Meeting Topic: AES42 Digital Microphones and Controllers - Technology and Application --
Moderator Name: Gabriel Leuzinger
Speaker Name: Dr. Helmut Wittek (SCHOEPS Mikrofone GmbH) and Stephan Flock (DirectOut GmbH)
Meeting Location: tpc HD studio 6, Zurich
About 25 audio professionals gathered at tpc's HD studio 6, which hosted the AES42 event with Helmut Wittek (SCHOEPS) and Stefan Flock (DirectOut).
Helmut Wittek started his presentation with the often-raised questions concerning the advantages of digital microphones. In his opinion, the dynamic capability of digital microphones is often overestimated and that in practice, it is not easy to reach the dynamic performance of analogue microphones feeding separate converters. In fact, the signal chain of a digital mic is the same as with its analogue predecessor but with the challenge of less power and limited space in the actual microphone housing. To handle the more or less 120 dB dynamic range of the capsule, the amplifier/converter combination is the bottleneck and needs to provide more than 140 dB dynamic range in order not to limit headroom. One solution to this problem is the well-known gain ranging technique, such as that implemented in StageTec's TrueMatch converter.
Stefan Flock delved deeper into the AES42 standard, as well as providing the history of the standard and how it has developed to its current release. As design engineer of the RME DMC842 (during his former position at RME), he concentrated on the control side of AES42, of which an important part is the control and status of the microphone. Control signals are modulated onto the power supply voltage and run at a maximum of 750 bps. These signals are organized in a simple control set with basic features and an extended control set providing 31 further parameters. For Mode2, a special "distributed" PLL (Phase Locked Loop) is used, as the discriminator is located in the controller and the VCXO (Voltage Controlled Crystal Oscillator) is located in the microphone. Control signals for the VCXO are sent as control pulses, which are then integrated and smoothed to control the VCXO. Stefan Flock visualized the locking process of the PLL with a short video of his oscilloscope. The result of this architecture is a very low jitter, phase aligned sampling clock used for the AD converter inside the microphone.
Following the presentations and Q+A, the discussion continued in a nearby restaurant. As well as our two speakers, we would like to thank tpc ag for providing HD studio 6, Alfio Di Fazio and Gerard Koch for their support and Decibel S.A., MusicNetwork AG, SLG Broadcast AG and DirectOut GmbH for providing demo equipment.
Written By: Gabriel Leuzinger