scarr
Experienced
In case there is some confusion in the language, you can shift a polyphonic signal by a fixed amount. You can not shift the individual notes in a polyphonic signal by different amounts, at least with the current technology. This is because the technology to break the combination sound down into its respective notes in realtime isn't available yet (at least that I know about / on a commercial level).
That isn't to say it's impossible. It's just very tricky to do, especially if you don't know what type of signal is coming into the processor. (You have to account for the overtone series from one note influencing the loudness of other peaks in the frequency spectrum, to figure out what is the same note versus a second note. It gets even harder with a distorted signal, if you want to ignore things like intermodular distortion.)
That isn't to say it's impossible. It's just very tricky to do, especially if you don't know what type of signal is coming into the processor. (You have to account for the overtone series from one note influencing the loudness of other peaks in the frequency spectrum, to figure out what is the same note versus a second note. It gets even harder with a distorted signal, if you want to ignore things like intermodular distortion.)