in an electronic musical instrument which obtains waveshape amplitude values at sampling points by calculation for producing a desired waveshape , there are provided first means for calculating a waveshape varying with time and second means for calculating a waveshape undergoing no variations with time . normally the waveshape calculation by the first means is repeated but , in the case of a new key depression or a change in the state of a tone select switch , the waveshape calculation by the second means is performed .