Dolby AC-4: Better TV Sound For the Internet Age

Coming to a TV near you soon

While traditional broadcast TV is not, in truth, anywhere near as dead as many pundits like to tell you it is, there’s also no doubt that more and more of us are streaming television and movies from the internet. Being able to watch content from services like Amazon Instant Video, Netflix and Hulu whenever and, increasingly, wherever we like is an irresistible lure. There’s a problem, though. Namely that unless you happen to have both a super-fast and super-consistent broadband connection, the quality of the AV experience you get with streamed video is often sub-optimal to say the least, as digital pictures and sound have to be compressed to squeeze them through the available broadband ‘pipe’.

Fortunately, with streamed video likely to make up an ever-bigger chunk of our viewing lives, moves are afoot to make the experience better. I’ll be looking at a system for improving the quality of streamed video soon, but here I want to focus on a new technology that its creator, Dolby, thinks will put the quality back into streamed audio. 

Introducing AC-4

Dubbed AC-4, the still in development technology is essentially a system for improving the way audio is compressed for distribution over digital networks (compression being the chief cause of poor streamed sound quality). It seeks to accomplish this by upping both the efficiency of the compression used and its flexibility - as in, the way it responds to changes in both the content it’s handling and the network conditions it’s working over.

We’re not talking small improvements here, either. Dolby claims that already AC-4’s compression techniques can achieve the same audio quality as existing digital audio distribution systems while requiring barely half as much bandwidth.

Which also means, by extension, that AC-4 has the potential to if not fully double sound quality then at least greatly improve it when working with the same broadband capacity current sound systems use.

4K's friend

The enormous extra compression efficiencies AC-4 is apparently capable of also make it a potentially invaluable technology for the new era of 4K/UHD video.

After all, while few doubt the impact good quality 4K UHD sources seen on a good 4K UHD TV can have on picture quality, the amount of data involved in a 4K stream has made distributing the new picture format a serious headache. So any new technology that can take some of the pressure off the streaming process without compromising audio quality can only be a good thing.

The AC-4 system clearly has the potential to deliver its maximum impact with television sets, where large screens highlight any compression issues with under-strain video streams, and large speakers can highlight audio frailties. However, Dolby is sensibly designing the AC-4 system to work across multiple device types - mobile phones, tablet computers and so on - so that you can benefit from its audio improvements no matter what you’re consuming your media on at any given moment.

Not just for streaming

While I’ve stressed the usefulness of AC-4 for streamed AV playback, it’s important to say that it can also work over other distribution systems - including traditional broadcasts, mobile data networks and the relatively new hybrid system, which divides the content burden across a combination of streamed and broadcast streams.

Dolby’s AC-4 system isn’t just about improved compression. It also supports channel-based and object-based programming, which essentially means it can offer consumers multiple audio stream options - for instance, a choice over which commentary track they listen to while enjoying a sporting event. Or maybe a different commentary language.

It currently additionally looks set to offer some user-selectable features, such as a dynamic range control, an intelligent loudness option, and a circuit for boosting dialogue in the mix. 

As you might expect, AC-4 is an end to end solution, requiring adoption at both the content creation and hardware playback stages of the AV chain.

With this in mind - and to make the system as straightforward and affordable as possible for potential clients to adopt - Dolby has equipped AC-4 with a sophisticated self-configuration system.

Waiting in the wings

At the time of writing Dolby AC-4 has yet to be introduced to any consumer device - be it a TV, smartphone or tablet computer. But such devices are certainly coming, with Vizio, Sony and TP Vision (the brand which now builds Philips TV for European territories) already committed to integrating AC-4 into future TVs. Crucially, too, the AC-4 standard has already been accepted by both the key Digital Video Broadcast (DVB) group, and the European Telecommunications Standards Institute (ETSI).

It’s also already been implemented at the content creation/delivery end of the chain. San Francisco’s KQED-TV broadcaster has completed trials of Dolby’s new audio technology in conjunction with AV delivery specialist Harmonic, and it was also used in the encoding of both the 2015 French Open tennis tournament and the 2015 UEFA Champion’s League final. 

In short, AC-4 is much more than just some ‘out there’ concept technology. It’s already starting to feature in the content creation plans of forward-thinking sectors of the AV industry, and it seems almost certain we’ll be able to get our ears on it ourselves with some TVs and personal smart devices in 2016.