Neumann.Berlin Digital Microphones User Manual

SCHNEIDER DIGITAL MICROPHONES FOR HIGH RESOLUTION AUDIO

DIGITAL MICROPHONES FOR HIGH RESOLUTION AUDIO

MARTIN SCHNEIDER1

1

Georg Neumann GmbH, Berlin, Germany

[e-mail]

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 acoustical­mechanical-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 120­130 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 XLR­connector and dedicated supply, delivering a multitude
of supply voltages to the circuit. A later commercialised
version by Beyerdynamic (MCD100) simplified this set­up with the adoption of phantom power, similar in

AES 31st International Conference, London, UK, 2007 June 25–27

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SCHNEIDER DIGITAL MICROPHONES FOR HIGH RESOLUTION AUDIO

principle to P48 defined in IEC61938 [6], but adapted to
the lower voltage and higher current requirements of
ADC components. It already featured a gain ranging
ADC, to be discussed later, and limited remote control
functions (pre-attenuation) but yielded sub-optimal
noise figures, compared to standard analogue micro­phones. Another proprietary solution was presented by
Milab [7].
Although the mentioned developments could not fully
compete technically with state-of-the-art analogue
microphones, they were helpful in starting discussions
amongst manufacturers on the future of digitisation in
microphones. It was found that, before presenting
microphones with digital output to a wide public, all
questions of power supply, interfacing, connector types,
remote control etc. should be put into a public standard,
to allow future products to interconnect between
manufacturers. Accordingly, the German DKE 742.6
committee served as a starting basis, then handing over
to an AES standardization committee to publish the
AES 42-2001 standard [8,9], currently revised to the
2006 edition. Almost ten international microphone
manufacturers were actively or passively involved,
guaranteeing a common consensus. First microphones
complying with the new standard were presented in
2001, as a full-feature large diaphragm microphone
[10], later followed by a measurement microphone [11]
and small diaphragm capsule systems [12,13].
In contrast to the professional audio approach, trying to
provide highest possible audio quality, recently other
solutions have been presented, driven by computer
technology, i.e. mainly USB-powered microphones,
with currently in comparison very limited specification
ranges [14].

2 REASONS AND REQUIREMENTS FOR

DIGITAL MICROPHONES

Analogue output condenser microphones are now, 90
years after their invention by E.C. Wente, certainly a
mature technology. In a professional set-up, with
appropriate cabling and limited outside interferences, a
very high dynamic range of up to 130 dB-A can be
transduced [15,16]. To reduce effects of cable length
and interferences on the comparatively small
microphone output signal, preamplifiers are often
located in close proximity to the microphones. In any
case, proper level matching of all analogue components
is necessary to guarantee optimal signal transmission,
allowing for sufficient head-room and foot-room in the
process. On the other hand, digital technology provides
potentially loss-less transmission, once the analogue-to­digital conversion has taken place. Accordingly, the
interest for microphones with digital output arose when
high quality ADC technology became available,
allowing conversion only minimally affecting micro­phone specifications.

Some of the requirements on digital microphones [8]
later realized in the AES 42 standard [9] were

- physical layer interface & protocol
compatibility: AES3 protocol with overlaid
phantom power, using 3-pin XLR connectors,

- control information from

the microphone: via

user bits in the AES3 data stream,

- control information to

the microphone: via low
frequent modulation of the phantom power
voltage.

With the chosen interface, loss-less transmission can be
performed over approximately 100 m also with high­quality “analogue” microphone cable, approximately
300 m with AES3 “digital” cable. This compares well
with typical values for high-quality analogue set-ups.
An essential point in digital technology is proper
synchronization of all audio streams to a reference
clock. In a minimal set-up a receiver can synchronize to
a single microphone, although this would be in contrast
to typical studio procedures, where either the mixing
console, or a dedicated reference clock provide the
clocking reference. But, with multiple digital micro­phones one needs to either work with sample rate
converters in every channel at the receiver side (AES42
mode1), or preferably synchronize the microphones to
the reference clock (AES42 mode2). High quality
sample rate converters do increase the cost, and even
though in their current embodiments [17,18] they might
not influence the signal much, they will increase
processing time and thus add to the overall latency,
which can become prohibitive in some applications, e.g.
where direct monitoring is called for.
Sending the clock signal directly to the microphone
would imply multi-lead cables, incompatible with
standard 2-wire+ground/return studio wiring. The
solution adopted by AES42, after extensive tests, was to
integrate a voltage controlled crystal oscillator (VCXO)
inside the microphone, yielding an already very stable
data stream but where the frequency is dynamically fine
tuned from the receiver side via the control information
sent to the microphone (Fig. 1).

Figure 1: Connection of a digital microphone, with

synchronization using AES42 interface specification.

Microphone sample rate is controlled (CTL), comparing

extracted microphone rate and external word clock.

AES 31st International Conference, London, UK, 2007 June 25–27

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