Digital removal of pulse-width-modulation-induced distortion in class-D audio amplifiers

In an all-digital, class-D audio amplifier, pulse-width-modulation (PWM) of a digital signal source is usually followed by a low-order analog low-pass filter to construct the analog audio waveform. This study shows how to remove the non-linear distortion usually associated with PWM, by prefiltering the digital signal prior to the PWM mapping in such a manner that the overall result is distortion-free. The prefiltering is done using computationally effective infinite-impulse-response filters combined with short-kernel anticausal finite-impulse-response filters, and relies on the interpretation of PWM as a Volterra filter. A case study is presented where a second-order Butterworth analog low-pass filter is used for reconstruction of the analog audio signal. A complete amplifier system is modelled, including upsampling, Volterra prefiltering and noise feedback coding. Computer simulations on CD music signals were performed. Using a third-order prefilter, a signal-to-noise ratio of 97-102 dB was obtained for the music signals tested. All necessary filter data needed for realisation of the prefilter are given in the Appendix.