AES Standards News Blog

The recent publication of the new multi-part AES3-2009 has meant that some references in other AES documents became hard to find in the new AES3. In order that these documents can continue to provide useful guidance, a number of them have been reprinted with relevant editorial addenda to indicate references to AES3-2009 in addition to their original normative references.  The documents concerned are:

AES-2id-2006    Guidelines for the use of the AES3 interface
AES10-2008    Serial Multichannel Audio Digital Interface (MADI)
AES11-2009    Synchronization of digital audio equipment in studio operations
AES47-2006    Transmission of digital audio over asynchronous transfer mode (ATM) networks
AES52-2006    Insertion of unique identifiers into the AES3 transport stream
AES55-2007    Carriage of MPEG Surround in an AES3 bitstream
AES42-2006    Digital interface for microphones
 

Charges for AES standards were reviewed at the end of last year and, with the support of the AES Board of Governors, it has been decided to make AES standards available to AES members free of charge as a benefit of membership. This will provide individual standards in their full printable form as downloadable PDF files. Select the documents you need in the Standards Store (http://www.aes.org/publications/standards/) and proceed to checkout as normal; make sure you provide the required proof of membership and the invoice will show no charge!  Non-members will continue to be able to access informational previews of standards for free and also purchase full printable copies.

AES3-2009 AES standard for digital audio — Digital input-output interfacing — Serial transmission format for two-channel linearly represented digital audio data has been published in a new multi-part revision at the the AES on-line store at http://www.aes.org/publications/standards/.

AES3 has been under constant review since the standard was first issued in 1985, and this multi-part edition reflects the collective experience and opinions of many users, manufacturers, and organizations familiar with equipment or systems employing AES3. Its technical content is intended to be identical to the relevant parts of the 2003 edition as amended by Amendments 5 and 6. The language has been reviewed to remove potential ambiguities -  occasionally introduced because modern technology now offers many more options that were historically available - and to improve readability.

Separating AES3 into four independently-maintainable parts in this way allows, for example, additional transmission media to be introduced in the future by revising AES3 Part 4 without affecting the other parts of AES3. Interfaces will also be able to claim compliance with AES3 Parts 1 to 3, even though they use physical and electrical techniques beyond those described in Part 4.

Part 1 Audio Content: defines the format for coding audio used for the audio content. This Part specifies the semantics of the audio data, including the "validity" flag. It also specifies the sampling frequency by reference to AES5, "AES recommended practice for professional digital audio — Preferred sampling frequencies for applications employing pulse-code modulation".

Part 2 Metadata and Subcode: specifies the format for information, metadata, or subcode transmitted with the audio data: principally the "channel status" but also User Data and the auxiliary bits. Implementors wil note that, since the "Minimum Implementation level" was retired in Amendment 6 (2008), the current implementation options ("Standard" and "Enhanced") both require that AES3 status data be implemented correctly in compliant equipment.

Part 3 Transport: defines the format for transport of an AES3 digital audio interface. 
It specifies the framing and channel coding for transmission on a unidirectional point-to-point physical link. The specified format minimizes the direct-current (DC) component on the transmission line, facilitates clock recovery from the data stream, and makes the interface insensitive to the polarity of connections.  Specific synchronization issues are covered in AES11 "AES recommended practice for digital audio engineering - Synchronization of digital audio equipment in studio operations". 

Part 4 Physical and Electrical: specifies the physical signals that convey the bit stream specified in Part 3. The transport format is intended for use with shielded twisted-pair cable of conventional design over distances of up to 100 m at frame rates of up to 50 kHz. Longer cable lengths and higher frame rates may be used, but with a rapidly increasing requirement for care in cable selection and possible receiver equalization, or the use of active repeaters. Provision is made in this standard for adapting the balanced terminals to use 75 Ohm coaxial cable, incorporating information previously published in AES-3id. Transmission by fibre-optic cable is under consideration.

Because these documents are individually maintained, they are published independently.  However, because the contents of these four documents are related, they are also available as a set. An engineering guideline document to accompany AES3, "AES-2id AES information document for digital audio engineering - Guidelines for the use of the AES3 interface", carries references to the previous edition, AES3-2003, but is in the process of amendment to refer to the new multi-part standard. Other AES documents that carry references to AES3 will also be amended where necessary.

As predicted in the foreword to AES41 in 2000, digital compression techniques now dominate the broadcast television environment. In addition to the problems foreseen relating to cascaded compression, new problems have arisen because of the use of loudness control and surround sound with those digital compression techniques.

Metadata within the compressed audio bit-stream is used to control loudness and the mixing down of multichannel surround sound to two-channel stereo. These metadata are usually known by terms such as "dialnorm", "prog_ref_level", and "down-mix coefficients".

Whilst this might seem unrelated to the original scope of AES41, dealing with bit allocations and scale factors, it is simply another form of data that can affect a later encoding of the audio: this time is it is more macroscopic than microscopic.

The metadata is lost when the bit-stream is uncompressed unless provision is made to transport it or store it somewhere. Existing methods rely on non-audio mechanisms to convey the metadata alongside the audio, for example a serial data link like RS-422 and serial digital video SMPTE 259M, or a "chunk" in an audio file (for metadata that does not change).

This revision extends AES41 to include data formats for carrying this loudness and down-mix metadata with the uncompressed PCM using the same transport mechanism as before. The metadata can therefore be carried in the audio to which it relates.

A Call for Comment on DRAFT AES59-xxxx, AES standard for professional audio - Audio application of 25-way D-type connectors in balanced circuits has been published: 2009-12-08.  http://www.aes.org/standards/comments/cfc-draft-aes59-xxxx.cfm.

This document describes a standard contact assignment and gender convention for users of the 25-contact D-type connector as a multi-channel balanced audio interconnection, in analogue or AES3 digital form, to facilitate interconnection of equipment from different suppliers using standardized cables.

A Call for Comment on reaffirmation of AES14-1992 (r2004), AES standard for professional audio equipment - Application of connectors, part 1, XLR-type polarity and gender was published in accordance with our 5-year review policy on 2009-12-07.  http://www.aes.org/standards/comments/cfc-reaffirm-aes14-xxxx.cfm

A Call for Comment on REAFFIRMATION of AES17-1998 (r2004), AES standard method for digital audio equipment - Measurement of digital audio equipment has been published in accordance with our 5-year review policy on 2009-12-07.  http://www.aes.org/standards/comments/cfc-reaffirm-aes17-xxxx.cfm

The synchronization of audio and video (A-V sync, or lip sync) in television continues to pose a problem for
producers and broadcasters, because errors in synchronization continue to be a source of audience
dissatisfaction.

Although digital television includes timing information that should keep audio and video properly
synchronized, this in itself can simply be another source of problems. New display technologies, and separate
audio and video signal paths within the living room, present new challenges for lip sync.

This project was started in an effort to help with creating the solution to the problem. It had the aim of
standardizing a simple method of measuring lip sync. The task proved more difficult than anticipated and the
project was subsequently closed. However this report gathers together two relevant documents that were
produced by the British Broadcasting Corporation during the life-time of the project. One is a study of factors
that affect perception of lip sync, the other is a study of operational practices in a real broadcast chain.

A Call for Comment on DRAFT REVISED AES3, AES standard for digital audio — Digital input-output interfacing — Serial transmission format for two-channel linearly represented digital audio data, has been published at http://www.aes.org/standards/comments/

AES3 has been under constant review since the standard was first issued in 1985. The present revision - divided into four complementary parts to simplify implementation over an increasingly wide range of application - reflects the collective experience and opinions of many users, manufacturers, and organizations familiar with equipment or systems employing AES3.

Part 1 defines the format for coding audio used for the audio content.

Part 2 specifies the format for information, metadata, or subcode transmitted with the audio data: principally the "channel status" but also user data and the use of the auxiliary bits to carry a co-ordination signal.

Part 3 specifies the framing and channel coding for transmission on a unidirectional point-to-point physical link. The specified format minimizes the direct-current (DC) component on the transmission line, facilitates clock recovery from the data stream, and makes the interface insensitive to the polarity of connections. 

Part 4 specifies the physical signals that convey the bit stream specified in Part 3. The current revision covers electrical signals on twisted-pair and co-axial cables. Other media, including fibre optic connections, are under consideration.  

Separating AES3 into independently maintainable parts in this way would allow, for example, the revision of part 4 to introduce additional media in the future without affecting the other parts of AES3. Interfaces would also be able to claim compliance with AES3-1 to AES3-3 even though they use physical and electrical techniques beyond those described in part 4. Its technical content is believed to be consistent with the relevant parts of the 2003 edition as amended by Amendments 5 and 6. The language has been reviewed to remove potential ambiguities— occasionally introduced because modern technology now offers many more options that were historically available—and to improve readability.

Because the contents of these four documents are related, they are published for comment as a set. Any comments arising will be considered for each document independently.

A new AES standards project, AES-X188, "Screen-less navigation for high-resolution audio on Blu-ray Discs" has been initiated. The project aims to specify a method for authoring an audio-only Blu-ray Disc to enable playback in screen-less consumer systems, and to provide simple stream and track selection using the remote control of a normal Blu-ray Disc player.

High-resolution audio, presented as uncompressed LPCM, has been waiting for a suitable transport format for some time. The Blu-ray disc format offers such a transport and supports the necessary linear and lossless codecs as part of its basic specification. While many Blu-ray players can be found in home theatre and games environments, there are some issues that need to be addressed before they can be introduced into a hi-fi environment that does not have a screen to present visual menus for audio stream setup and track selection.

Development of this standard has been assigned to the AES standards working group SC-02-08 on Audio-file transfer and exchange.  More information can be found at http://www.aes.org/standards/meetings/init-projects/aes-x188-init.cfm