I'm starting a new series of projects relating to music, electronics, art, and other random hobby things. I'm going to start with a project I've started recently, with a goal of designing and building a solid body banjo with the capability to output both a normal analog output as well as polyphonic MIDI.
Since you're looking at this, you probably have a sense of what I'm looking to build already, but just in case it's still fuzzy, I need to explain what's normal and commonly available before I can explain my particular version of fun. Most electric instruments, and in particular a solid body guitar, use inductive pickups. The signal is them amplified and then driven out of a speaker or recorded. Since the coil used for the pickup spans all of the strings, this coil picks up a blend of all of the strings that are being played.
Now MIDI is a completely different beast. That is a digital protocol that's used to communicate between computers and various digital music devices. It can signal things like when notes start and end, how loud they are, and so on. There's no audio in MIDI - it's like a computer program that needs to be interpreted by a synth of something similar to turn it into something we can hear and enjoy.
Where things get interesting is when you want to take a live non-digital instrument and generate the equivalent MIDI instructions. In some cases, like a piano this is fairly straightforward - you track what keys are pressed, when and how hard, and you can get a pretty good approximation of the original performance. With stringed instruments, it's generally been more difficult, since these don't have keys.
One solution is to replace the strings with the equivalent of keys, which is what some designers have done, such as the SynthAxe and the more recent You Rock Guitar. In this case, you're using the guitar (or whatever) as a digital input, but you potentially lose a lot of the look and feel of the original instrument.
The other route is to take the audio signals that the instrument normally produces from a mic or a pickup, and convert that into the note information. This simplifies the use of the instrument, but you lose some of the expression information. It can also be fairly complex to extract discrete pitch information for each string what it's all blended together.
Next time I'll talk about some of the specifics that I'm working on, including a version of a hexphonic pickup for a banjo (which makes it pentaphonic!), and some planned experiments with hall effect sensors instead of the traditional coil pickup.