US Patent 7723598 Electronic proof system and musical instrument equipped with the same

An electronic proof system is provided in association with an automatic player piano; while a human player is practicing a music tune on the acoustic piano, the electronic proof system monitors the pedals of piano to see whether or not the player brings the pedals to target pedal positions; if the current pedal position is spaced from the target pedal position, the electronic proof system gives rise to fine vibrations of the pedal, and makes the player notice the pedal staying at incorrect pedal position through the tactile impression.