Digital values representing an audio signal are formed in a FM transmitter. The deviation of the amplitude of the audio signal as represented by the digital values, from a reference amplitude level, is measured. The audio signal is filtered based on the deviation measured by processing the digital values, with the filtering designed to filter different frequency components of the audio signal with different respective magnitudes. Another set of digital values representing the filtered audio signal is generated, and used to frequency modulate a carrier signal to generate a FM signal. Over-modulation of the FM signal is thereby controlled.