Surround Formats Today's home theater receivers can decode many surround sound formats and offer various kinds of multichannel processing. These are the most common.

Dolby Digital This format should be familiar, since it's used in most movie theaters, on most DVDs, and in HDTV broadcasts. It can deliver up to five separate main channels - left, center, and right front and left and right surround - plus a deep-bass-only ".1" low-frequency effects (LFE) channel.

Dolby Digital EX Extracts an additional back surround channel from the regular left and right surround channels of a Dolby Digital EX soundtrack for an improved sense of sound effects moving from front to back (or vice versa) and to make sounds placed directly behind the listener more realistic. Dolby Digital EX is often called a 6.1-channel format (as opposed to the standard 5.1). The back surround channel may be reproduced by one or two speakers at the back of the room. A system set up for two back surround speakers is usually called "7.1-channel" even when both back surrounds play the same thing.

Dolby Pro Logic II (DPL II) Enhanced successor to the original Dolby Pro Logic system. Decodes soundtracks on VHS tapes, DVDs, and many TV broadcasts that use the older Dolby Surround format, providing front left, center, and right channels plus a single surround channel (normally reproduced by two speakers at the sides of the room). DPL II can also generate 5.1-channel sound from any stereo or Dolby Surround source and can provide separate listening modes for music and movies as well as several parameters than you can adjust yourself (but not all receivers offer all of these options).

Dolby Pro Logic IIx (DPL IIx) An extension of DPL II that generates one or two additional back surround channels to create 6.1- or 7.1-channel surround sound. Works with any stereo, Dolby Surround, or Dolby Digital 5.1 soundtrack.

Dolby Virtual Speaker and Dolby Headphone Can simulate 5.1-channel playback with just two speakers and a subwoofer or multichannel surround sound from a set of headphones.

DSP Digital signal processing surround modes. Most surround receivers provide several DSP modes meant to enhance music listening with artificially generated ambience.

DTS Digital Surround Popular in movie theaters and similar to Dolby Digital 5.1, DTS is found as an alternative to Dolby Digital on some DVDs.

DTS-ES Adds a back surround channel to DTS 5.1 soundtracks, but unlike Dolby Digital EX, DTS-ES includes the back surround as a discrete channel rather than matrixing it into the left and right surround channels.

DTS Neo:6 Similar to DPL II, except that it can also generate a back surround channel for 6.1-channel sound from two- and four-channel sources.

THX Surround EX Decodes Dolby Digital EX 6.1-channel soundtracks with various enhancements, such as "decorrelating" the single back surround channel for 7.1-channel systems so it becomes a pair of distinct channels.

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