At last, the final component in the recording chain is digital.
However, in order to realize the dream of a digital microphone, two
major problems had to be solved, and that has taken some time. The
first requirement was a converter capable of doing justice to a mic
capsule. It was 70 years ago when Georg Neumann perfected the condenser capsule and at
once eliminated the mic as the weak link in the recording
chain—at least in terms of dynamic range. That remains true today, even in
digital systems. The industry also required standards for creating,
controlling and transmitting digital signals. Now it appears that both
of these problems have been solved, and Neumann is again leading the
way with what they call “Solution D.”
There have been previous digital microphone attempts; some of them
started before the ink was dry on the AES/EBU digital signal
transmission standard. The method is obvious: Place a preamp and A/D
converter inside the mic body. There are several reasons why this does
not work well.
First, mic capsules have a dynamic range of at least 125 to 135 dB, while 16-bit
converters are capable of only 98 dB at best. Just which portion of the
capsule’s range needs to be converted depends on the sound source. A
preamp gain setting that works for distant pickup of a classical trio
will overload when the mic is placed near the bell of a trumpet. This
is why recording consoles have a mic preamp gain trim—a control
not available on the preamp in previous digital mics.
Second, there is a clocking problem. All the converters need to be
synchronous. This usually requires some sort of house clock, but there
was no way to send that upstream to the mic. It would be possible to
have all other components in the system sync to the mic, but this only
works if there was just one mic. Alternatively, we could resample at
the console end and let the sources run asynchronously, but that is a
lossy step with some sonic penalties, and, even worse, it requires a
certain amount of latency, which adds phasing and delays that we would
rather not deal with.
Finally, we end up with a mic that is not truly digital. The preamp
and converter have imperfect analog stages, especially the high-gain
mic preamp. Generally, as the gain boosting increases, a preamp’s sonic
fingerprint becomes more audible. Mic preamps tend to be the
most-colored stages in a recording console and typically have gains
from 30 to 60 dB. These circuits would have to be even less than ideal
when you consider that they’re designed to work within the voltage and
current limitations of a remote power source. On top of that, current
converters have 24 bits of resolution at best, and that is usually only
on paper anyway. Practical products available today have a range that
is around 100 to 115 dB, not nearly enough to pass along the
extraordinary range of a condenser capsule.
HOW IT WORKS
With its Solution D system, Neumann proposed a revolutionary approach
to converting a capsule’s signal that does not rely on intermediate
analog stages and is not limited to present-day PCM standards. In order
to capture the full dynamic range of the capsule, two delta sigma
converters are employed that share the dynamic range of the signal, as
shown in Fig. 1. The capsule feeds one converter though a nonlinear
network that is transparent to high-level signals. The capsule also
feeds an amplifier
that boosts the signal 24 dB for detection by a second converter
dedicated to the low-level portions of the signal. The amplifier and
nonlinear network are incorporated in a feedback loop that ensures that, no matter how
imprecise this network may be, whatever portion of the signal that is
not sifted out to the high-level converter will be unconditionally sent
to the low-level converter. Figure 2 shows the resulting wave shapes
and what they look like once combined.
Once in the digital domain, the two converter outputs are summed,
with the high-level converter’s noise gated out. Both converters are in
play the entire time. The advantage of this approach is providing a
seamless hand-off between the two paths, while avoiding any distortions
that would result from switching between two converters. The resulting
signal has an astonishing resolution of 28 bits.
Figure 3 shows the spectra of the component signals and the net
result. While each component is highly distorted, this distortion is
fully predictable and completely canceled after summation.
THE DIGITAL ADVANTAGE
Besides eliminating the sonic colorations of a mic preamp and removing
the possibility of noise pickup in the interconnections, there are
other benefits to a digital mic. Most obvious are the cost savings from
eliminating mic preamps and A/D converters in the console. Depending on
the specific console architecture, this could represent a significant
savings.
Another useful feature is the ability to include some simple DSP
functions in the microphone itself. Certainly, the standard features
such as gain, low cut and pattern selection could be implemented. The
processing can also include a signature response curve designed for a
particular broadcast station or talent. It would also be possible, with
a dual-diaphragm capsule, to derive a polar pattern that is, say, omni at low frequencies
but gradually changes to cardioid at high frequencies. I sure could use that
one! Peak limiting or compression could be inserted there, and anyone
who records live would be interested in a clip-proof channel. Other
applications are possible outside of music recording. A noise-canceling
mic can be derived with much more control than has previously been
possible, for instance.
Stereo mics can include M-S processing. Ambisonic arrays (such as in
the Soundfield mic) can be realized inexpensively and with greater
accuracy. As with any new technology, we may not yet be aware of the
“killer app,” but the hooks are there to implement ideas we
have not yet considered.
The AES 42-2001 standard (which Neumann participated in creating)
offers other advantages as well. The signal sent downstream is in the
familiar AES/EBU signal (aka AES3) format—a stereo signal
containing status and user bits. These bits can be utilized to tell the
receiver what mic is present. This tells the console what sort of
control is available in that particular model of mic. From this data,
the console knows what the mic’s settings are. The bits can also
indicate if the mic is synched and ready. Because the data is stereo, a
mono mic could send pre- and post-processed signal versions to separate
inputs, or deliver each side of a dual-diaphragm capsule to independent
tracks, allowing its pattern to be adjusted after recording.
The AES 42 standard provides for powering the mic and its circuitry
with a common-mode DC voltage that is placed on the cable. Also
included is a common-mode AC signal, which consists of data that can be
used to control the mic from the console end and even provide a tally
that indicates when the mic is live.
Contained in this upstream data is the signal that allows the mic to
sync with the recording system. Rather than send the actual clock
itself, which could be significantly degraded after passing though the
cabling, an 8-bit data word is sent that describes the amount of error
in the mic’s clock. The microphone then interprets this to adjust its
own clock accordingly. This allows for very low jitter—less than
1 picosecond in the audio range. Figure 4 shows one possible
implementation of the standard.
Although the capacity of the upstream channel is limited, it is
possible to use it when the mic is off-line to program specialized DSP
functions in the mic. The specialized functions mentioned previously
can be sent to the mic, as long as they are within the processing
capability designed into that particular microphone model.
THINGS TO COME
Release of the D-01 mic is imminent. In addition to implementing the
digital technology, its all-new capsule design represents the cutting
edge in dual-diaphragm mics. Other digital models are sure to follow.
Neumann is also providing an AES 42 interface (the DMI-2) and
remote-control software (RCS), which will be necessary until compatible
console inputs become available.
Clearly, the D-01 represents an important first step in advancing
digital microphone technology, offering the promise of powerful, new
tools that could change the way we record.
Neumann USA, www.neumannusa.com
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