Dolby DigitalDolby Digital is a digital encoding system for audio signals that can be decoded by a receiver or preamplifier with a Dolby Digital decoder.
Dolby Digital is often referred to as a 5.1 channel surround system. However, it must be noted that term "Dolby Digital" refers to the digital encoding of the audio signal, not how many channels it has. In other words, Dolby Digital can be Monophonic, 2-channel, 4-channel, 5.1 channels, or 6.1 channels. However, in its most common applications, Dolby Digital 5.1 and 6.1 are often referred to as just Dolby Digital.
For details and technical information on Dolby Digital, check out the Official Dolby Digital Page.
Dolby Digital EXDolby Digital EX is based on the technology already developed for Dolby Digital 5.1. This process adds a third surround channel that is placed directly behind the listener.
In other words, the listener has both a front center channel and, with Dolby Digital EX, a rear center channel. If you are losing count, the channels are labeled: Left Front, Center, Right Front, Surround Left, Surround Right, Subwoofer, with a Surround Back Center (6.1) or Surround Back Left and Surround Back Right (which would actually be a single channel - in terms of Dolby Digital EX decoding). This requires another amplifier and a special decoder in the A/V Surround Receiver.
Official Dolby EX Website
Dolby Pro LogicDolby Pro Logic extracts a dedicated Center Channel and Rear Channel from two channel content. The Center Channel more accurately centers the dialog (this necessitates a center channel speaker for full effect) in a movie soundtrack. Also there is a rear channel, but although the rear surround channel employs two speakers, they are still passing a monophonic signal, limiting rear-to-front and side-to-front motion and sound placement cues.
Dolby Pro Logic IIDolby Pro Logic II is a surround sound decoding technology, developed by jointly by Jim Fosgate and Dolby Labs.
Dolby Pro-Logic II technology can create a "simulated" 5.1 channel surround environment from any two channel source (such as stereo CDs and Vinyl Records) as well as from a 4-Channel Dolby Surround signal.
Although not a discrete format, such as Dolby Digital 5.1 or DTS, in which each channel goes though its own encoding/decoding process, Pro Logic II makes an effective use of matrixing to deliver an adequate 5.1 representation of a stereo film or music soundtrack.
Dolby Pro Logic II Website
Dolby Pro Logic IIxAn enhancement to Dolby Pro-Logic II, that includes the addition of two back channels, in addition to Dolby Pro-logic II's 5.1 channels, thus making Dolby Pro-logic IIx a 7.1 channel surround processing system.
Dolby Pro Logic IIz
Dolby Pro Logic IIz is a surround sound processing format that is an enhancement that extends surround sound vertically. Dolby Pro Logic IIz offers the option of adding two more front speakers that are placed above the left and right main speakers. This feature adds a "vertical" or overhead component to the surround sound field (great for rain, helicopter, plane flyover effects). Dolby Prologic IIz can be added to either a 5.1 channel or 7.1 channel setup.
For more details, check out the official Dolby Pro Logic IIz page.
Yamaha offers a similar technology on some of its home theater receivers called Presence.
Dolby TrueHDDolby TrueHD is a high definition digital-based surround sound format that supports up to 8-channels of surround decoding and is bit-for-bit identical to a studio master recording. Dolby TrueHD is one of the several audio formats designed and employed by Blu-ray Disc and HD-DVD technologies. Dolby TrueHD is compatible with the audio portion of the HDMI interface. For more technical details, go to the Official Dolby TrueHD page.
Dolby Virtual SpeakerDolby Labs has developed a way to create a fairly accurate surround experience that gives the illusion that you are listening to a complete surround speaker system, but is utilizing just two speakers and a subwoofer. Dolby Virtual Speaker, when used with standard stereo sources, such as CD, creates a wider sound stage. However, when stereo sources are combined with Dolby Prologic II, or Dolby Digital encoded DVDs are played, Dolby Virtual speaker creates a 5.1 channel sound image using technology that takes into account sound reflection and how humans hear sound in a natural environment, enabling the surround sound signal to be reproduced without needing five or six speakers.
Dolby Virtual Speaker Website
DTS (also referred to as DTS Digital Surround)DTS is a 5.1 channel encoding and decoding surround sound format that is similar to Dolby Digital 5.1, but DTS uses less compression in the encoding process. As a result, many feel that DTS has a better result on the listening end.
In addition, while Dolby Digital is mainly intended for the Movie Soundtrack experience, DTS is also being used in the mixing and reproduction of Musical performances.
To access DTS encoded information on CDs and DVDs, you must have an AV receiver or preamplifier with a built-in DTS decoder, as well as a CD and/or DVD player with DTS-pass through.
For additional information on DTS, go to the DTS Digital Surround Page.