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What is Surround Sound?

By definition, surround sound is sound that surrounds you—in particular, music or movie soundtracks that come at you from all sides, thanks to multiple audio channels fed through speakers positioned in all corners of the room. In a home theater system, the surround sound is initially encoded onto a programming source—a DVD, a cable or satellite broadcast, and so on. When the programming is played, the surround sound tracks are extracted from the source by a piece of electronics called a surround decoder. The individual surround channels are then amplified and fed to the appropriate speakers. The result is sound that envelops you as you watch your movie or listen to your CD.


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How do all those channels of audio get from their source to your ears? It's all a matter of technology. Two primary technologies are used for surround sound today: the older matrix surround, typified by Dolby Pro Logic, and the newer discrete surround, as used in the Dolby Digital system. We'll look at both, starting with the oldest first.

Matrix Surround
The original home surround technology tried to pull off the impossible—turning two things into four. That's because, at the time (we're talking the early 1990s here—a virtual lifetime ago), the videotapes and laserdiscs we were watching barely had space to carry two stereophonic channels, let alone the four channels necessary for a complete surround experience. Engineers could fit right and left channels onto the tape or the disc, but no more than that. So where to put the extra channels?

To stuff four channels of information into the space normally used by two, the engineers at Dolby Laboratories used a matrixing technology. This enabled them to combine four streams of information into two tracks, and then retrieve the original four channels on playback—sometimes referred to as a 4-2-4 processing system. (They took four channels, crammed them into two channels, and then separated them into two channels again: 4-2-4.)

Does that sound a little tricky? It was. Imagine mixing streams of red, green, blue, and purple sand together in a bucket, carrying the bucket across a room, and then trying to extract the individual colors at the other end. Messy at best, and perhaps even impossible to separate into the original colors.

The Dolby engineers figured out how to do it, however. By analyzing the audio information contained within the left and right channels, they could identify the information that was the same in both channels. They realized that they could mix information from a third channel into both the left and right channels, compare the two channels, and then extract that information (the third channel) that was identical in the two channels. They could even get a fourth channel into the mix, by recording it out of phase with the third channel and then also feeding it to the left and right channels.

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