Aug 3, 2009 12:00 PM, By Scott Wilkinson
Enhancing multichannel audio immersion.
When recorded sound expanded from monaural to stereo in 1957, it was a revelation. Forty years later, DVD entered the scene with 5.1-channel surround soundfive main channels and a low-frequency subwooferand the sonic landscape expanded again. Today, many consumer AV receivers can artificially derive two extra channels from a 5.1 source for so-called 7.1, though no one mixes content with seven discrete main channels.
A 5.1 system places three loudspeakers in front of the listenera center loudspeaker directly in front and left and right loudspeakers at ±30 degreesand two loudspeakers to the sides and just behind the listener at ±100 to 120 degrees from the center (see Figure 1). A 7.1 system adds two more loudspeakers farther behind the listener, though the angles are not well defined.
Unfortunately, five main channels are insufficient to create a truly immersive soundstage. For example, when panning a sound from the left-front channel to the left-surround channel, the apparent source of the sound becomes quite indistinct between the two loudspeakers. Adding two more back-surround channels does little to improve the sense of immersion because our awareness of sonic directionality is far less acute behind us than it is in front. Thus, to increase the immersive quality of a multichannel audio system, it's much more effective to add channels in front of listeners rather than behind them.
Tomlinson Holman (the "TH" in THX) is a longtime advocate of this approach. His research at the University of Southern California's Immersive Audio Lab (USC IAL) clearly indicates that adding two more front loudspeakers at ±60 degreescalled the left- and right-wide channelssignificantly enhances the sense of immersion, allowing pans along the sides to remain much more cohesive than they are with five channels.
He has also found that adding two more loudspeakers at ±45 degrees and raised 45 degrees above the plane of the other loudspeakersthe left- and right-height channelsadds a sense of height that further enhances the immersive experience. Finally, putting a single back-surround loudspeaker directly behind the listener is all that's needed to complete a truly immersive 10.1-channel surround system.
Unfortunately, no 10.1-channel commercial content exists, nor is it likely to exist anytime soon. So a company called Audyssey Laboratories, founded by Holman and others from the USC IAL, has come up with an ingenious workaround. Dubbed DSX (Dynamic Surround eXpansion), this DSP algorithm accepts a 5.1-channel signal and derives the left- and right-wide, height, and back-surround channels.
The system is completely scalable so you don't need to install 10 or 11 loudspeakers to enjoy an enhanced surround experience. For example, the first products with DSX are 7.1-channel AV receivers that let you designate the back-surround channels as left- and right-wide or height channels instead.
I recently heard a demo of DSX at Audyssey's facilities in Los Angeles, and I was very impressed. One clip was a scene from WALL•E, the Pixar animated feature about a waste-reclamation robot on a deserted Earth. In the scene, the little robot rolls across the screen from left to right, then ascends a ramp, and the sound panned to the right-height loudspeaker. I asked how the system knew that's where the sound should go and was informed that they could tell me, but then they'd have to kill me.
However it's done, the sense of immersion was significantly enhanced as compared with conventional 5.1. DSX represents the next step in the evolution of surround sound and provides an effective bridge to the day when multichannel content will be delivered with 10.1 discrete channels.
Scott Wilkinson is a contributing editor for EM, in which this article first appeared.
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