Turn your guitar to MIDI and Guitar Synthesizer I don't know if this question is related to DSP, but I'll give a chance.

Introduction:
These days there are a Guitar Synthesizers which allow the guitarist
to play another instruments like: brass, drums, strings, pianos and other guitar models
from his guitar. 

It possible to turn any guitar with the MIDI Pickup, that sends the all data
that the synth needs. 
Now, from what I understand the pickup turns the guitar into MIDI one. 

My question is how a signals like those that the guitar makes can be converted into 
MIDI instructions ? 

There is a sampling before this conversion? I think that it should be, because if not.
How the Synth will know in which level (volume) or duration to produce the sounds?
There is a sensor for each string which individually records the string vibration. This signal is converted to digital, analyzed for estimating its fundamental frequency and its amplitude (envelope detector). Whenever a sharp increase in amplitude is detected, a MIDI note on message is sent, at the detected frequency. Whenever the amplitude falls below a threshold, a MIDI note off message is sent.