Report of the SC-04-04 Working Group on Microphone Measurement and Characterization of the SC-04 Subcommittee on Acoustics meeting, held in Vienna, Austria, 2007-05-06
The meeting was convened by chair D. Josephson.
The agenda and the report of the previous meeting, held in San Francisco, 2006-10-07, were approved as written.
Open projects
AES42-R Review of AES42-2006 Digital Interface for Microphones
AES42, the standard for digitally-interfaced microphones, is under active review in its revision cycle and work continues to incorporate input from several organizations to keep the standard current.
At the exhibition in Vienna, the first power and control device compliant with AES42 modes 1 and 2 was introduced, with active support from some of the manufacturers of AES42-compliant microphones.
At the time of the meeting, SC-04-04-D scheduled its meeting for the following day, May 7. Its report will follow from the task-group reflector.
Development projects
AES-X085 Detailed Professional Microphone Specifications
Work continues in AES-X085 around the development of detailed professional microphone specifications. The present task is the development of consensus on the measurement and reporting of microphone self-noise. Eddy Brixen from DPA provided a sample "dummy microphone" example noise source which is now being sent around to the ten microphone makers who participated in the earlier round-robin test of microphone performance. Noise was the specification where the most variation was seen in the results of the various microphone labs, due mostly to the lack of a precise standard for such measurement.
The first testing lab for the example noise source was Listen, Inc. and S. Temme presented the results of their tests in graphic form, with spectra and summed wideband and A-weighted rms presentations. Unfortunately the measurements included some mid-band noise and power supply hum components, but these can probably be edited out of the data for later comparisons. Quasi-peak measurements compliant with the IEC 60268-4 standard were also not presented. There was some discussion of the need to use a good mic preamplifier because of the difficulty of commercial test gear to match the noise performance of modern microphones.
There was a brief discussion of whether the new standard might include maximum or typical figures for noise, and it was made clear that no decision has been made on that aspect.
The important and the history of the ITU-R (formerly CCIR, formerly CCITT) BS.468 quasi-peak noise testing specification was reviewed, with comments from E. Werner, J. Woodgate and J. Wuttke. Further development of this standard is not expected, because its primary application (measuring the audibility of switching transients cross-coupled from one channel of an analog telephone multiplex system to another) no longer exists. The standard was developed with a great deal of research and analysis on the audibility of low-amplitude impulse noises in the presence of louder signal sources, and it would be desirable to retain this knowledge base in the progression of the microphone standard. Woodgate volunteered to research some of his sources for material on the intent of the CCITT quasi-peak measurement so that we can develop an optimally precise definition for use in evaluating "popcorn" or impulse noise in microphones.
It was pointed out that maintenance of IEC 60268-4 was managed by IEC TC100 and that active liaison was underway with that group.
AES-X093 Recommendations for Revisions of IEC 61938 Clause 7
There was no progress to report on projects AES-X093 as the task group for this project is still working on the details. Reports are expected to be available at the New York meeting in October.
AES-X160, Powering and analog interface for constant-current line drive (CCLD) transducers
There was no progress to report on project AES-X160 as the task group for this project is still working on the initial draft. A report is expected to be available at the New York meeting in October.
New projects
Under new projects, C. Woolf brought up the issue of the proliferation of USB microphones. J. Breitlow reported on the work being done by the German national committee contributing to IEC TC100, with the summary that all normal microphone specifications should be adhered to in principle, with the reference level being redefined to be 0 dBFS. Breitlow will forward a summary of this work to the SC-04-04 reflector.
New business
There was no new business.
The next meeting was scheduled to be held in conjunction with the 123rd AES convention in New York, 2007-10.
