Report of the meeting of the SC-02-02 working group on digital audio interfaces, of the subcommittee on digital audio, held in Budapest, Hungary, on 2012-04-26
The meeting was convenened by chair J. Grant.
The agenda was approved as written. The report of the previous meeting had previously been approved via the e-mail reflector and there were no further comments.
Open projects:
AES41-R: Review of AES41-2009: AES standard for digital audio - Recoding data set for audio bit-rate reduction
The revised text is now in the process of a formal Call for Comment (CFC). A further amendment to add support for MPEG AAC has been proposed by A. Borsum, but the text was not available at the time of the meeting.
AES55-R: Review of AES55-2007: AES standard for digital audio engineering - Carriage of MPEG Surround in an AES3 bitstream
A revised text, with editorial changes such that this standard can be used for MPEG Spatial Audio Object Coding as well as MPEG Surround, has been uploaded to the document site by A. Borsum. Comments have been requested via the WG reflector.
AES-2id-R: Review of AES-2id-2006: AES information document for digital audio engineering - Guidelines for the use of the AES3 interface
The new draft and recent comments were reviewed. There was much discussion on how the V (validity) bit should be used; one issue was whether it can be set to indicate that the sample is the result of error concealment, so not a true copy of the original signal although still suitable for conversion to analogue. A. Mason and S. Heinzmann proposed that equipment manufacturers and users should be asked how they actually use the V bit, and also to provide horror stories to illustrate use cases for which the V bit does not work.
Another issue, raised by M. Law, was the recommendation in clause 4.7 to "mute" the signal (i.e. set the sample value to zero) when a sample with the V bit set is encountered, rather than the more friendly measures described in clause 8.4. A new draft has been uploaded with filename aes-2id-r-120426.doc incorporating changes agreed by the meeting.
The Secretariat suggested this new draft should go for CFC, with a further amendment being made later to incorporate the results of the poll of users regarding the V bit and also any changes needed for AES-X196 (see below).
AES-R8: AES standards project report - Synchronisation of digital audio over wide areas
No proposals for changes were made in the meeting.
However, during a Workshop later in the Convention it was noticed that clause 3.2 does not state that if multiple signals contribute to the same sound field they must be synchronised locally; clause 2.4 points out that such signals can drift relative to each other, but does not make clear the effect of this on (for instance) a stereo image.
Development projects:
AES-X139: Liaison with EBU ECA (Expert Committee on Audio)
A. Mason reported that PLOUD is still active, and work on 3D audio is starting. There is also a group studying synchronisation, both audio-video (lip sync) and audio-audio, e.g. coherence of signals from multiple microphones. An up- and down-mix group is being formed, but does not have a chair yet. Also, BWF is being revised. More details are available from http://tech.ebu.ch/ .
AES-X182: AES/Ethernet Simple Open Protocol
U. Zanghieri reported that the specification is being changed to use Rapid Spanning Tree Protocol instead of a proprietary protocol which uses MAC addresses in an illegal way. The question of whether Ethernet transformers would perform well enough at lower temperatures [corrected text - originally written as "lower frequencies"] to pass AES3 signals was also discussed.
AES-X186A,B,C: Liaisons with ITU-R re drafts derived from AES10-2008, AES3-2009, and AES11
No input had been received from ITU-R.
AES-X196: Revision of AES3 to use high sampling rates to carry multi-channel audio
The issue of how to ensure that channels are correctly identified immediately after sources are switched in a cross-point router was discussed. The options were identified as: (a) synchronise sources to house clock in a way that ensures they are all correctly aligned with each other; (b) use Y preamble for all channels except the first; and (c) set the C bit in the second subframe of a frame to 1 for the last channel, 0 otherwise. Option (c) was suggested by M. Law, and the rationale is that the first subframe can carry the channel status for the signal, which is unlikely to require more than one set of channel status. The channel status block for the second subframe will always begin with a zero, and this apparent setting of professional use in the first subframe and consumer use in the other will be easy to recognise.
A new Task Group, SC-02-02-K, is being set up to enquire of EBU members and router manufacturers which of these options would work with their systems.
Liaisons:
J. Yoshio reported that an amendment to IEC 60958-3 Edition 3 has been proposed by SMPTE to add metadata including loudness, and would be discussed in the IEC meeting on the Saturday.
New Project:
The proposal (on replay gain, to normalise playback levels), provisionally identified as AES-X202, is awaiting feedback from proposer S. Lyman prior to initialization.
New business:
There was no new business
The meeting closed at 1655. The next meeting will be held in conjunction with the 133rd Convention in San Francisco.
