Dolby is a deep company that owns many technologies. Keeping the terminology straight is challenging. Here is my version:

Dolby Surround (LCRS-->Lt/Rt-->files/tape/broadcast-->LCRS) 4:2:4 matrix process only. The mono surround channel is bandwidth limited between 100hz and 7khz. The term was originally used to mean , but now the term is used for non-film delivery formats. See

Dolby Stereo (LCRS-->LtRt-->Dolby A encode-->OpticalTrack-->Dolby A decode-- >LCRS) 4:2:4 matrix described above with Dolby A type noise reduction. Dolby A is no longer favored, instead use Dolby SR.

Dolby SR (LCRS-->LtRt-->Dolby SR encode-->OpticalTrack-->Dolby SR decode-- >LCRS) 4:2:4 matrix described above with Dolby SR type noise reduction. This is the same as Dolby Stereo except that the optical track is encoded with SR instead of A type noise reduction. This format is still used for surround audio from film optical and as the backup track for . The terms Dolby SR and Dolby Stereo are often interchanged, incorrectly.

Dolby Digital a.k.a. AC-3 5.1-->AC-3 data stream-->5.1 The compressed data stream supports up to 5 full bandwidth plus one low frequency effects channel. It is designed for film, broadcast, and DVD final audio delivery. It is NOT considered "editable", which means you cannot just "punch in" on the data stream without consequences. This is the digital audio part of Dolby SR-D and Digitial Cinema.

Dolby SR-D is a dual-stream audio delivery system printed on the film, with a Dolby SR optical track and blocks of bits printed between sprocket holes that, when reconstructed, form the AC-3 digital audio stream. Since both technologies (Dolby Digital and Dolby SR)are present on the same piece of film, the print will play in any theater, whether or not equiped with Dolby Digital equipment.

Dolby Digital EX is like Dolby Digital with the addition of a center surround channel. The AC-3 stream is identical to regular Dolby Digital, and backward compatible, except that the two surround channels are matrix encoded in a way very similar to the original Dolby Surround, to provide three rear channels instead of only two.

Dolby Pro Logic (LCRS-->Lt/Rt-->files/tape/broadcast-->LCRS) The same as Dolby Surround. The matrix process used with Dolby Pro Logic and Dolby SR are identical.

Dolby Pro Logic II LCRSS-->Lt/Rt-->LCRSS Note there is no LFE channel, and the surround channels are full bandwidth. Dolby claims a PLII Lt/Rt is fully backward compatible with Dolby Pro Logic. Because there are significant differences in the Pro Logic and Pro Logic II processes, most apparent in the surround channels, an LtRt program will not sound the same on both decoders! Results should be checked on both systems during mixing for best compatibility. Pro Logic II was developed primarily as an enhancement for MUSIC listening, not dramatic voice-heavy content. This distinction is not apparent to the general public.

Dolby E A compressed digital delivery method that supports up to 8 full bandwidth channels of audio. Unlike AC-3, it was developed for use with broadcast and video delivery systems and during post production interchange, and is considered "editible".

Lt/Rt A surround recording processed into a two channel recording. It has the quality and essence of a regular stereo recording and is created when a surround program’s various channels (left, center, right, surrounds) are encoded and combined through one of several special processes (patented by Dolby and sometimes called “matrixing”) that results in a stereo recording. When properly decoded, the surround signals are extracted and played through their original speakers (left, center, right, surrounds). The term applies to several Dolby technologies including Dolby Surround, Dolby Pro Logic, and Dolby Pro Logic II.

Discrete or discrete stereo: stereo that is NOT an Lt/Rt

Now go to http://www.dolby.com/ and start reading!