Mix Magazine held an impressive event on September 6 titled “Immersive Sound: From Production to Playback” on the Sony studio lot in Culver City, California. Top sound mixers with impressive movie credits shared their experiences when working with the two major immersive sound systems on the market. They also provided impressive insight into how immersive sound helps movie directors in telling stories. The information should lead to better-informed buying decisions by cinema owners.
Apart from the core distribution technology, there are qualitative differences between Atmos and Auro that deserve attention. The term most frequently applied by mixers is “resolution.” Atmos is both channel and object-based, individually amplifing each speaker in dense surround and overhead arrays which provide impressive resolution for sound position, at least on paper. In practice, our ears face the screen, making it difficult to precisely localize sounds from above and behind, raising doubt about the value of high front-to-back resolution. Even when localization in the surround field is available, it appears to have limited creative value, prompting sound mixer and author Larry Blake to state in this month’s issue of Mix Magazine “What object-based mixing does uniquely—move sounds either down the length of the theater or to specific locations away from the screen—is precisely what I don’t like.” Auro 11.1, in contrast to Atmos, is purely channel-based, and focuses less on surround resolution, and more on surround dispersion through multiple height layers of surround speakers, rather than high front-to-back resolution. However, when pressed on these distinctions, sound mixers state that it can be useful to place sound in a particular location in the room for certain effects – a task that requires sound objects, versus sound channels, and high front-to-back resolution. The occasional but creative use of sound objects and the preference for sound channels is documented by Dolby’s Scott Robinson, who states in the August issue of the SMPTE Newsletter “We are finding that mixers continue to specify much of the mix using audio channels. In 12 out of 12 soundtracks that we analyzed, more sound energy was carried by channels than audio objects.”
More striking differences lie behind the screen. Atmos sticks to the tried and true three-speaker Left/Center/Right arrangement, also including support for Left Extra/Right Extra speakers made popular with Cinerama and much later SDDS. Auro 11.1 departs from the norm by placing six speakers behind the screen, arranged as an upper layer of Left/Center/Right, and a lower layer of three channels. Mixers using the Auro layout report that the big benefit has been in dialog clarity. Directing music and effects to the upper row of speakers, and dialog to the lower row, limits intermodulation distortion and provides greater clarity of dialog during loud passages – a result one might not intuitively expect.
The killer feature of immersive sound, however, is not the localization of sound objects, but the innovation of full-range surrounds, the new “black” of surround sound. Full-range surrounds depart from the conventional “THX” style surround speaker, noted for its limited bandwidth. With full-range surrounds, directors are able to creatively pull sound away from the screen and into the audience, without changing the timbre of the sound. This was by far the most valued feature discussed by sound mixers at the Mix Magazine conference. Full-range surrounds are encouraged by all immersive sound systems, but most frequently associated with Dolby Atmos. Dolby is well-known for its end-to-end management of quality, from mixing room to installation, and diligently applies this ethic to its Atmos installations. Unfortunately, Auro has licensed quality management of its installations to Barco, which has shown little interest in the task. Further, full-range surrounds require bass management, where low frequencies are shuffled-off to a subwoofer system, as it is not practical to invest in a hanging array of truly full-range speakers on cinema walls. Dolby’s CP850 Atmos processor includes bass management of surrounds, while Barco’s Auro 11.1 processor does not.
There is another feature of immersive sound that was not discussed at length at the conference: that of vertical resolution. Mixers placed value on the vertical capability of immersive sound, but more discussion is needed to understand where resolution is useful. For example, Atmos places high value on front-to-back resolution, and less on height resolution. Where Auro places more value on height resolution than on front-to-back resolution. Unfortunately, no one system does it all, which makes comparisons spotty at best. If Mix Magazine were to conduct a follow-up conference, this is an area I would like to see explored.
My conclusion from the conference was quite simple: both full-range surrounds and high-resolution surrounds are important features of immersive sound, but if ranking priorities, full-range ranks higher than high-resolution. A full-range 7.1 surround system, for example, would be viewed as a significant improvement by many mixers. The “immersive” nature of a sound system, where vertical resolution is added to the conventional lateral array of surround speakers, is effective, but it’s not clear as to the degree of resolution needed by the creative community. This is significant as a major cost to exhibitors is the structural re-engineering required to install the dense Atmos ceiling array. How can an exhibitor value such financial investments if the creative value isn’t clear?
In a private presentation on this subject given to major Russian exhibitors at KinoExpo this month, I was asked for my recommendation of immersive sound systems. My response was simple: I would wait. Better and more cost-effective systems are bound to emerge.