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7.1 Placement and Acoustics

Aug 12, 2010 12:28 PM, By Dan Daley

The release of 8-channel audio creates an expanded home theater soundscape with more opportunity and possibly more confusion.

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7.1 Format

The home theater format war has a new front, and it’s on the side walls of the theater—or not, depending on which loudspeaker placement philosophy you subscribe to. Surround in 7.1 had been hinted at for years, such as with the SDDS 8 format variant. It arrived confidently earlier this year in a coming-out party at the Consumer Electronics Show that saw virtually every major CE brand offering an 8-channel AV decoder. What made this debut more potent was that it was joined on the video side by the emergence of 3D as a consumer format.

Figure 1: Dolby Labs has released a web feature that will calibrate 7.1 loudspeaker placement and positioning. See larger image.

If James Cameron’s Avatar is putatively regarded as the primary stimulus for 3D, Toy Story 3, which came out in July, will likely be looked back on as the impetus for the synergy of both 3D and 7.1 sound. Toy Story 3 is the first film to be shown in Dolby Surround 7.1, and 240 cinemas have already upgraded to 7.1 capability, according to Dolby, which is offering a free upgrade to those already using digital systems and Dolby’s Surround and EX systems. The major U.S. film studios are already mixing in 7.1, with Disney saying that it will use it on forthcoming releases such as Step Up 3D.
Getting eight discrete channels of sound into theaters requires that cinemas convert to digital, since the old sprocket-borne delivery of theater sound is limited to six discrete channels. However, the timely arrival of Blu-ray gives home theaters a ready-made 8-channel discrete delivery solution. It’s serendipity rare in the consumer entertainment formats business.

As mentioned, 7.1 has some variation as to what can be done with those two extra loudspeakers, and 3D will undergo its own internal competitions to determine how consumers want to use the technology—glasses or not? passive anaglyphic or electronic shutter?—but it’s also clear that the two new formats will feed off each other: They literally add dimension to the home viewing experience.

Figure 2: DTS lists a total of seven loudspeaker configurations for the 7.1 format, including using side channels to create a wider L-C-R array as seen here. See larger image.

Dolby and DTS

Dolby Labs has a helpful web feature at that will calibrate 7.1 loudspeaker placement and positioning (Figure 1). Though the distances vary, the placement relationships between loudspeakers, viewer location, and video screen don’t.

The world of 7.1 is far more complex place as far as DTS is concerned, however. DTS’ site lists a total of seven loudspeaker configurations for the format, including using the side channels to create a wider L-C-R array (Figure 2), using them to create a tight rear-side surround arrangement (Figure 3), and an overhead configuration (Figure 4).

DTS’ more complex configuration possibilities stem from the technology’s ability to let the AV receiver electronically remap the loudspeakers locations using metadata on the disc deciphered by the DTS chip in the receiver to reflect the film sound mixer’s intent, says Tom Dixon, DTS’ vice president of marketing.

Figure 3: DTS’ 7.1 tight rear-side surround arrangement. See larger image.

“7.1 is offering film studios a lot of new flexibility in terms of their sound mixes, and we’re providing a way for listeners to access those mixes the way they were intended to be listened to,” he says. DTS will also put installer calibration material that will check phase coherence, level, and other 7.1 parameters on its next DTS MasterAudio demo disc.

In 2009, the CEA and CEDIA devised their “Home Theater Recommended Practice: Audio Design” white paper (CES/CEDIA-CEB-22). In differentiating between a typical 5.1 layout and a 7.1 configuration, it specifies that the surround loudspeakers be placed within the angular range of ±110 degrees to ±120 degrees. However, in a 7.1-channel system, the side loudspeakers can be placed symmetrically within the angular range, spanning ±60 degrees to about ±100 degrees, and the rear loudspeakers within the angular range spanning approximately ±135 degrees to ±150 degrees.

Figure 4: DTS’ 7.1 overhead configuration. See larger image.

The positioning formula was devised with input from the gurus of home theater sound, including CEDIA’s 2008 Lifetime Achievement Award winner Floyd Toole, Ph.D., and Dolby veteran and consultant Anthony Grimani. It’s based on the ITU-R BS.775-2 specification widely used in film sound mixing stage design, which emphasizes a true radial configuration. Each individual loudspeaker (excluding the LFE) should be optimally placed an equal distance from the listening position so that all phase relationships among all channels are proper. The thinking is that reproducing as closely as possible the environment in which the audio content is created makes the most sense for the design of the playback environment.

The reality, of course, is based on the actual environment of a client’s media room, where the sweet spot is often the couch up against the rear wall of the room. But the intent was to take the partisanship out of multichannel audio systems design, says Dave Pedigo, CEDIA’s senior director of technology. That’s important, he emphasizes, not solely to create predictable listening environments, but also to lay the groundwork for what’s shaping up to be another turning point in home theater standards. “The penetration of 7.1 systems in homes is likely relatively small at this stage,” he says, “but the number of receivers capable of playing back 7.1 is enormous, and CE manufacturers are starting to [differentiate] themselves by putting out products that can do 9.1 and 9.2 speaker configurations. Dolby’s Logic IIz encoding is taking the audio envelope out of just the horizontal axis into a more spherical one by elevating additional speaker locations. At some point, I wouldn’t be surprised to see a voice-of-God speaker placed directly overhead.” With the saturation of 5.1 home-theater-in-a-box products, Pedigo says the market might be ripe for the next round of market-leading, aspirational home theater installations that will drive commoditization of 7.1.

Precise loudspeaker placement will become more critical in the future as Hollywood begins to ramp up discrete 7.1 soundtracks for home video. “Much of the 7.1 content presently available on Blu-ray Disc has been remixed for 7.1 from the original 5.1 mix,” explains Craig Eggers, senior manager of consumer electronics partner marketing at Dolby. “But what systems integrators need to know is that in the future, 7.1 audio content [on home video media] that was released as 7.1 in the cinema will be completely consistent with and true to the mixing artist intent and vision. Dolby 7.1 surround sound enables artists to mix specifically for a 7.1 soundstage from the beginning of their work effort and that mix will be communicated directly to the home theater on a 7.1 Blu-ray Disc.”

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