Abstract: A player piano recording system has photosensor flags secured to the undersides of the piano keys, vertical movement of which is detected by horizontally adjustable photosensors to produce "key played" and key velocity signals which supplied to a microprocessor for deriving expression signals for recording on magnetic tape. The microprocessor provides output expression values and key play information. According to the invention, the expression values are a direct function of key velocity and key play information and switch selected boost (an enhanced initial frame expression for overcoming solenoid inertia) and add (for trill) values. Key play data is dependent upon key play inputs and the frame extension switch value. The unique structure of the key flag permits horizontal adjustment of the photosensors for vertical misalignments etc. of the piano keys.