Audio Engineering Society Convention Paper 7228 Presented at the 123rd Convention 2007 October 5–8 New York, NY, USA This convention paper has been reproduced from the author's advance manuscript, without editing, corrections, or consideration by the Review Board. The AES takes no responsibility for the contents. Additional papers may be obtained by sending request and remittance to Audio Engineering Society, 60 East 42 nd Street, New York, New York 10165-2520, USA; also see www.aes.org. All rights reserved. Reproduction of this paper, or any portion thereof, is not permitted without direct permission from the Journal of the Audio Engineering Society. Recursive Natural Sampling for Digital PWM Pallab Midya 1 , Bill Roeckner 2 , and Theresa Paulo 3 1 Freescale Semiconductor, Inc., Lake Zurich, IL 60047, USA pallab.midya@freescale.com 2 Freescale Semiconductor, Inc., Lake Zurich, IL 60047, USA bill.roeckner@freescale.com 3 Freescale Semiconductor, Inc., Lake Zurich, IL 60047, USA theresa.paulo@freescale.com ABSTRACT This paper presents a highly accurate and computationally efficient method for digital-domain computation of naturally sampled digital pulse width modulation (PWM) signals. This method is used in a switching digital audio amplifier. The method is scalable for performance versus calculation complexity. Using a second order version of the algorithm with no iteration, intermodulation linearity of better than 113 dB is obtained with a full scale input at 19 kHz and 20 kHz. Matlab simulation and measured results from a digital amplifier implemented with this algorithm are presented. Overall system performance is not limited by the accuracy of the natural sampling method. 1. INTRODUCTION Directly translating digital pulse code modulated (PCM) audio samples to a digital PWM duty ratio is called uniformly sampled PWM (UPWM). UPWM has a mathematical nonlinearity. Naturally sampled PWM (NPWM) is the signal which is determined by the intersection of a ramp and an analog signal. Natural sampling corrects for the mathematical nonlinearity of the PWM process. There exist numerous digital-domain methods for generating/approximating a naturally sampled digital PWM signal from a digital PCM input [1-3]. Some of these methods target digital audio amplifier applications [4-6] while others target inverter applications [7]. The method described in this paper achieves very high linearity with low computational overhead relative to previous work [8-9]. Further, the method is less sensitive to the presence of out-of-band noise in the incoming PCM signal. This method of natural sampling has been patented [10] but performance results have not been previously published. 2. IMPLEMENTATION 2.1. Description Naturally sampled PWM is best described in the analog domain as comparing an analog input signal to a ramp waveform signal to produce a naturally sampled PWM