Surround standards can take more than one form. Some are codecs, or encode/decode processes - that means they are encoded into software and decoded by hardware. Dolby Digital, Dolby Digital Plus, and Dolby TrueHD are codecs.

Codecs can be "lossy" or "lossless". Lossy codecs (like Dolby Digital) omit some data, using sophisticated psychoacoustic principles, to provide greater efficiency. Lossless codecs (like Dolby TrueHD) omit no data and are considered the new high-end choice.

Not all surround standards are straight codecs. Some (like Dolby Pro Logic II,) are listening modes. Therefore they can work with stereo signals that may or may not have surround encoded in them. Others (like Dolby Pro Logic IIx or IIz) can also start with surround signals and add additional back-surround or height channels.

Dolby Digital: Used in Blu-ray, DVD, digital cable, satellite video, video game consoles, and over-the-air DTV broadcasting, this is a 5.1-channel lossy codec. It records 5.1 discrete (wholly separate) channels: front left/center/right, surround left/right, and subwoofer.

Dolby Digital EX: Used in Blu-ray and DVD, this is a 6.1-channel codec that adds a single back-surround channel to the standard 5.1-channel array. It uses a matrixing process, deriving the back-surround channel from the side-surround channels. Though mono, the back-surround channel is typically played by a pair of speakers, and thus would be compatible with a 7.1 speaker package.

Dolby Digital Plus: Used in Blu-ray and European over-the-air DTV broadcasting, this codec is an updated version of Dolby Digital. At its best, by using a higher data rate and more efficient compression, it pr0vides a finer-grained sound than traditional Dolby Digital. At the lower data rates used in broadcasting, it can provide DVD-quality audio. The signal can be decoded by a Blu-ray player or by an HDMI-connected surround receiver. When decoded by the player, it can be passed to the receiver as a high-resolution PCM signal via HDMI 1.1 or as a high-resolution analog signal via the multi-channel analog interface. When decoded by the receiver, the player must pass the bitstream to the receiver via HDMI 1.3 or higher.

Dolby Digital Live: Converts any audio signal from PC or game console to Dolby Digital for compatible playback through a home theater system.

Dolby TrueHD: Used in Blu-ray, this is Dolby's state-of-the-art lossless codec. By omitting no data, it provides a bit-for-bit replica of the studio master soundtrack. The signal can be decoded by a Blu-ray player or by an HDMI-connected surround receiver. When decoded by the player, it can be passed to the receiver as a high-resolution PCM signal via HDMI 1.1 or as a high-resolution analog signal via the multi-channel analog interface. When decoded by the receiver, the player must pass the bitstream to the receiver via HDMI 1.3 or higher.

Dolby Surround: Used primarily in VHS, laserdisc, and now-discontinued analog TV broadcasting, this is the oldest of Dolby's home surround standards. This analog codec uses a technique called "matrixing" to encode surround sound into stereo sources such as videotapes, laserdiscs, and DVDs—but it is decoded by any of the Pro Logic modes (see below). It serves the two rear speakers with only a mono signal.

Dolby Pro Logic: Decodes a two-channel Dolby Surround signal into a 5.1-channel array, reversing the matrixing process to add the center, surround, and subwoofer channels. As the oldest member of the Pro Logic family, it has a few shortcomings, especially in the rear channels, where it preserves the mono surround signal and truncates frequency response. Today Pro Logic is provided in an "emulation" mode but rarely used, having been superseded by the superior Pro Logic II and IIx.

Dolby Pro Logic II: An improved form of Pro Logic. It operates on two-channel signals with or without Dolby Surround encoding, and can therefore be helpful with CDs, LPs, and other stereo sources. It decodes the mono rear signal into two independent channels, provides user adjustment of the side-to-side and front/rear balances, and does so with simpler processing that provides full frequency response in all channels and a more stable soundfield. DPLII has four user modes: movie, music, games, and the rarely used old-style Pro Logic emulation.

Dolby Pro Logic IIx: The 7.1-channel version of Dolby Pro Logic II. Expands either stereo or 5.1-channel signals to 7.1 channels, including front left/center/right, side-surround left/right, back-surround left/right, and sub.

Dolby Pro Logic IIz: Builds on Dolby Pro Logic II and IIx by adding a pair of "height" channels above the front left and right, totaling 7.1 channels without back-surrounds, or 9.1 channels with back-surrounds.

Dolby Volume: Provides consistent volume levels across all sources and content (based on consumer preference and settings). Also restores perception of the original audio mix at low listening levels.

Dolby Virtual Speaker: Used in compact systems, DTVs, and PCs, simulates a 5.1-speaker listening environment with as few as two speakers.

Dolby Headphone: Intended for devices with headphone outputs, provides a 5.1- or 7.1-channel listening experience with two-channel or 5.1-channel sources. Compatible with any decoded 5.1-channel audio codec. Used in conjunction with Dolby Pro Logic II for two-channel sources decoded to 5.1.

Dolby Mobile: Used in cell phones, personal media players, and portable audio devices, this suite of technologies enables provides surround listening via headphones. It offers better bass, restores high-frequency elements lost during compression, and manipulates the soundstage for more subjectively perceived realism from downloaded music, movie, and video content.

Dolby Axon: Adds 360-degree surround effects to online voice communications in gaming and virtual worlds.

Other Standards
Here are a few other companies and technologies floating around in the surround sound world.

Audyssey: This company licenses an automatic setup and room correction systems incorporated into more than one major brand of surround receiver. Most receiver manufacturers create their own auto-setup circuits—this is one of the rare licensed exceptions. These systems typically co-exist with Dolby and DTS surround standards. However, Audyssey has also come up with technologies that compete with the surround standard setters, such as Audyssey Dynamic EQ, which competes with THX Loudness Plus and Dolby Volume.

Circle Surround: Developed by SRS Labs, Circle includes both movie and music modes. Like Dolby Pro Logic II/IIx and DTS Neo:6, it operates on Dolby Surround and stereo signals, but produces a different and "wetter" sound with more reverb.

Logic 7: Developed by Lexicon, a division of Harman International, and used in Lexicon products. Though it is technically an encode/decode process, in practice there is little Logic 7 encoded software, so the process operates mainly on Dolby Surround and stereo signals.