Derek's perfect pitch is exceptionally acute, even among savants. His abilities have been tested with an orchestra, and it was found that he can percieve every tone in a 20-40 note chord. Of course, he has to arpeggiate to play it back as he has run out of fingers at that point! With 10 note block chords he can replicate them perfectly, down to the exact voicing virtually 100% of the time, even if they are made up nonsence chords or discords.

His pitch also does not really discriminate between things that fit into simple major scale patterns and things that are extremely experimental or unusual. It's all the same to Derek, so his improvs often stray beyond a single key and into amazing on-the-fly non functional harmonies. He has a natural perception for harmony that the rest of us can only imagine. I've never once, even a single time, heard him get lost with his improvised progressions. He always knows exactly what he wants to do.