By Jonathan Abrams
Dolby, founded over 50 years ago as an audio company, is elevating the experience of watching movies and TV content through new technologies in audio and video, the latter of which is a relatively new area for the company’s offerings. This is being done with Dolby AC-4 and Dolby Atmos for audio, and Dolby Vision for video. In this post, the focus will be on Dolby’s audio technologies.
Why would Dolby create AC-4? Dolby AC-3 is over 20 years old, and as a function of its age, it does not do new things well. What are those new things and how will Dolby AC-4 elevate your audio experience?
First, let’s define some acronyms, as they are part of the past and present of Dolby audio in broadcasting. OTA stands for Over The Air, as in what you can receive with an antenna. ATSC stands for Advanced Television Systems Committee, an organization based in the US that standardized HDTV (ATSC 1.0) in the US 20 years ago and is working to standardize Ultra HDTV broadcasts as ATSC 3.0. Ultra HD is referred to as UHD.
Now, some math. Dolby AC-3, which is used with ATSC 1.0, uses up to 384 kbps for 5.1 audio. Dolby AC-4 needs only 128 kbps for 5.1 audio. That increased coding efficiency, along with a maximum bit rate of 640 kbps, leaves 512 kbps to work with. What can be done with that extra 512 kbps?
If you are watching sporting events, Dolby AC-4 allows broadcasters to provide you with the option to select which audio stream you are listening to. You can choose which team’s audio broadcast to listen to, listen to another language, hear what is happening on the field of play, or listen to the audio description of what is happening. This could be applicable to other types of broadcasts, though the demos I have heard, including one at this year’s NAB Show, have all been for sporting events.
Dolby AC-4 allows the viewer to select from three types of dialog enhancement: none, low and high. The dialog enhancement processing is done at the encoder, where it runs a sophisticated dialog identification algorithm and then creates a parametric description that is included as metadata in the Dolby AC-4 bit stream.
What if I told you that after implementing what I described above in a Dolby AC-4 bit stream that there were still bits available for other audio content? It is true, and Dolby AC-4 is what allows Dolby Atmos, a next-generation, rich, and complex object audio system, to be inside ATSC 3.0 audio streams in the US, At my NAB demo, I heard a clip of Mad Max: Fury Road, which was mixed in Dolby Atmos, from a Yamaha sound bar. I perceived elements of the mix coming from places other than the screen, even though the sound bar was where all of the sound waves originated from. Whatever is being done with psychoacoustics to make the experience of surround sound from a sound bar possible is convincing.
The advancements in both the coding and presentation of audio have applications beyond broadcasting. The next challenge that Dolby is taking on is mobile. Dolby’s audio codecs are being licensed to mobile applications, which allows them to be pushed out via apps, which in turn removes the dependency from the mobile device’s OS. I heard a Dolby Atmos clip from a Samsung mobile device. While the device had to be centered in front of me to perceive surround sound, I did perceive it.
Years of R&D at Dolby have yielded efficiencies in coding and new ways of presenting audio that will elevate your experience. From home theater, to mobile, and once broadcasters adopt ATSC 3.0, Ultra HDTV.
Check out my coverage of Dolby’s Dolby Vision offerings at NAB as well.
Jonathan S. Abrams is the Chief Technical Engineer at Nutmeg, a creative marketing, production and post resource.