In Phase vs Out of Phase
A number of techniques are used to create the illusion that certain sounds originate from particular spatial locations. The most obvious technique is using more than two speakers, which literally moves the location a sound originates from. Another important component is the phase relationships between different channels. Because sound reaches different spatial locations at different times, by having channels out of phase with each other, like having a delay between the time a sound is played on the Left, Right, and Center channels and the time it's played on the Rear (Surround) Channels, an important component of space and movement is added. It also avoids the problem of soundwaves projected in opposite directions physically interfering with each other.
LFE
The .1 in 5.1 Surround Sound represents a LFE, or Low Frequency Effects, Channel. Although this has become a popular configuration for many consumers, many Surround mixes don't include a separate Channel for this because they prefer to leave it up to the listener's equipment. Additionally, using six channels doesn't guarantee that one will be dedicated to LFE, as is the case with DVD-Audio where producers aren't restricted to LFE frequencies for the sixth channel. In many cases the mixes are basic 5.0 or 3-2 Surround, indicating three front (Left, Right, Center) Channels and two Surround Channels.
Matrix Surround
Although there are a number of technical differences between any two Surround Sound formats, the most basic distinction is between Matrix and Discrete Surround. Matrix Surround was developed as a way to Encode audio that would play in stereo on legacy audio equipment, but provide additional channels for equipment designed to handle it. There are two basic techniques used to add surround information to a stereo signal. The first is to base it purely on the contents of the Left and Right channels. Rather than adding separate information, the audio from the standard stereo channels is combined to form additional channels. This is used for analog Dolby Surround and Dolby Pro Logic.
Discrete Surround
Discrete Surround, on the other hand, is similar to regular stereo audio in that each channel is separate (discrete), rather than being created by compositing a stereo signal. Because digital formats like Dolby Digital and DTS already require decoders to output the analog sound transmitted to speakers, they also use Discrete Surround. The drawback to Discrete Surround is that it may require substantially higher bitrates due to carrying information for more channels. With newer formats that use high capacity disks, like Blu-ray and HD DVD, this isn't an issue, but due to the storage limitations of DVD media, the Discrete Surround formats used also include Lossy Compression to reduce audio stream sizes.
Surround Configurations
There are two standard Surround configurations, as well as a number that are less well known. The first, which is based on Dolby's Pro Logic, used primarily on LaserDisc players, has Left, Right, Center, and Surround Channels. Although only a single Surround (rear) Channel is used, it's designed for two back speakers. Being a Matrix Surround format, no specific provisions are made for a LFE (Low Frequency Effects) channel. This configuration may be referred to as 3/1 Surround due to the three Channels at the front and one at the back.
3/2 Surround
When the average consumer thinks about Surround Sound, this is probably what they have in mind. Unlike the 3/1 Channel configuration used in Dolby Pro Logic, 3/2 Surround Sound systems have the standard three front speakers (Left, Right, and Center), as well as two Surround Channels (Left Surround and Right Surround). This obviously leads to better spatial effects than a single Surround Channel. Although 3/2 Surround is often called 5.1, this isn't really accurate as the standard is essentially the same whether a LFE channel is included in either the signal or speaker configuration.
Other Configurations
Although five channel configurations are by far the most popular, systems have been designed with six or more unique channels. In some cases, such as Dolby Digital EX, there's complete backwards compatibility with 3/2 configurations. In other cases the new configurations are only suitable if you have content specially encoded for them. Despite some support among receiver manufacturers for 6.1 and 7.1 digital audio, the standard for the analog connections required for high Resolution audio, such as uncompressed LPCM, or even Losslessly Compressed MLP, Dolby DigitalHD, or DTS-HD is still only six channels. As lossless audio has become the standard for both Blu-ray and HD DVD, more than 5.1 Audio isn't likely to become mainstream any time soon. For the average consumer this is a good thing as the extra speakers add more in cost than value to most home theaters.
6.1 Surround
Below is a diagram of a 6.1 Surround system. Notice that the Left and Right Surround Channels have been re-located to the sides of the listeners, while a single Back Surround Channel is behind them. Although it's possible to encode the Back Surround Channel as a Discrete Channel, in most cases it's actually Matrixed from Left Surround Channel and Right Surround Channel of the 3/2 Channel Surround.
7.1 Surround
Like 6.1 Surround, 7.1 Surround uses Left and Right Surround Channels located to the sides of the listener, but two Back Surround Channels are added instead of one. Also like 6.1 Surround, the Back Surround Channels are typically Matrixed rather than Discrete.
Front Speaker Placement
Although there are specific guidelines suggested for various Surround Sound systems, in reality most people are Limited by the accoustics and space / contour limitations of their viewing environment, as well as the technical limits of every piece of electronics involved, including the speakers themselves. As a rule, try to line up the Left, Right, and Center Channel speakers in a single line. This plane should be either at, or just in front of the screen. The Center Channel should be centered either directly above or directly below the screen, pointed directly forward. While the exact facing of the Left and Right Channels is generally suggested to be at a 60 degree angle to one another, in reality it will depend on how far apart you're able to put them. They should be pointed toward the primary viewing area, which may or may not match the recommended angle. For larger viewing areas, or further seating distances from the screen, setting the Left and Right Channel speakers to wider Angles (sometimes facing straight forward) is generally preferable. Always let your own ears be the judge.
Rear Speaker Placement
Even more than the front Channels, placement of Surround Channels varies greatly from one room and set of speakers to another. The ideal configuration would be setting the Left and Right Surround speakers either directly to the left and right, or just behind and to the left and right. In either case, the ideal height is considered to be above ear level for the listener by up to 1 meter (3 feet). Usually the two big variables are the location of walls, availability / desirability of speaker stands, and the quality of the speakers themselves. Once again, your own ears should be the ultimate arbiter of speaker placement.
LFE Placement
A Subwoofer can greatly increase the dynamics of movie sound. Since they're made to reproduce sounds that are theoretically not directional (because of the low frequency of the soundwaves), only a single (.1) Channel is used for the LFE Channel. In reality the LFE Channel isn't even required for a decent receiver to make use of a Subwoofer. In reality, the quality of some Subwoofers, especially those found in so-called "Home Theater In A Box" products, are often not up to the task of filling a room with rumbling bass. Assuming you have a Subwoofer that is up to the task, make sure not to place it too close to walls, or especially in corners. You want to feel it vibrate your body, not hear it rattle your windows.
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Additional Reading
Dolby Pro Logic II (Matrix Surround)
