As I mentioned in my AVguide blog from last week, I recently had an opportunity to visit both Audyssey Laboratories as well as Audyssey’s associated academic think-tank, the USC Immersive Audio Laboratory. But a major purpose of the visit was to get an introduction to Audyssey’s newest technology innovation, known as DSX (Dynamic Surround Expansion).
The DSX Concept: Adding Surround Channels Where They’ll Do the Most Good
Audyssey co-founder and USC professor of Film Sound Tomlinson Holman (whose initials contribute the “TH” in THX) gave a basic introduction to and a historical review of commercial surround sound offerings, up through and including Dolby Surround, Dolby Digital and Dolby TrueHD (as well as DTS equivalents). Holman indicated that present 5.1-channel (or 7.1-channel) surround-sound solutions, though good, represent a sonic compromise that can be improved upon, though not necessarily through the means you might normally expected. Contrary to what some listeners might expect, and especially in light of the new high resolution, 24-bit/192kHz-capable codecs from Dolby and DTS, Holman argued that at this stage in the evolution of surround sound, the real frontier does not involve going to higher bit rates or longer digital word lengths but rather involves adding more surround channels and in exactly the right places. This is precisely what DSX aims to do.
Holman explained that Audyssey’s research leading up to DSX involved three areas of study:
Physical Acoustics—to understand what specific factors make great listening spaces (and in particular, great concert halls) sound so spacious and enveloping.
Audyssey’s core conclusion is that two of the most important—but sometimes overlooked—factors involve early reflections from the sidewalls and ceilings of great listening spaces. As you’ll see in a moment, DSX takes into account the importance of sonic information arriving from the sides of the soundstage, and from overhead
Psychoacoustics—to understand how humans process the surround sound cues they hear.
Audyssey’s core conclusion—and one that makes perfect intuitive sense—is that human hearing is optimized for processing sounds arriving from the front and from above the listener, with acuity falling off sharply when sounds arrive from behind the listener.
Specifically, Holman pointed out that, when trying to pinpoint the sources of sounds to the front of them, listeners can detect angular differences as small as 1 degree in the horizontal axis and 3 degrees in the vertical axis, but with precision falling off dramatically as sound sources sweep to the sides of, and then to the rear of, the listener. Interesting, this implies that most current 7.1 channel systems, with three front channels and four surround channels, essentially have their priorities wrong (because if anything the additional channels are needed up front—not to the rear).
Accordingly, DSX proposes adding up to four more channels placed to the front of the listeners: two “Wide” channels placed about 60 degrees to either side of the centerline, and two “Height” channels placed about 45 degrees to either side of the centerline and at a 45 degree up-angle.
Holman said the greatest benefits come from adding DSX Wide channels first, and then DSX Height channels. Surround information can be well handled by two Side Surround channels, though there is some extra benefit that comes with adding one (or even two) Center Surround channels directly behind the listeners.
Film Sound/Sound Design
—studying the types of surround effects sound designers would like to create, and understanding the techniques they are likely to use to achieve their goals.
Holman pointed out that, at present, the majority of soundtracks are originally mastered in 5.1 channels (including even those that are billeded as “7.1-channel” soundtracks, where—Holman observed—the sixth and seventh channels are often added after the 5.1 master has been created).