A prefilter equalizer for pulse width modulation

This article demonstrates the feasibility of pulse-width-modulation (PWM) of discrete time signals in a digital audio amplifier without inducing the nonlinear distortion usually associated with straightforward PWM. This is achieved by prefiltering the signal prior to the PWM mapping in such a manner that the overall result is distortion-free. All processing is done in the discrete time domain and relies on the interpretation of PWM as a Volterra filter.Computer simulations on CD music signals were performed. With no upsampling the distortion due to PWM gave a signal-to-noise-ratio (SNR) of approximately 45–50 dB. Using the prefiltering scheme developed in this work, dramatic improvements of the reconstructed signal quality was demonstrated. The 9th order prefilter gave an SNR of well above 90 dB for the music signals tested.Experiments using the same signals upsampled with N=8 showed that a 3rd order prefilter was sufficient for removing PWM-induced distortion.

► Straightforward PCM to PWM conversion in a digital audio amplifier produces nonlinear distortion.
► It is shown how to remove the distortion using a precompensation technique before PWM.
► The precompensation is based on interpreting the PWM process as a Volterra filter.
► The solution is fully digital.