The Basics
Here's an explanation of the basic surround encoding systems in general use today.
Dolby Surround: This is a "matrix" encoding scheme, which means that it folds four channels (front left, front right, center, and surround) down into two channels. With the proper decoder, it can be reproduced as four-channel surround sound; without decoding, it will play as ordinary stereo. As a result, Dolby Surround is extremely versatile, since it can be applied to any analog or digital audio transmission or storage medium (such as a CD) or to any device that supports ordinary two-channel stereo. The downside is that the four original channels can never be extracted perfectly, although the latest decoding techniques (Dolby Pro Logic II, for example) come amazingly close.
Dolby Digital: This is technically a digital data-reduction, or compression, system. Although similar in concept to MP3 encoding, Dolby Digital is more efficient and can carry up to six channels of high-quality audio. The most common formats are 5.1-channel (five full-range channels--front left, front right, center, left surround, and right surround--plus a bass-only channel for low-frequency effects, or LFE) and 2.0-channel, which is simply good old stereo. (And like any other stereo soundtrack, Dolby Digital 2.0 may be Dolby Surround encoded.) Dolby Digital is the audio format used in HDTV broadcasts and the standard format for DVDs. To decode it using your A/V receiver, you will have to make an optical or coaxial digital connection from the source.
Dolby Digital EX: This format, an extension of Dolby Digital, matrix-encodes a back surround channel into the regular left and right surround channels; the additional channel can be extracted on playback to create 6.1-channel surround.
DTS: Short for Digital Theater Systems, DTS is like Dolby Digital but less efficient. DTS is an optional soundtrack format on some DVDs. As with Dolby Digital, a digital connection is required for decoding in your A/V receiver.
DTS-ES: This is the DTS equivalent of Dolby Digital EX, though it allows inclusion of a fully discrete (as opposed to matrix-encoded) back surround channel.
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