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, 1 Enterprise Drive, Old Lyme, CT 06371; 860/434-5220; www.neumannusa.com.
John Monforte lives in the virtual world atwww.personal.umich.edu/~monforte/, where you can find audio, semiconductor design, holography, vintage tractors and more.