In a computor organ of the type wherein a musical waveshape is synthesized by computing in real time the amplitude contributions of the constituent Fourier components and summing these to obtain each waveshape sample point amplitude, the tonal quality or voice of the produced note is established by a set of harmonic coefficients that specify the relative amplitude of each Fourier component. Herein circuitry is disclosed for modifying the harmonic coefficient values to accomplish transient voice insertion including ''''chiff'''' and percussive transients, to modulate the harmonic content as a function of time during attack and decay, and to facilitate the external insertion of additional voices for the instrument.