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Session O Tuesday, May 15 13:30 - 18:00 hr Room B

Signal Processing for Audio, Part 3

Chair: Rhonda Wilson, Meridian Audio, Huntingdon, UK

13:30 hr O-1
Investigation of Practical 1-Bit Delta-Sigma Conversion for Professional Audio Applications
Hiroshi Takahashi & Ayataka Nishio
Sony Corporation, Tokyo, Japan

The digital encoding method known as "Direct Stream Digital (DSD)", which is based upon direct recording of the 1-bit output signal of a delta-sigma modulated analog to digital (A to D) converter, provides an analog-like sound quality for both professional audio applications and the new consumer audio delivery format known as "Super Audio CD (SACD)". In this paper, an investigation of 1-bit delta-sigma conversion in some basic DSD signal processing is given to show the practical performance of DSD compatible production tools.
Paper 5392

14:00 hr O-2
Achieving Effective Dither in Delta-Sigma Modulation Systems
James Angus
University of Salford, Manchester, UK

This paper clarifies some of the confusion which has arisen over the efficacy of dither in PCM and Sigma-Delta Modulation systems. It describes a fair means of comparison between them. It presents results which show that dither is effective in Sigma-Delta Modulation systems and proposes methods for achieving optimum performance in both systems.
Paper 5393

14:30 hr O-3
The Practical Performance Limits of Multi-Bit Sigma-Delta Modulation
James Angus
University of Salford, Manchester, UK

This paper presents a novel method which allows a direct comparison of the benefits of multi-bit Sigma-Delta modulation versus PCM in audio systems, when physically implemented. It then goes on to examine the effect of various errors, in particular non-linearity and jitter in the two systems, and shows that the effect of component tolerances strongly limit the maximum performance of multi-bit quantizers in such systems.
Paper 5394

15:00 hr O-4
Why 1-Bit Sigma-Delta Conversion is Unsuitable for High-Quality Applications
Stanley Lipshitz & John Vanderkooy
University of Waterloo, Waterloo, Canada

Single-stage, 1-bit sigma-delta converters are in principle imperfectible. We prove this fact. The reason, simply stated, is that, when properly dithered, they are in constant overload. Prevention of overload allows only partial dithering to be performed. The consequence is that distortion, limit cycles, instability, and noise modulation can never be totally avoided. We demonstrate these effects, and using coherent averaging techniques, are able to display the consequent profusion of nonlinear artifacts which are usually hidden in the noise floor. Recording, editing, storage, or conversion systems using single-stage, 1-bit sigma-delta modulators, are thus inimical to audio of the highest quality. In contrast, multi-bit sigma-delta converters, which output linear PCM code, are in principle infinitely perfectible. (Here, multi-bit refers to at least two bits in the converter.) They can be properly dithered so as to guarantee the absence of all distortion, limit cycles, and noise modulation. The audio industry is misguided if it adopts 1-bit sigma-delta conversion as the basis for any high-quality processing, archiving, or distribution format to replace multi-bit, linear PCM.
Paper 5395

15:30 hr O-5
Towards a Better Understanding of 1-Bit Sigma-Delta Modulators
John Vanderkooy & Stanley Lipshitz
University of Waterloo, Waterloo, Canada

Although 1-bit Sigma-Delta modulators cannot be adequately dithered to make their performance perfect, the application of substantial noise shaping gives them modest performance while maintaining reasonable stability. Partial dithering has been shown to improve these devices, but spuriae remain. By simulation we study: (a) the nature of the idle tone, (b) the effect of the order of the shaping filter, (c) the influence of shaper stability, and (d) coherence and incoherence of the shaper's output. Calculation of these features demands high numerical precision and is greatly aided by coherent averaging.
Paper 5398

16:00 hr O-6
Why Direct Stream Digital (DSD) is the Best Choice as a Digital Audio Format
Derk Reefman & Peter Nuijten
Philips, Eindhoven, The Netherlands

In this paper, an overview of Direct Stream Digital (DSD) signal processing is given. It is shown that 1-bit DSD signals can be dithered properly, so the resulting dithered DSD stream does not contain audible artifacts in a band from 0-100kHz. It is also shown that signal processing can be done best in a high rate, multi-bit domain. Arguments are given that the minimal frequency span needed to comply with the human auditory system is roughly 0-300kHz. Following the signal processing, final conversion to DSD is made. It is demonstrated that Super Audio CD (SACD) is a very efficient consumer format: it is the format which, while maintaining all necessary psycho-acoustical characteristics such as high band width, filtering with wide transition bands etc, uses the least bits from the disk; hence offering the longest playing time.
Paper 5396

16:30 hr O-7
Editing and Switching in 1-Bit Audio Streams
Derk Reefman & Peter Nuijten
Philips, Eindhoven, The Netherlands

This paper addresses the issues around switching 1-bit audio streams, such as used in Direct Stream Digital (DSD) in the Super Audio CD format. A theoretical description is derived which shows how these streams can be switched, independent of the Sigma Delta topologies that are used. Also a simplification will be discussed, which is technically much simpler, while still achieving high quality cross-overs between 1-bit audio streams without any audible artifacts.
Paper 5399

17:00 hr O-8
SDM versus LPCM: The Debate Continues
Malcolm Hawksford
University of Essex, Colchester, UK

Significant misrepresentation of both 1-bit SDM and multi-bit LPCM coding paradigms persist within both professional and commercial arenas that impacts directly upon the perception of DVD-A and SACD formats. A balanced appraisal of these schemes is presented in order to expose the core differences in the technology both in the theoretical and instrumentation domains. Some observations are made about the fallacy of performance comparisons and the consequence of misinformation that subsequently is derived.
Paper 5397

17:30 hr O-9
Digital Audio Power Amplifiers using Sigma Delta Modulation - Linearity Problems in the Class-D Power Stage
Rolf Esslinger (1), Gerhard Gruhler (1) & Robert W. Stewart (2)
University of Applied Sciences, Heilbronn, Germany
University of Strathclyde, Glasgow, UK

A completely digital audio power amplifier uses binary (two-level) signals up to the power stage. Often MOSFETs are used as power devices, as they can provide a high output power together with a fast switching speed. The generation of accurate, rectangular pulses in the analogue power amplifier circuit is a problem, especially at pulses with an extremely short duration, as they may come from high resolution Sigma-Delta Modulation. In this paper, some of the typical pulse signal waveform distortions introduced by the realistic switching power stage are shown by simple circuit models. Additionally it will be explained how certain errors can affect the linearity of pulse signals from Sigma-Delta Modulation.
Paper 5400


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