Task: Question Answering
Input (English): Passage: Returning home to Brantford after six months abroad, Bell continued his experiments with his "harmonic telegraph".[N 12] The basic concept behind his device was that messages could be sent through a single wire if each message was transmitted at a different pitch, but work on both the transmitter and receiver was needed. Question: For  his system to work, Bell had to change what about each code?
Output (English): pitch