AES42 digital microphone interface - PSNEurope

AES42 digital microphone interface

The first in a series of regular ‘Tech Notes’ designed to offer 5-minute ‘cut-out-and-keep’ explanations of how existing technology and standards work
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Microphones with digital interfaces enable analog-to-digital conversion to take place as close to the capsule as possible. The audio signal is then transferred digitally from the microphone to the console, helping to avoid typical problems with interference and noise. The AES42 digital mic interface consists of a standard AES3 interface with various added features, such as power for the microphone and control information. Unlike analogue microphone interfaces, most of which use 48 volt phantom power, the ‘digital phantom power’ used here is at a level of 10 volts applied to both legs of the balanced AES3 cable via a centre tap on the cable side of the transformer. Although the standard currently includes the option to use a modified ‘XLD’ connector for microphones using AES42, instead of the conventional XLR, these have not been implemented in practice. Instead a more recent AES standards project, known as X105, is in the final draft stage. It will specify a modified XLR-3 connector for digital audio that has a number of different keying arrangements for connector variants. Although using the normal XLR connector on digitally interfaced microphones allows for the possibility of operational errors when plugging, say, a digital mic signal into an analogue desk, this is little different from the situation that already exists with AES3 digital connections. However you will probably get a rather nasty noise if you try it. Remote control data is sent to the mic using pulsed modulation of the power supply voltage. Positive-going pulses of 2 ±0.2 volts carry data at a rate of 750 bits per second when the interface is operating at the 48 kHz sampling frequency. You can use this data stream to control settings such as the mic’s directivity (cardioid, omni, etc.), attenuation (pads), limiting, gain, muting and high-pass filtering. Manufacturer-specific settings are also possible. The microphone’s status can also be sent back to the receiver by means of the user bit channel in the AES3 data stream. There are two modes of sampling frequency synchronisation for AES42-interfaced mics. In Mode 1 the microphone is self-clocking and generates its own sampling frequency reference, whereas in Mode 2 mics can be synchronised to a common reference signal using remote control information. The AES42 receiver is supposed to continuously evaluate the sampling frequency of the incoming information from the mic, sending control information back to it in order to adjust any errors. In Mode 1, on the other hand, it’s necessary to use some form of sampling frequency conversion at the receiver end to synchronise the different rates coming from multiple microphones. The interface is not very widely used yet, but it’s likely to gain ground over the next few years. Some mics that use it include Neumann’s KM184D, D-01, and Schoeps’ SuperCMIT digital shotgun microphone. Schoeps’ CMD2 digital microphone amplifier also allows any of its Colette series capsules to be used with AES42. Neumann manufactures the DMI-2 and DMI-8 digital microphone interfaces, which handle the power, remote control and sync for its AES42 mics. RME’s DMC-842 interface is also an 8-channel AES42 interface and controller for digital microphones. A useful ‘white paper’ on ‘Digital Microphones and AES42’, containing much more detailed discussion, can be downloaded from http://www.hauptmikrofon.de/. The AES42 standard can be downloaded from http://www.aes.org/publications/standards/, free to AES members.

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