Report of the meeting of the SC-02-01 Working Group on Digital audio measurement techniques of the SC-02 Subcommittee on Digital audio, held in Berlin, Germany, 2014-04-28
The meeting was convened by chair T. Kite.
The agenda and the report of the previous meeting, held in New York, 2013-10-18, were approved as written.
AES-X102 Liaison with IEC TC100 MT 61606
The secretariat will draft a liaison statement to IEC TC100. We expect to offer proposals for updating IEC 61606-3 once the update to AES17 is complete.
AES-X187A: Revision of AES17
Kite reported that the document is very close to completion. Kite and M. Yonge had several remote editorial meetings before the Berlin meeting to achieve this. Note that project AES-X217 (below) has prompted one comparatively minor update.
Before this Berlin meeting, there had been some activity on the reflector regarding the long-standing problem of the confusion of the 'FS' unit in AES17. There have been requests for clarification, particularly of the relationship between the FS unit and peak levels.
The confusion in the field might be expressed as follows: traditional level meters for broadcast and music studios are concerned with quasi-peaks, whereas AES17 uses only rms values. A sine wave can hit 1.0 FS, and that coincides with an instantaneous peak of the maximum code in the representation, but for higher crest factor signals (such as music or speech) an rms level of 1.0 FS implies overload.
Kite described the situation as follows: AES17 defines 1.0 FS to be the rms level of a 997 Hz sine wave whose peak level touches the maximum code in the representation. AES17 defines neither a unit for peak level, nor a means for measuring it. It is unfortunate that a "1.0 FS" digital sine wave (akin to a 1V rms analog sine wave) has a peak level of 1.0 when expressed as a fraction (compared to 1.414 V for analog). From a measurement perspective, it is necessary to scale rms values differently for digital and analog.
The issues with AES17 might be summarized as either: 1. The rms definition is not consistent with the analog domain. 2. There is no peak unit.
Ultimately, what is in question is the definition of FS (clause 3.12 in AES-X187A). J. Woodgate feels that the existing definition is inadequate. He offered to make a proposal for alternative wording. Given the schedule for AES-X187A (see later), this should be done in the next two weeks.
P. Treleaven felt that the definition in 3.12 was correct and consistent, and suggested that Woodgate's suggestion be treated as one of the public comments on the document. Given Woodgate's clear discomfort with the current wording, Kite felt that Woodgate should have an opportunity before public comment to improve on it. The draft was posted to the working group via the reflector on 2014-04-10. Working-group members should comment, if necessary, on the Proposed Task-group Draft (PTD) within two weeks from this report (that is, before 2014-05-16).
AES-X217: Performance standard for archival ADCs
After the New York meeting, Yonge began the work of separating C. Lacinak's document into the methods of measurement, and the performance criteria for those measurements.
A concern expressed in New York was that not all the tests in the AES-X217 proposal were contained in AES17. Kite reported:
1. The crosstalk method given in Lacinak's proposal was similar to the AES17 crosstalk measurement, but carried out at a test signal level of -1 dBFS rather than the -20 dBFS of AES-17. Kite and the group felt that -1 dBFS was better, because crosstalk is inherently a noisy measurement. AES-X187A will be changed to use this stimulus level. Note that this will not change measurement results (other than making them less noisy) because crosstalk is measured as a ratio [and expressed in decibels].
2. Lacinak's proposal has a CMRR measurement. Kite asked whether this was needed in AES-X187A. The group felt that this was the purview of IEC 60268-3, and that AES-X187A should simply refer to that standard.
3. Lacinak's proposal has a measurement of "spurious and aharmonic signals" which drives the EUT with a 997 Hz sine wave and determines the highest tone in the EUT output by using a 32k FFT. Since AES-X187A adds an FFT analyzer to the equipment list, this measurement could potentially be added to it. However Yonge suggested that it would be better to leave this to a future revision of the standard, given the late date and the rather nebulous nature of the measurement.
Liaisons
Liaisons with other groups were discussed in other projects, as reported above.
New Projects
In New York, Lacinak brought up the issue of errors that occur between the audio inputs and the mass-storage device when using general-purpose computers as digital recorders. The Working Group then decided to explore this problem.
At this meeting the Working Group felt that they were not the right group to address this problem. Whilst audio is obviously involved, loss of data across a network or inside a computer is the purview of computer science. The group therefore proposed to pass on this project.
New Business
There was no new business.
The next meeting will be held in conjunction with the AES 137th Convention in Los Angeles, USA, 9 to 12 October 2014.
