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SCHNEIDER DIGITAL MICROPHONES FOR HIGH RESOLUTION AUDIO
AES 31st International Conference, London, UK, 2007 June 25–27
1
DIGITAL MICROPHONES FOR HIGH RESOLUTION AUDIO
MARTIN SCHNEIDER
1
1
Georg Neumann GmbH, Berlin, Germany
schneidm@neumann.com
Microphones with digital output format have appeared on the market in the last few years. They integrate the functions
of microphone, preamplifier, and analogue-to-digital converter in one device. Properly designed, the microphone
dynamic range can thus be optimally adapted to the intended application. The need to adjust gain settings and trim
levels is reduced to a minimum. Dynamic range issues inside and outside the microphone are discussed. Advantages of
digital microphones complying with AES 42, with a wide dynamic range and 24-bit resolution are shown.
INTRODUCTION
One should first define the term “digital microphone” in
the context of this article. A possible classification
could comprehend:
- a transducer where the underlying acoustical-
mechanical-eletrical transduction principle
contains a quantization,
- a combination of separate transducers, each
responsible for certain quantization steps,
- a microphone integrating an analog-to-digital
converter (ADC).
The first category describes the “purely digital”
transducer. The first microphone by Philipp Reis [1], a
single contact transducer, represented such a transducer,
albeit with very low quality due to the 1-bit resolution.
This is the only purely digital transducer known to the
author.
In the second category we find e.g. an optical
microphone, where the position-dependant displacement
of a diaphragm is traced with distinct light rays. The
reflected rays excite separate sensors, whose outputs are
combined into a single signal [2]. Another, electrostatic
transducer experiment shows the diaphragm as part of
the ADC, as component for the electrical / acoustical
summation in the feedback loop of a Σ∆-converter [3].
To obtain dynamic ranges comparable to the 120-
130 dB of standard analogue microphones, these
principles would need to be scaleable over 6 orders of
magnitude, a feat hardly achievable due to the extreme
mechanical precision involved.
Current microphone technology thus focuses on the
third category: microphones with integrated ADC. Here,
a purist could further differentiate between
- microphones with ADC output modules,
- microphones with ADC in closest proximity to
the transducer,
where the first subcategory would describe a complete
microphone, just with an added ADC module; the
second subcategory represents transducers where the
transducing element itself is closely integrated with the
analogue-to-digital conversion process. In the context of
high resolution audio it will be clear that the preferred
transducer should be of the electrostatic (condenser)
type, as this principle still yields the highest
performance regarding parameters like linearity,
dynamic range and frequency range.
1 HISTORICAL DEVELOPMENT
Possibly the first realization, in 1989, incorporating an
ADC in the same housing with an electro-acoustical
transducer is mentioned in [4]. The corresponding
electret condenser microphone by Ariel company was
intended for use with the now defunct NeXT computer,
with the then available 16 bit transducers and a stated
dynamic range of 92 dB. A 1995 prototype by Konrath
[5] put an ADC circuit inside the housing of a
commercial microphone. It featured a 7-pin XLR-
connector and dedicated supply, delivering a multitude
of supply voltages to the circuit. A later commercialised
version by Beyerdynamic (MCD100) simplified this set-
up with the adoption of phantom power, similar in