "guitar synths". These will analyze the GK signals and figure out what note is being played, and transmit this as a MIDI signal to a sound module - an internal sound module in most cases (the exeption being the Roland GI-20 and the Yamaha G-50). This would enable a guitar player or a bass guitar player (or a banjo player - look at Béla Fleck) to play any MIDI sound using their guitar type instrument. This can be quite fun and useful, as it expands the sonic palette quite a lot beyond even the V-Guitar systems. But the process of transforming the string sound to a MIDI signal isn't painless. The nuances of playing style are lost, what you are left with is information on what note is played and how loud it is. Also, for the bass guitar there is another catch. The low frequencies have a longer wave cycle and since the computer measures the length of the wave cycle it'll take longer time to know for sure what note is being played - thus a latency on the lower notes on the bass guitar. This makes normal rythmic bass playing very inaccurate and not good. So the guitar synth technology is not well suited for traditional bass playing.
Obviously it isn't done yet.