The present invention relates to a rotor control system for actuating pitch of pitch adjustable rotor blades in order to reduce vibrations of a wind turbine element, e.g. tower vibrations. A pitch modification signal is determined which is based on an m-blade coordinate transformation, such as the Coleman transformation. The m-blade coordinate transformation taking as input a first signal and a second signal. The second signal is determined by filtering the first signal with a signal filter with a quadrature phase shift filter phase response.