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
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.
14:00 hr O-2
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.
14:30 hr O-3
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.
15:00 hr O-4
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.
15:30 hr O-5
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.
16:00 hr O-6
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.
16:30 hr O-7
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.
17:00 hr O-8
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.
17:30 hr O-9
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.