Mics in a digital world
Digital equipment is working its way into all corners of the broadcast facility. Even the loudspeakers are digitally controlled to some extent. However, the vast majority of microphones are basically still analog. Furthermore, the marketplace is loaded with new microphone brands presenting vintage designs. Has modern technology nothing to offer when it comes to microphones? The answer: Of course it has. However, it seems like broadcasters and others in the pro-audio field are reluctant to implement these technologies.
What makes a mic digital?
The microphone is a transducer. Analog microphones have two basic parts. The first is the membrane that vibrates in accordance with the surrounding sound field. The second is the power plant that transforms the vibrations of the membrane to electricity, basically a voltage.
If we convert the output of the microphone to digital code, we may call it a digital microphone. It is just a question of how close to the membrane we can place the analog-to-digital conversion.
It should be mentioned that digital semiconductor microphone elements that produce digital code directly do exist. However, the sonic qualities of these products are not yet sufficient for high-quality audio production.
The digital microphones made for pro audio in general have a condenser capsule followed by the least possible amount of analog circuitry. In major designs, an attenuator is introduced as an important interstage before the A/D conversion. It is there to optimize the dynamic range of the membrane system to that of the digital system.
Today, two different interfaces are used with digital output from microphones. The first is AES42, which covers digital microphones connected to digital audio input. (See Figure 1.) The second is the USB interface, which is regarded as a device connected to a computer. The USB connection is not really a microphone-related standard as there is no common agreement on interpretation of signaled values. These are defined by the individual manufacturer.
The Standards Committee of the AES has an active working group that prepared the AES42 first published in 2001. The latest version is from 2006. This standard describes an extension of the existing digital interface AES3 (formerly known as AES/EBU) to provide a digital interface for microphones. The major features — besides the audio transfer — are microphone signaling and remote control functions.
In the receiving end, it is possible to get information that identifies the microphone, the actual settings and so on. Also it is possible to remotely control the microphone. The typical options here include gain settings, directivity pattern and low cut. Synchronization is another issue of great importance also addressed within the AES3. Two modes are possible, the master being either the transmitter or the receiver.
The connection involves a combination of the standard AES3 and the phantom-powered microphone connection. A DC supply for the microphone and the electronics is needed. The DC is modulated, providing the controlling of the microphone. A standard XLR is mandatory. A special XLD connector is described for the use with digital microphones in order to prevent connection errors.
What is the advantage?
Remote control of microphones is nothing new. It has been done for more than 50 years. So what are the advantages of this technology from the broadcast perspective?
First, the degradation of the (analog) signal is not related to the cabling. Line losses will not directly influence the signal.
The optimization of the dynamic range is an obvious advantage. Remember that the dynamic range of a high-quality microphone basically is larger than what is obtainable with most A/D converters. The use of digital signal processing can improve the microphone and enlarge the scope of application.
The automation routines offer a major advantage in that they make it possible to add microphone settings to the scene memory of the console. Of course, there is no physical positioning involved. But still it makes large-scale live setups faster to operate when the optimization of the microphone signals is a part of the automation.
On the negative side, high-quality digital microphones are few and expensive. Besides, only a small number of mixing consoles offer input for digital microphones. These facts are the major reasons for the relatively limited distribution.
Digitally processed signals
A group of microphones regarded as digital does in principle have an analog output but involve a certain amount of DSP. These microphones are already widely in use.
For example, in array microphones, more capsules are built into one microphone housing. By processing the multiple signals, interesting features can be obtained, including direction steering and directivity control of a much higher order than known from any single or double capsule microphone. (See Figure 2.) Other possibilities are wind noise reduction and automatic sound source tracking. Much effort is put into the research of a specific area, especially for sports broadcast.
Another group of microphones involving more or less digital processing is for surround sound production. Surround sound is a major issue in digital broadcast, especially for the car industry, with its increasing number of vehicles installed with 5.1 systems.
Future of analog mics
There is a future for analog microphones. And there is a future in the manufacturing of good input stages, preamps, optimized for the different designs in order to get the preferred sound. The output of these devices might eventually be converted into digital streams.
The sound of the voice and the traditional musical instruments are still analog. The perception of sound is far from the digital domain.
However, money is also an issue here. It is a question of cost vs. benefit. We must realize that if there had been no use of business computers, digital audio would not have reached the dominating position we know today.
Concerning microphones, too much hardware is required to make the digital ones prized equal to the analog. We will pick our microphones — digital or analog — depending on the application and sound quality. Now and then we also look at the pricing.
Eddy B. Brixen is a consultant of EBB-consult of Denmark.
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