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Next gen audio standardised for Ultra HD TV systems

ETSI has ratified DVB's choice of MPEG-H 3D and Dolby AC-4 as the audio options for Ultra High Delimitation TV services and products

The European Telecommunications Standards Institute (ETSI) has published the DVB (Digital Video Broadcasting) specification for upcoming Ultra High Definition UHD) technologies and TV/streaming services as an official standard. This includes Next Generation (NGA), with features such as immersive sound offering options for personalisation, interactivity and the ability to select alternative languages and other versions.

The DVB Steering Board approved TS 101 154 V2.3.1 (‘Specification for the use of Video and Audio Coding in Broadcasting Applications based on the MPEG-2 Transport Stream’) in November 2016. The decision by ETSI to standardise those recommendations will make the interoperability necessary for UHD-1 Phase 2 products and broadcasting widely available to both consumer electronics manufacturers and broadcasters.

The vision aspects of this, primarily HDR (high dynamic range, which delivers greater luminosity than other imaging formats, including 4K), have received most attention but future UHD systems will also offer a comprehensive selection of audio features. Since UHD was first announced there has been discussion over what sound formats should be used with this higher resolution, more immersive video system.

Many developers, broadcasters and manufacturers – both consumer and professional – view spatial surround systems, such as Dolby Atmos and Auro3D with the extra dimension of height, as the obvious accompaniment to UHD. Others considered established discrete 5.1/7.1 formats such as Dolby Digital adequate for the purpose. This divergence of opinion surfaced in DVB discussions but eventually it was decided to recommend NGA technologies.

“There were some DVB members who were not convinced NGA was needed,” comments David Wood, chair of the DVB Commercial Module group on UHDTV. “But members in general wanted a ‘tool box’ approach for NGA where both the standardised options are included in the specification.”

The two audio formats DVB decided on are MPEG-H 3D Audio and Dolby AC-4. Both technologies deliver the desired immersive experience, using object-based techniques instead of relying wholly on the more traditional channel approach. There is also scope for personalisation and customisation, allowing viewers to select different audio versions and features. Important among these in terms of accessibility is Audio Description (AD).

MPEG-H 3D Audio comprises a mixture of channels, objects and scene-based technologies. Writing in the March edition of DVB Scene, Stefan Meltzer, a technology consultant to Fraunhofer IIS, which was involved in the development of MPEG-H 3D Audio, explains that the channels add the height component to existing surround systems, producing 5.1+4 and 7.1+4. The objects are not tied to a specific loudspeaker configuration, with their position and function identified by associated metadata. Scene-based audio is made possible through High Order Ambisonics, which allow the sound picture to be manipulated prior to playback. This last feature is seen as particularly suited to virtual reality (VR) projects.

Dolby AC-4 is an open standard defined by ETSI and became part of the DVB’s codec spec in 2014. AC-4 was designed round Dolby’s Atmos immersive system but also includes options for selecting different audio versions, such as alternative commentaries and altering the balance between dialogue and sound effects and music, as well as AD (in a similar way to MPEG-H 3D). Also writing in DVB Scene, Elfed Howells, business group director at Dolby, comments that AC-4 is suitable for broadcast transmission, OTT and mobile streaming services (and their hybrids), as well as VR.

Because both systems derive from the same basic principles, Wood (pictured) says they offer “similar features”, which, he adds, means there will be “no major difference in the experience” for the listener/viewer.

Wood explains that the NGA systems have two major objectives: “One is to provide the option of an immersive audio experience. The other is to allow the broadcaster to provide separate audio channels, which we term ‘objects’ and that can be selected and adjusted by the user. These allow for different languages, dialogue and audience groups, which could be a very exciting part of NGA.”

There is the possibility that NGA could in the future be used to alleviate some audibility problems, although it is unlikely to help with ‘mumbling’ actors. “Both NGA systems would offer considerable flexibility in terms of the audio offering,” Wood says. “This could include a separate dialogue channel that could be adjusted in relative volume to the background sounds. This could be helpful for many, particularly the elderly. But that would be for the NGA broadcaster to decide.”

The NGA and DVB UHD Phase 2 specifications were being processed by ETSI at the time PSNEurope was going to press. Wood expected publication of the standard in the following weeks. While these are still early days for the implementation of NGA, the UHDTV service in South Korea is set to become the first broadcaster to use MPEG-H Audio as its sole sound source. Spanish national broadcaster RTVE has been testing AC-4 in live transmissions over the last six months. Other trials of the system have taken place in France at TDF, France Television and Arte and at PBS and KQED in the US.