The classical technique of sinusoidal PWM entails modifying the width of the pulses forming the output voltage (current), by comparing a given voltage signal (the reference signal) with a triangular voltage signal of higher frequency (the carrier signal). The reference signal is the modulating signal and determines the required waveform of the output voltage (current). In the present case, this signal is sinusoidal and is at the same frequency as the fundamental component of the voltage or current. Many versions of this method exist, with special nonsinusoidal waveforms of the modulating signals. By such means, the level of specific harmonics may be successfully reduced.