Dual-band PWM audio coding for ultrasound artefact reduction

A novel approach to PWM coding is introduced based on generating two complementary PWM streams out of the in-band signal's spectrum. The 2 streams are then recombined in a suitable way, so that out-of-phase cancellation of the carrier frequency harmonics is achieved. The approach suppresses the strong out-of-band frequencies of the carrier signal without introducing distortion of the in-band coded signal. Such method can achieve superior reduction compared to the out-of-band artefact suppression induced by traditional analog low-pass filters employed in typical Class-D audio amplifiers or other switching power delivery systems, hence allowing designs with reduced filter requirements or even filterless implementations.