Freeing memory safely with low performance overhead in a concurrent environment is described. The modulator and the controller receive an external voltage indication signal indicating whether the pre-stage output DC voltage is the first DC voltage or the second DC voltage. In this event, Ra is representative of a center-line mean roughness, Rmax is defined as a maximum height representative of a difference between a highest point and a lowest point and Rq is representative of a root mean square roughness.