max@uwm.UUCP (Max Hauser) (05/17/91)
In article <12047@uwm.edu>, smithjh@math.orst.edu (Jeremy Smith) inquires:
(1) what is the MASH 1-bit D/A conversion?
(2) Is this better than oversampling?
Intelligent questions deserve an intelligent answer. In fact these and
related questions arise regularly, so I will try a more general response.
I last did so on the "MASH" topic in (I think) summer 1989, on the group
rec.audio. I will limit myself to the definitions and the technical context
and thus leave the field open to others to comment intelligently on personal
experience and impressions of specific products containing these techniques.
"MASH" is a commercial acronym to describe a class of oversampling data
converters (either A-to-D or D-to-A) introduced and popularized by T. Hayashi
of Nippon Telephone and Telegraph in February 1986 at an integrated-circuits
conference [Note 1]. Thus "MASH" denotes a subset of "oversampling" rather
than a competing technique. The acronym derives from and means "MultistAge
noise SHaping." While the technique was popularized by Hayashi, he did not
use the actual acronym "MASH," which came somewhat later, from K. Uchimura
et al. [Note 2]. Also, the underlying idea is much older still [Note 3].
The technique now dubbed "MASH" is a series of small one-bit oversampling
data converters each of which does a partial, and noise-shaped, conversion
and then passes a residue (conversion error) to the next stage. The
individual outputs of all of the stages are then combined (this is
nontrivial) to form a composite output. That output is passed to a digital
filter (for an A-to-D function) or an analog filter (for a D-to-A function).
(Yes, that's right -- I've proofread it.) This brutally short explanation
does not do the subject justice and may even leave unfamiliar readers with
the mistaken impression that MASH resembles successive-approximation or
pipelined data conversion (well-defined in the art), which it does not,
differing deeply in philosophy. However it's an accurate nutshell
description in that it will endure deeper scrutiny. For further details
you may wish to look in one of my survey papers, such as the AES Journal
January/February 1991.
The technique dubbed "MASH" by NTT is also called, less commercially,
"feedforward" or (less precisely) "cascaded" higher-order noise-shaping
[Note 3]. It is one of several different topologies that can implement the
"noise-shaping modulator" portion of an oversampling data converter. Other
such topologies include multibit and one-bit pure-feedback modulators. The
latter (one-bit) class are called delta-sigma in the research literature,
and "Bitstream" by NV Philips in digital-audio products (these techniques
also have many applications outside of audio). "MASH" data converters are
themselves not truly "one-bit" data converters in a meaningful sense, as I
have explained in more depth in print, although they are made up of one-bit
subsections and this often causes confusion.
Each of these competing modulator topologies has technical strengths and
weaknesses that are very involved and do not lend themselves to reductive
explanation. The signal fidelity in the "MASH" technique can be very good
but depends on a different set of circuit elements than in the one-bit
schemes. It is all a matter of "second-order" electrical effects; if the
components are all perfect (as they invariably are assumed to be, in popular
"explanations" of this subject matter), then all the techniques work well.
When Hayashi introduced the "MASH" technique to the international community
in 1986 (I was present), it was suggested from the floor that this method,
although a viable alternative to one-bit oversampling, did introduce certain
additional vulnerabilities to circuit imperfections, and moreover that the
real justification for the method was not clear (Hayashi's actual motivating
statement at the time was dubious). It has since been suggested, perhaps
insightfully, that the sheer novelty of the "MASH" method might afford a
commercial, rather than a performance, advantage since the other basic
oversampling methods occupy expired patents and are therefore in the public
domain. (My use of passive verbs in this paragraph is not accidental.)
But I think it even more important, in the context of this newsgroup and the
usual questions, that none of these considerations need correlate at all with
audible differences among finished audio products that employ these data
converters! Offhand I would suggest that audible quality could easily be
affected far more by such prosaic outboard factors as the choice of output
buffer amplifiers, and the degree of analog-digital isolation in the
conversion-reconstruction circuitry, than by whether the internal modulator
is realized via "MASH," or "Bitstream," or some other topology. This has
not, of course, prevented legions of advertising copywriters and cult audio
pundits from pontificating about the "revolutionary" aspects of this or that
topology and deploying buzzwords like "noise-shaping" and "one-bit" with
impressive vacuity. They apparently prefer this to focusing on the
sonically and technically more vital, but less glamorous, issues.
There is my terse explanation of the relationship between "MASH," "one-bit,"
"Bitstream," "delta-sigma," and "oversampling" data converters. Be
skeptical of anyone who asserts glibly that one of these techniques is
"better" than the others. You might be able to hear differences among
CD players using the competing schemes, but almost certainly not for the
reasons everybody talks about. Such differences could easily be due to
any of the numerous important factors other than the particular choice of
data-converter topology.
Notes from text:
Note 1: Hayashi et al., ISSCC 1986. Printed version in that year's ISSCC
digest, pages 182-183.
Note 2: Uchimura et al., ICASSP 1986. Printed version in 1986 ICASSP
Proceedings pp. 1545-1548. Research papers quite regularly misattribute
the modern origin of these circuits to this ICASSP paper rather than to the
earlier ISSCC paper [Note 1] by co-workers.
Note 3: The technique has existed in various forms, including a small
paper in "Electronics Letters" in 1969. Candy and Temes have told me that
they would cite this in their forthcoming broad research overview of
oversampling. I find it still more intriguing that the technique is also
a data-conversion implementation of Black's multistage linear amplifier
(US patent 1,686,792, issued 1928), a classic invention actually predating
the invention and patent of negative feedback, also by Black.
Max Hauser {mips,philabs,pyramid}!prls!max prls!max@mips.com
Copyright (c) 1991 by Max W. Hauser. All rights reserved.
"A lot of things can be solved by the use of jargon -- for example, the
effort of thinking, or the danger of saying something that someone else
may not like. You don't have to be clever, and you're always on the side
of whoever has the money or power ..."
-- Stanislav Andreski, author of "Social Sciences as Sorcery"