Mic Arrays

Transducers that are directly integrated with DSP are the next big thing in loudspeakers. While this is the focus of many of the major audio manufacturers, these are not the only transducers that can benefit from thoughtful use of integrated signal processing. Microphones can use DSP to offer improvements that have thwarted the mechanical efforts of the past.

Microphones have always used mechanical systems to achieve the directivity we need to achieve gain-before-feedback. The labyrinths within the cardioid mic form the frequency-dependent cancellation that yields the microphone pattern. In more expensive multi-capsule microphones, the physical relationship of the capsules and the phase of their signals, when combined, create the resulting pickup pattern. A few highly specialized mics use external processors to achieve sophisticated patterns. Processing the individual signals from an array of microphones is not new and there are other variations on this approach: from the noise-cancelling line-array microphones sold to replace headset mics in computer applications; to the Soundfield microphone developed by Calrec for surround recording.

In sound reinforcement, it always comes down to polar pattern. Without being able to reject the sound of stage monitors, other instruments and the noisy venue, a microphone won't find wide use in sound reinforcement applications. While the mechanical approach offers simple and rugged devices, they are limited in their ability to reject low-frequency sound. Highly directional microphones, such as the shotgun mic, were never popular for sound reinforcement because of this significant limitation. The long interference tube provides the high-frequency directivity but often requires so much EQ for low/mid frequency feedback reduction that the result rarely sounds natural when used for sound reinforcement.

However, by adding some digital processing and a few more capsules, Audio-Technica was able to achieve a practical shotgun microphone that is relatively small and has incredible directivity through the vocal range. Of course, it is not the price or size of an SM-58, so don't tip out the mic case just yet. The lack of good rejection (typically 15 to 20 dB) at low/mid frequencies has always prevented the typical shotgun from getting the "reach" you need when forced to use distant microphones in sound reinforcement.

The AT895 adaptive array microphone uses a complement of four cardioid capsules mounted at the base of a single capsule mounted on a short interference tube. This combines the advantage of the mechanical interference tube at higher frequencies with the ability to process low-frequency directivity from the mic array. The resulting combination produces a ratio between on and off axis of up to 70 dB at low and mid frequencies. The lack of peaks in the digitally-processed mic array's off-axis response is significant when gain-before-feedback is important. The microphone also offers other modes including limiting the rejection to one axis (Planar Adaptive mode) so that a wide horizontal area can be picked up while extremely high rejection is achieved on the vertical axis.

You probably won't have a budget to fill a mic cabinet with these devices, but when you are backed into a "production" corner, this could be the solution. Much the same way that wireless microphones are more expensive than wired versions but are worth the expense in some applications. Unlike some other digital microphones, like the Beyer MCD 800 series, which have digital audio outputs, the Audio Technica adaptive array provides a single channel of analogue audio at the output of the control pack.

We should expect to see significant new products making use of active signal processing and multiple microphone transducers. The use of an array of small diaphragms to sample the sound field and produce highly defined (and even remotely controlled) directivity is not new. However the convergence of other technologies, such as extremely small signal processors (from cell phone development) and digital audio and control over simple cables (currently being standardized) make this an excellent time to develop new microphone arrays with extended capabilities.

The Audio Engineering Society is currently calling for comment on the standards for digital microphones. Currently, an AES Standards Committee SC-04-04 has drafted DRAFT AES42-xxxx and is looking for input from the audio community. If you share my surprise and horror that they may standardize on the use of conventional XLR mic connectors for this utterly incompatible signal, you may wish to visit the AES web site at www.aessc.org, check out AES42-xxxx and voice your concerns. I can imagine the problems of getting sound systems to work with these microphones connected through regular mic cables (because no one in the field reads the manual) and having intermittent audio (or worse) at the mixer input.

We can all benefit from good ideas in the processing of signals right at the microphone. Many of the developments focus on the needs of broadcasters and recording studios but some are also suitable for sound reinforcement. Plug one in and listen.

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