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AC-4 Audio Delivery Picks Up Pace

By Steve Harvey. This year has seen a significant uptick in activity for AC-4, Dolby Laboratories’ end-to-end audio solution for next-generation broadcast and streaming services—and here’s why.

San Francisco, CA—This year has seen a significant uptick in activity for AC-4, Dolby Laboratories’ end-to-end audio solution for next-generation broadcast and streaming services. Offering improved compression efficiency, immersive audio and features including dialogue enhancement, intelligent loudness and advanced dynamic range control, Dolby AC-4 is already standardized with the European Telecommunications Standards Institute (ETSI) and has been adopted—alongside the MPEG-H TV Audio System—by the Digital Video Broadcasting Project (DVB) and ATSC 3.0.

The year kicked off with the ATSC 3.0 transmission of various Winter Olympics events, including the closing ceremonies, in a trial at WRAL-TV in Raleigh, NC, reports Mathias Bendull, vice president, Multi-Screen Services Audio, Dolby Laboratories. “It was the first AC-4 transmission over ATSC 3.0, delivered over the air to a very small number of receivers in the market—but it was a proof point.”

More significantly, perhaps, is the Phoenix Model Market, a trial of next-gen broadcast services being led by the Pearl TV group of network and broadcasters currently underway in Arizona. “Phoenix is very interesting because it has a very high percentage of households watching television sets over the air, so ATSC 1.0 is very important there,” Bendull explains. “We are happy to report that the signals that are on the air in Phoenix, which are two channels, are encoded in AC-4.” The broadcasts are 5.1, he notes, “but we now have equipment out there that runs 24/7 and will expand beyond those configurations.”

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Since ATSC 3.0 is IP-based, “We need to make sure that the audio part, which in many instances is just a small piece of the overall broadcast chain, is very well integrated with all the other pieces, whether that’s advertising, scheduling or just the connection with the video stream. That’s why we are very supportive and strongly involved in the Phoenix Model Market.”

Potential benefits of Dolby AC-4 exist throughout the content lifecycle, from capture to distribution.

The AC-4 codec is a tool to deliver next-generation audio (NGA) experiences, Bendull comments. And NGA is reinventing how audio is produced, moving from a channel-based to an object-based paradigm. “That allows us to separate individual sound sources as objects, and only at the playback device do we combine them into what the listener would like to listen to.”

One benefit of objects is that they can be used to deliver alternate languages or commentaries, or potentially enable viewers to adjust the relative background and dialogue levels. “That brings benefits to the consumer as well as to the service provider or broadcaster,” he says. Which NGA features are adopted first will depend on the operational costs, commercial value or content availability relevant to individual broadcasters, TV stations or streaming companies.

“If I start producing object-based audio—and very few do that today in the way where you separate the dialogue, for example—that changes the production workflow. Unless the industry comes together and really creates tools that make that easy, this may be an obstacle” to wider or faster NGA adoption, Bendull cautions.

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The value of Dolby Atmos to service providers currently is typically in attracting top-tier subscriptions, he notes. “Netflix makes Atmos content available only to their highest tier. They use that as a differentiation.”

But there is certainly plenty of content available. There are some 600 Dolby Atmos movie titles today, says Bendull, and BT Sports and Sky in the UK produce 250 English Premier League games every year, plus other content, in Dolby Atmos.

For all the hoopla surrounding immersive audio’s potential, dialogue intelligibility remains high on the list for many consumers. “Even if you take existing content and run it through an AC-4 encoder, we are able to identify the dialogue frequency band and can flag it so that an AC-4 decoder will boost the dialogue a little bit,” he says. He adds that dialogue enhancement is a feature unique to the codec.

It’s all very well being able to broadcast using AC-4, but there must be receivers in the market for consumers to experience the benefits. Currently 18 TV manufacturers support AC-4 in their sets, says Bendull, but they have been slow in also integrating ATSC 3.0 tuners. “Because TV sets are not just used for broadcast services,” he observes.

He adds, “The greater the population of devices that support AC-4 out there in the market, the more likely it is that streaming services will launch in AC-4.”

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One attractive feature of AC-4 is its efficiency, Bendull believes. “When you think about today’s networks, MVPDs and TV stations, they want to reach as many devices as possible, and they want to deliver in the most efficient way. Therefore, by having low data rates, specifically for streaming, they can start, in an adaptive way, at the lowest possible bitrate that delivers quality.” In a multi-device household, for instance, “efficiency becomes something that allows access to services faster without compromising the quality.”

In Europe, Italy’s broadcast standards body has selected AC-4 as a mandatory feature in the next-gen UHD terrestrial specification; it’s the only NGA codec specified, says Bendull. A second European body is expected to announce AC-4 support soon, he reveals.

Dolby is also excited about two of the Big Three smartphone makers supporting AC-4, he says. “Huawei is shipping mobile phones with AC-4 decoding capabilities and Dolby Atmos, and Samsung has announced AC-4 support on their mobile devices in addition to the availability of Dolby Atmos rendering. You can have a Dolby Atmos experience on your headphones or through the built-in speakers on some of their models,” he says.

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“This is what Dolby has done again and again: working with the industry to solve complex problems and roll out ecosystems, from the content creation representing the artists’ intention all the way to delivering the most authentic listening experience to the consumer in the home or on the go,” Bendull says.

Dolby Laboratories •