Dolby Digital is the trademarked name for Dolby Laboratories' system of discrete, digital 6-channel surround sound used in professional movie theaters and consumer home theater surround sound systems. It is also the soundtrack standard for the DVD format. The most common version of Dolby Digital contains 5.1 channels—Left Front, Center, Right Front, Right Surround, Left Surround, and the “.1” subwoofer or Low Frequency Effects (LFE) channel. The format also supports Mono and Stereo (Dolby Digital 2.0) versions as well. Dolby Digital 5.1 is the soundtrack standard for digital HDTV broadcast, satellite and cable programming.

Dolby Pro Logic II (DPLII) is a digital matrix system that simulates a 5.1-channel output from any 2-channel analog or digital stereo source, including CDs, cassettes, LPs, MP3s, old Dolby Surround videotapes or any other stereo source. The latest “x” version of DPLII (DPLIIx) uses matrix technology to add one or two more surround channels (6.1 or 7.1) at the rear.

Dolby Digital EX (Extension) adds an additional rear center channel to the standard 5.1 channel Dolby Digital format by using matrix technology to extract rear-center channel information. However, the format is not considered a true, discrete 6.1 or 7.1 channel system because it lacks the capability to support a discrete 6th channel like the competing DTS-ES format, sometimes included as an optional soundtrack on DVDs.

Dolby Digital Plus is an enhanced discrete digital surround system developed for future use on High Definition DVDs. It offers increased bitrates (up to 3 Mbit/s), support for more audio channels (up to 13.1), improved coding techniques to reduce compression artifacts , and backward compatibility with existing Dolby Digital 5.1-channel hardware.
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