Surround sound (also known as multichannel sound) incorporates multiple speakers to envelop the listener, providing sound in front, to the sides, and behind. Movie theaters use surround sound to impact large audiences with the feeling of being in the middle of the action. Surround sound is an essential element in creating the home theater experience.
Dolby Surround is the earliest form of surround sound. It is a three-channel process meant to recreate a theater experience. The Dolby stereo track is channeled into the front left and front right speakers. A mono signal is then fed into both rear speakers.
Dolby Pro Logic
Dolby Pro Logic is an advanced version of Dolby Surround, adding a center channel speaker for music and effects. It is a four-channel system that directs the information to certain speakers. The four channels are the set of front speakers, one center channel speaker and one rear speaker.
Dolby Digital (AC-3)
Multichannel format introduced in 1996 presents 6 discrete audio channels. (Also described as 5.1) 5 of the 6 channels carry the entire bandwidth of sound with the 6 the speaker or LFE (Low Frequency Effects) Channel carries all the low-bass sounds. This enables you to maximize your action-adventure sequences with fuller explosions and sound effects. You are still able to hear bass sounds from your left and right front speakers even without a subwoofer, but you won't experience the full impact of audio without a subwoofer.
DTS Digital Surround
DTS Digital surround and Dolby Digital are nearly the same. The basic difference between these two formats is the method of compression or how the large audio data files are manipulated to fit in less space. In theory, this means more overall information available on the soundtrack. These two formats are not compatible, and require their own branded decoding chips on AV receivers and processors, as well as separate digital outputs on DVD players. DTS and Dolby Digital will continue to coexist in the marketplace.
THX is a set of technical specifications in order to standardize the performance of surround sound. George Lucas developed these standards shortly after the film "Star Wars" was created. He did so in order to standardize the audio and video experience in theaters across the world so that his films would be represented as they were made, essential trying to recreate the original. Manufacturers of all theater and audio/video products have been giving a set of performance specs that their products must to meet in order to meet the THX certification. THX is a certification that is can be an indication of how well a product has been built - although it should not be the only indication. Some manufacturers choose not to participate in the program preferring to build products to their own specs.