[Christien]

It's not really a synthesized voice - I'm not sure of how the electronics work, but I believe the guitar signal is routed through the talk box, which the gets encoded with the information the talk box gets from the tube going to the guitarist's mouth, then mixes the two and sends that signal as its output. The notes (pitches) come from the guitar, and the basic syllables from the lips get encoded onto it. There's no singing involved, just the resonance made by the voice and mouth. Google vocoding if you really want to learn more about it.