A non-contact continuous optical sensor system is used to register the depth and velocity at which keys are depressed. Along with the key sensors, grey-scale shutters are installed under each of the 88 keys. A grey-scale shutter is a small, thin plate whose optical transmission increases towards its bottom while an LED light beam is constantly radiated inside a key sensor. When a piano key is depressed, a grey-scale shutter blocks that light, with the amount of light blocked increasing in proportion to the depth of the keystroke. Based on the amount and traveling speed of the blocked light, the sensor registers keystroke depth and velocity as well as the timing at which muting should occur.