Distortion-Free 1-Bit PWM Coding for Digital Audio Signals
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Research Article Distortion-Free 1-Bit PWM Coding for Digital Audio Signals Andreas Floros1 and John Mourjopoulos2 1 Department 2 Audio
of Computer Science, Ionian University, Plateia Tsirigoti 7, 49 100 Corfu, Greece Technology Group, Department of Electrical and Computer Engineering, University of Patras, 265 00 Rio Patras, Greece
Received 15 June 2006; Revised 1 December 2006; Accepted 13 March 2007 Recommended by Sven Nordholm Although uniformly sampled pulse width modulation (UPWM) represents a very efficient digital audio coding scheme for digitalto-analog conversion and full-digital amplification, it suffers from strong harmonic distortions, as opposed to benign nonharmonic artifacts present in analog PWM (naturally sampled PWM, NPWM). Complete elimination of these distortions usually requires excessive oversampling of the source PCM audio signal, which results to impractical realizations of digital PWM systems. In this paper, a description of digital PWM distortion generation mechanism is given and a novel principle for their minimization is proposed, based on a process having some similarity to the dithering principle employed in multibit signal quantization. This conditioning signal is termed “jither” and it can be applied either in the PCM amplitude or the PWM time domain. It is shown that the proposed method achieves significant decrement of the harmonic distortions, rendering digital PWM performance equivalent to that of source PCM audio, for mild oversampling (e.g., ×4) resulting to typical PWM clock rates of 90 MHz. Copyright © 2007 A. Floros and J. Mourjopoulos. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
1.
INTRODUCTION
Over the last decades, the use of 1-bit audio signals has emerged as an attractive practical alternative to multibit pulse code modulation (PCM) audio, which up to now was considered as the de facto format for the representation of such data. The advantages of a pulse-stream representation for digital audio originate from the simpler hardware implementations with respect to the required audio performance. For example, analog-to-digital (ADC) and digital-to-analog (DAC) conversion systems with the increased requirements imposed in dynamic range and bandwidth can be efficiently implemented using 1-bit digital storage formats (i.e., in the form of direct stream digital—DSD [1], which is based upon sigma-delta modulation—SDM [2]). Similarly, conversion of audio to 1-bit pulse width modulation (PWM) streams introduces comparable practical implementation advantages for the realization of DACs [3] and other components in the audio chain, especially alldigital amplifiers, since the PWM pulse-stream can be directly amplified using power switch transistors [4]. Theoretically, any switching power stage has 100% efficiency. In practice, no ideal power switch exists and such implementations result into an amount of power loss tak
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