Report of the Meeting of the SC-04-04 Working Group on Microphone Measurement and Characterization of the SC-04 Subcommittee on Acoustics held in San Francisco, CA., US., 2012-10-28
The meeting was convened by chair D. Josephson.
The agenda and the report of the previous meeting, held in Budapest, 2012-04-27, were approved.
Open projects
AES42-R Review of AES42-2010: AES Standard for Acoustics - Digital interface for microphones
J. Breitlow prepared and has now uploaded to the doc site a report on known devices in the market offering interfaces compatible with AES42-compliant microphones. Many provide only Mode 1 function.
Development projects
AES-X085 Detailed Professional Microphone Specifications
Work on AES-X085 continues with evaluation of the results obtained by the various manufacturers' labs with the reference noise source provided by DPA. E. Brixen will report on the before-and-after measurements of the noise source, which will be added to the last revision of the measurement table uploaded during the Budapest meeting.
Most labs are in close agreement (0.3 dB peak-to-peak variation envelope) on measurement of A-weighted rms noise, with the exception of one lab whose variation is consistent with their use of a 1k ohm load resistor rather than the open circuit specified. The variation for ITU-R BS.468 measurements vary over a nearly 3 dB envelope. Discussion centered on the less than clear definition of the detector law used for these measurements as the algorithms used by the various instrument makers are different. S. Temme agreed to provide an algorithm for creation of a white noise reference file which could be used for testing detectors, and a sample file of its output with rms value at -40 dBFS, 24 bit, 48 kHz sample rate.
Liaisons
IEC 60268-4
The revision cycle of IEC 60268-4 continues with most of the members of its maintenance team also members of SC-04-04, so concerns and input from SC-04-04 are routinely considered in the revision of the IEC standard. Edition 5 is progressing to CDV in the next few months.
RTCA SC-226
There is no formal liaison established, but WG chair D. Josephson served on RTCA SC-226, which is revising the standard for audio interfaces for large aircraft including some difficult problems with cockpit area microphones and cabin loudspeakers. Now there is a lot of work on active noise-cancelling headsets and their impact on flight safety. RTCA and its European counterpart EUROCAE are now working to resolve these problems and are well on their way to a solution. The next few years will see additional work in this area both with RTCA and EUROCAE and there is opportunity for AESSC to be involved if it is so inclined.
New projects
wireless or radio microphones
There was a brief discussion at the meeting about opening a project to develop standards applying to wireless or radio microphones.
Wireless microphone makers almost universally prefer to sell entire systems, leaving the air interface proprietary. The only application for IEC 60268-4 in this case will be acoustic to electrical performance, as if the wireless link were not present. There are additional complications, as many radio mics now use nonlinear codecs in their digital links, resulting in latency and potential artifacts. The WG will discuss over the next few meetings whether there is any interest in pursuing AES standards activity in this area.
One aspect of standardization is already proceeding, the inclusion of wireless/radio mics in the EMC provisions of IEC 60268-4. It is a complex question and the IEC maintenance team is attempting, by reference to existing standards, to preserve manufacturers' options as much as possible rather than suggesting that new or different standards be adopted.
New Business
There was no new business.
The date of the next meeting will be scheduled in conjunction with the 134th Convention of the AES in Rome, Italy, 2013-05.
