A stator structure comprising at least one first magnetic conductive plate and at least one second magnetic conductive plate, each magnetic conductive plate having a plurality of radial arms and pole tooth provided on ends of the radial arms, wherein the pole teeth of the first and second magnetic conductive plates have different shapes. A gating signal is sent to a gating device connected to the header device. After comparing the first through fourth level-shifted signals with each other, the comparison results are used in determining the validity of the differential input signals. A melody parameter generator receives the operational signals from the control pad, and then decides on scales, sound production starting time, note lengths and other necessary conditions by considering the operational signals as sound producing factors for a melody. The majority by weight of the plurality of solid material impactors may have a mean diameter of at least 0.