Movie Surround Formats
There are three types of surround sound formats that can be enjoyed at home:



Dolby Pro Logic. An analog encoding/decoding system, used in most of the movies you buy or rent in the VHS format. A Pro Logic receiver processor converts two tracks of sound into four channels. It detects the presence of data that is different in the two tracks and sends the two separate sets of imformation to the left and right front channels as regular stereo sound (first and second channel). All the data that is identical in both tracks and in phase is sent to the center speaker (third channel). The data out of phase is sent to the rear speakers (fourth channel).

Dolby Digital. (DD or AC-3) A 5.1 system, with 5 full-bandwidth audio channels (left, center, right) front (left, right) rear and a low frequency channel (subwoofer) As you can imagine, a system of this nature can only be appreciated trough exceptional speakers.   Dolby Digital offers many benefits beyond the standart Dolby Pro Logic system: It`s a true digital soundtrack system. So you enjoy the dynamics of high quality digital audio. All five channels cover the full 20Hz to 20kHz range, with the "0.1" low-frequency effects channel operating at 20Hz to 120Hz. There is a dramatic improvement in overall sound quality, especially for the rear channels. It is very important that the three front speakers match each other tonally. Dolby Digital represents the future of home theater playback.

Upcoming  Dolby Digital Movies

DTS  Similar to DD but with a diferent compression rate. DD uses 11:1
and dts uses 3:1. Most of the receivers today have both.

THX  A set of standards by which home theater equipment is made. Similar to the "Good Housekepping seal of approve".