Digital Microphones (AES 42)

Recent advances in technology have enabled microphone manufacturers to incorporate the analogue to digital convertor into the microphone itself.

The advantage of this is that there is no quality loss due to impedance and induced distortion artefacts, especially where long cable runs are required. It can operate on standard 75 ohm XLR cables upto 100 metres and upto 400 metres on 110 ohm (AES 3) cable.

An added benefit of this system is that it is possible to remotely control all the functions of the microphone from the mixer or recorder. So you don’t need to keep going out to the microphone itself to change pickup pattern, bass cut, attenuation and any built-in processing and you can listen to the effect there and then. 

In addition information such as the microphone’s manufacturer, model, serial number, software version installed and a list of control functions are displayed also you can name the sound source for that microphone. 

The technology that supports this remote control is the AES 42 protocol, this is an agreed standard approved by the Audio Engineering Society that any manufacturer of digital microphones will have to conform to.

The digital microphone e.g. Neumann’s KM 183 D has a three pin XLR connector, however, unlike an analogue microphone this connector handles a bi-directional signal (AES 42) which takes a balanced audio feed from the microphone and supplies phantom power, the remote control data stream and a synchronising signal from the Master Clock to the microphone. N.B. unlike analogue systems, digital systems need a master clock to keep all the packets of digital information in the correct sequence.

A hardware controller such as the DMI-2 (in the case of Neumann’s two channel digital microphone system) and the remote control software (which can be used on a Windows PC or Mac’ computer) is used to operate the microphones, if more than two microphones are needed then more DMI-2’s can be cascaded and their output is fed (still in the digital domain) to a standalone digital recorder or to a software based recorder on the same computer that hosts the remote control software.

RME makes the DMC 842 which is an eight channel AES 42 controller which can be linked to seven more controllers via an ‘RME164’ MADI (multiplexed audio digital interface) card for a total of 64 channels.

Mode 1 and Mode 2 operation

AES 42 has two modes of synchronising digital microphones with the controller.

Mode 1 operates asynchronously i.e. the microphone generates its own internal clock and a sample rate convertor in the controller matches the sample rates and timing of each microphone’s signals.

In Mode 2 the controller sends timing (frequency and phase) information to each microphone to control the internal quartz oscillator in each microphone.

Mode 2 is the preferred option since mode 1 can impair the signal quality in terms of dynamic range and latency times.

Currently there are only a few manufacturers making digital microphones using AES 42 technology, these include Neumann with their KM-D range and Schoeps with their Colette range.

Pro’s

Excellent quality over very long distances.
Remote control of parameters and instant recall of settings from previous sessions.
If all the microphones are digital then it could work out cheaper than an analogue system.

Possible Cons

When using digital and analogue microphones at the same time there may be slight time delays between them due to the digitisation process; the analogue microphones will have to be delayed to match the timing, either when they are digitally converted or in the digital recorder - test record a handclap on all the microphones when they are physically next to each other and then line up the waveforms on the digital audio recorder’s arrange page using the plus or minus delay on the relevant tracks and save those settings for the actual recording.

Cost, although this might be offset by not having to buy separate pre-amplifiers and analogue to digital convertors.

USB Microphones

 Although technically these are digital microphones, they are not as versatile as AES42 types and some care needs to be taken with their gain structure (particularly those with 16 bit convertors) to avoid either hiss or distortion. Gain has to be set at the microphone itself and the signal can’t travel very long distances without employing USB powered hubs to amplify the signal.

However they are a compact and lightweight solution to getting audio into a computer since they don’t require a separate pre-amplifier and analogue to digital convertor.

If you want to use many microphones you will probably be better off using a mixer (analogue or digital) because you will soon run out of the number of USB ports you’ll need.  

Examples of USB microphones are Audio Technica’s AT2020-USB, Rode’s Podcaster and Samson’s G Track SA-GM1U.

Also in this category is the HHB DRM 85 which has an inbuilt Flash Ram recorder which can be connected to your DAW (Digital Audio Workstation) after you’ve finished recording for editing.