Report of the SC-04-04 Working Group on Microphone Measurement and Characterization of the SC-04 Subcommittee on Acoustics meeting, held in Munich, Germany, 2009-05-08
The meeting was convened by chair David Josephson.
The agenda approved with the addition of new projects discussion from E. Brixen, previous meeting report approved.
AES42-R, Review of AES42-2006: AES Standards for Acoustics - Digital interface for microphones
C. Langen, leader of the SC-04-04-D task group working on revisions to AES42-2006, reported on the meeting held the previous day. His report has been uploaded to the document site, showing consensus of the task group on a number of definition changes in the system command structure for AES42. The changes are summarized in "aes42-draft-080910-LANGEN-KERN-LESCHA.pdf" on the document site.
Consensus had not been reached on two items, one that is mentioned in the task group report and one that is not. Langen indicated that he was concerned that some task-group decisions had been made too arbitrarily to favor some companies using the standard, without due reference to existing standards which should apply. Discussion ensued in the working group on these topics.
AES42 contains definitions for several microphone parameters which may be switched using the command structure of the interface word. Several of them are not well defined, for example the low-frequency rolloff point and the compressor attack and release times. The standard does not describe these characteristics further than a frequency in Hertz for the rolloff, and a time in miliseconds for attack and release. Langen suggested that the standard should reference an existing standard where possible, such as IEC 60268-8 in the case of attack and release times. In the task group meeting there was no consensus on this topic, as members felt IEC 60268-8, being directly applicable only to automatic gain control devices, might not be appropriate for the attack and release times of a compressor in the microphone. No standard reference could be found to describe low frequency rolloff point.
After some discussion, the working group reached consensus that language be included in AES42 for each relevant parameter such as those mentioned here. The language should normatively reference an applicable standard if a suitable one can be found, or the parameter should be fully described in the manufacturer's manual for the AES42-compliant device. Graphs of rolloff curves would be sufficient, for example, as would a full description of the characterization of the attack and release time settings.
AES-X085, Detailed Professional Microphone Specifications
The chair gave a report on the progress of AES-X085. The round-robin test of the noise source is progressing, with four companies still to complete their tests.
The source of some uncertainty in the level measurements made in the round-robin tests so far was described, leading to a discussion in the working group of the preferred practice to be employed for the measurement of audio voltages for example for microphone sensitivity and noise levels. IEC 60268-4 requires that voltage levels be measured in open-circuit conditions (or at least with a load of at least 100 times the rated impedance of the microphone), and that the "true e.m.f." be calculated taking into account any load presented to the microphone by external equipment such as the power supply. The language remains unclear, however, since such correction is described in a Note with "should", and so may not be considered normative. It is also impossible in some cases to comply with the open-circuit requirements, as the load presented by phantom power resistors may be a lower value than the required 100 times the rated impedance of a microphone.
There was agreement that this situation was unsatisfactory, with some working-group members preferring that voltages be stated with a known load (Avenson, Josephson and Schneider preferring to state the voltage into a nominal 1000 ohm load, for example) and others preferring that the correction described in the standard be required.
It was the consensus of the working group that a comment be sent to IEC TC100 such that this issue is addressed in the next revision of 60268-4. Either a standard load impedance (such as 1000 ohms) should be used and stated, or the open circuit e.m.f. values be calculated taking into account the load impedance of the measurement setup.
Liaisons
J. Breitlow reported on the activity of the IEC maintenance group for 60268-4 (of which he is the leader) which considered comments contributed by SC-04-04, in 2008 in its Category D liaison role for revision of the standard. The working-group comments, found on the document site as "x85-iec60268-4-aes-comments.doc", had been referred to the MT in 100/1425/MCR. Several of SC-04-04's comments had been incorporated into the CDV issued recently by the maintenance group, while three were not accepted. No response from the maintenance group to these comments had been provided, and Breitlow reported that he was unaware that such response was required.
The working group discussed issues that remained unaddressed in 100/1549/CDV, and agreed to ask that the following comments once again be passed to TC100, with the assistance of J. Woodgate. The first three issues are from the working group's earlier comments, while the last is from the discussion at this meeting.
Clause 4.7 is unclear in its reference to "calibration". It should require that all measurements be no less accurate than +/- 2 dB.
Clause 11.1.1 should require that free field or plane progressive wave response be stated in all cases unless the microphone was not intended for use in the free field.
Clause 11.1.1 should require that the manufacturer state the maximum tolerance or variation from the given frequency response curves within which any microphone supplied under the specification will exhibit.
The several references to voltage measurements in open-circuit conditions should be conformed to a practice compatible with phantom-powered microphones, which may have a rated output impedance higher than 1/100 of the load presented by phantom powering resistors. Either a certain load should be required, with the voltages measured with such load, or an open-circuit e.m.f. correction should be required as normative rather than informative language. Otherwise significant errors will continue to be made due to the varying interpretations that may be applied to the present language.
AES-X093, Recommendations for Revisions of IEC 61938 Clause 7, Phantom power
There was a brief discussion of AES-X093. At its 2008-10 meeting, the working group passed comments to IEC TC100 within this project, which comments are on the document site as "x93-josephson-081013a". It is not clear whether TC100 has received these comments but they still represent the consensus of the working group and it was agreed to follow up on this work.
New projects
Brixen suggested that the working group initiate a new project to standardize the pinout of surround microphone connectors and he will initiate the project request. There was some discussion about the TC on spatial audio which already may have worked out such a standard. Mr. Brixen also asked that the working group consider establishing a standard for measurement of the diffuse field response of a microphone but deferred discussion until a later meeting.
There was some discussion about initiating a project to work on the characterization of wireless microphone systems, including the performance of the receiver. This will be discussed as a potential new project at the next meeting.
New business
There was no new business.
The date of the next meeting was noted to be in conjunction with the AES 127th Convention in New York, NY., US., 2009-10.
