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When you walk into a movie theater, you usually see 12 to 16 speakers hanging on the walls. In addition, there are typically four systems behind the screen and hidden from view. What’s up with that? How do we get today’s 5.1 channels distributed to them and what’s changing?
The movies pioneered multichannel sound. Fantasia in 1940 had five loudspeaker channels[i], and that was the first introduction of surround sound to the public. But even in 1940, the engineers found that if they played the two surround channels out of just two loudspeakers, those sitting nearby would get creamed with level, while those sitting closer to the front hardly heard them in a mix. So the idea of an array of loudspeakers to play the surrounds has old roots. In fact, the two theaters equipped for Fantasound used many loudspeakers to reproduce the two surround channels.
So the idea that the number of loudspeakers you see represents the number of channels is wrong; many may be driven by one input signal. The most common configuration today is 5.1, where there are three main channels, left, center, and right, behind the screen, along with one or more subwoofers. So that’s 3.1 of the channels counted. The other two channels drive the left and right halves of the array of loudspeakers in the room, with the back wall speakers split between left and right.
Several years ago, pressure to get sound literally to come from behind you led sound designer (and multiple Oscar®[ii] winner) Gary Rydstrom to ask for a separate rear channel[iii]. The problem for cinema is that if you sit in the middle of the house and sound pans from the fronts to the surrounds, it only seems to come from your sides, because those are closer to you than those on the back wall. So it’s not really surround. By having a separate back surround, the effect can be overcome. Commercial implementations of this are Dolby Surround EX and dts Surround ES in theaters. These might be called “quasi 6.1” systems, because there is not perfect separation of back from sides, but usually good enough for the effect to work.[iv] [v]
Which leads us to Summer 2010 and Toy Story III. In digital cinemas this will be offered in a “7.1” channel format, because it’s easy to drive the left side, left back, right back and right side separately. Added to three screen channels and one subwoofer channel that makes 7.1.
There have been previous 7.1-channel systems. In fact, Cinerama used 7 channels, five across the front and two surround channels. The two recorded surround channels could be switched by the “theater engineer” between left/right or front/back surround arrays, thus anticipating many of the results today. And that was 1952!
The Todd AO format had five channels behind the screen and a surround channel on 70mm film, and this was the system—once surrounds were split and sub were added—that was started for Superman I in 1978.[vi] It was the system that I called “5.1” in a meeting in 1987 and the name stuck.
“7.1” with surround will be with us this summer. However, it is the result of a gradual crossfade to more channels and, frankly, hasn’t been completely thought out. Having more surround than screen channels, for instance, is a little weird, since our perception of localization is better in front than back. But more channels are always merrier and welcome to the fold.
We have been working for some years on a 10.2 system, which is seeing its first introduction to the marketplace through our upmixing system called Audyssey DSX. So long as the infrastructure of distribution is based on 5.1- and 7.1-channel systems, going beyond those will require upmixing. But like the quasi-6.1-channel systems, these will find a place in the pantheon that is film sound.
The distribution 1, 2, 5, 10 represents equal steps along a logarithmic scale, so may be expected to result in steady improvements: mono, stereo, 5.1 channel, and 10.2 channel sound.
[i] Although Fantasound actually recorded only three channels. The five were derived by steering those three out to the five channels L, C, R, LS, and RS.
[ii] Oscar is a registered trademark of A.M.P.A.S.
[iii] Through the use of matrix technology (like Dolby Pro Logic) applied to the two surround channels, a center back was derived from the LS and RS channels. This was important psychoacoustically due to something called the precedence effect. In this effect of hearing, also known as the Law of the First Wavefront, first arriving sound will cause localization at the source of the first wave.
[iv] By the way, sharp readers will note that using a Pro Logic style decoder on two surround channels results in four outputs, not just the three used for left, right, and back surrounds. And indeed they’d be right. One picture tried using the fourth surround channel as a center overhead one. It was the Vietnam war picture We Were Soldiers. However, it played in the format in only a very few theaters due to the difficulty of putting loudspeakers on the ceiling (of what are, after all, only lay-in acoustical tile ceilings!).
[v] In order for these 6.1 systems to work, the back surround loudspeakers were divided in two halves, left and right. For standard 5.1, the left back is allied with the left surround, and vice versa. For 6.1, the left and right backs are wired together, and driven separately from left and right surround.
[vi] In the shameless plug department, these items and more are covered in the first chapter of my book Surround Sound Up and Running, 2nd. Ed. Focal Press.