Meeting of SC-04-04 in San Francisco, 2006-10-07

Report of the SC-04-04 Working Group on Microphone Measurement and Characterization of the SC-04 Subcommittee on Acoustics meeting, held in San Francisco, CA., US., 2006-10-07

The meeting was convened by chair D. Josephson.

The agenda and the report of the previous meeting, held in Paris, 2006-05-21, were approved as written.

Open Projects

AES42-R Review of AES42-2006 Digital Interface for Microphones
SC-04-04-D chair C. Langen reported on the task-group meeting held 2006-10-05 which was principally concerned with reviewing suggestions for updates to AES42. There was some discussion about harmonization of standard terms including the definition of "compressor" and "limiter." Before the next revision cycle, IEC standard terms will be incorporated where possible and additional definitions added where needed. Revisions are also planned in the informative sections of the standard to make the control theory of the two clock synchronization methods easier to understand.

Finally there was discussion about whether to extend AES42 to incorporate other digital microphone interfaces such as USB or Firewire (IEEE 1394). There was consensus among the WG members present that while SC-04-04-D might well be the appropriate task group to address these issues, they were distinct from AES42 and should be handled as separate projects, AES42 being a single defined interface.

Development Projects

AES-X085 Detailed Professional Microphone Specifications
Toward the goal of reaching a common graphics format for presentation of manufacturer specifications, J. Segota presented a data template consisting of a family of frequency response curves and data items. There was some discussion about the parameters of the sound source used to measure microphone response, which in common practice differs greatly from that specified in IEC 60268-4. Those members of SC-04-04-E who have been participating in the round-robin tests agreed to discuss on the email reflector and again at the next meeting the parameters of the sound sources they find most helpful in evaluating microphone response.

S. Temme (Listen, Inc.) and P. Pegas (Audio Engineering Associates) were added to the SC-04-04-E task group for development of test methods.

The first sample of the reference noise source produced by DPA Microphones appears to have been lost in transit, and DPA has produced another example which will be tested in turn by each of the members of SC-04-04-E beginning with Listen, Inc, then to AKG whence it will proceed in the same order as the microphone samples did in the earlier round-robin tests. Each participant will measure and report the A-weighted rms noise and psophometric quasi-peak noise of the noise source, and its noise spectrum, and report to the Working Group. M. Opitz of AKG will tabulate the results as they appear on the email reflector, as he did for the microphone measurements.

There was no consensus on the method to be used for reporting noise spectrum, several options being possible. One-third octave noise bands probably present the most useful and readily comparable metric for this characteristic.

AES-X093 Recommendations for Revisions of IEC 61938 Clause 7
The 2001 PWD of an Information Document on a revised phantom power standard was revised to incorporate concerns raised since it was written, and is now released by its writing group to the Working Group. It will be discussed over the next months, with a goal of voting on its acceptance as an Information Document at the next meeting. This information can then be passed to IEC TC100 as AESSC's recommendation for the revision of the IEC standard.

The basic change is to adopt 7 milliamperes as the "typical" maximum current, with the note that this value represents the maximum power transfer that is possible with standard P48 current limiting resistors. There is also support for noting that P24 phantom power is deprecated, perhaps using stronger language than the proposed "not recommended for new designs." It is understood that the present language in IEC 61938 (Ed.1 1996-12) preferring P24 may have been incorporated in error.

New projects

The chair briefly introduced a microphone interface standard common in the instrumentation industry, which is an extension of IEEE 1451.4 "Standard for a Smart Transducer Interface for Sensors and Actuators." While the "transducer electronic data sheet" (TEDS) format is well defined in the IEEE standard, the powering format of the "Constant Current Line Drive" (CCLD) analog interface is not, and there is the potential for damage to transducers whose design doesn't anticipate some of the other extensions of 1451.4 that are present in the market. The chair will initiate a new project request to develop an AES Information Document more clearly defining the powering aspect of the analog interface. This can be passed on to IEEE for suggested incorporation in their standard.

The IEEE standard exists and is in wide use; the AESSC's proposed role is simply to define its implementation for microphones a bit more clearly. There is a good possibility that this may become an alternative to phantom power where a low cost unbalanced powered interface is desired.

New business

There was no new business.

The next meeting will be scheduled in conjunction with the AES 122nd convention in Vienna, Austria, 2007-05.

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