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What is Surround Sound?

Movie theaters began upgrading their sound systems years ago to make the audio experience more dramatic by surrounding the audience with sound. Dolby Laboratories developed a system to encode multiple tracks of sound onto movie films. Later home audio equipment was developed to allow this same improved audio experience for the home theater.

Surround sound simply routes sound to individual speakers, which when placed around the room, make the sound seem to come from the proper position relative to the film. As a plane flies from on screen, over your head and into the distance behind you, the sound follows the movement of the plane from the front speakers to the rear speakers.

What is the Difference Between the Formats?

Dolby Surround is the original multichannel analog film sound format used in thousands of commercially available videocassettes, laser discs, DVDs, and television programs. When a Dolby Surround soundtrack is produced, four channels of audio information are matrix-encoded onto two audio tracks. The two encoded tracks are then carried on stereo program sources such as videotapes and TV broadcasts into the home. When played back, the signal is decoded by Dolby Pro Logic® to recreate the original four channel surround sound experience. The four channels provide left, center, right, and mono surround (which can be run through two surround speakers). Without a Pro Logic decoder, the encoded program plays in stereo.

Dolby Pro Logic® is a matrix decoder built into home theater receivers that decodes the four channels of Dolby Surround sound that have been encoded onto the stereo soundtracks of material such as VHS movies and TV shows. It delivers four channels of sound: left, center, right and mono surround to two rear surround speakers.

Dolby Pro Logic II replaced the earlier version of Pro Logic. It added the ability to derive five-channel surround (left, center, right, left surround, and right surround) from any stereo program, whether or not it had been encoded with Dolby Surround. Encoded material yields results more like Dolby Digital 5.1, including two discrete surround channels rather than a single mono channel and an LFE (low frequency effects) channel for the subwoofer.

Dolby Pro Logic IIx builds on Dolby Pro Logic II technology. It processes native stereo and 5.1 channel material to produce 6.1 or 7.1 output channels. This differs from Dolby Digital EX in that rather than decoding a 6.1 or 7.1 signal, it synthesizes the additional channels from non-encoded material. The extra one or two channels add rear surround to the left and right surround channels.

Dolby Digital 5.1
takes the encoding digital for DVDs, digital cable and digital television (DTV). Each of the 5.1 channels is discrete, meaning each channel is encoded, transmitted and played back without ever being matrixed as is done in Dolby Surround. This discrete handling results in improved range and channel separation and so yields greater fidelity. Dolby Digital is backwards compatible and will decode older Dolby formats.

Dolby Digital EX adds a center surround channel to DD 5.1 which can be played through one or two speakers. This differs from ProLogic IIx in that DD EX uses an encoded channel while PL IIx has to synthesize a channel from the other channels.

Newer Dolby Digital Surround EX soundtracks contain a digital flag that can automatically activate the EX decoding in a home theater receiver. For titles released prior to late 2001, you must turn on the EX decoding manually.

THX Surround EX is not a technology per se, but a set of standards for playback. Lucasfilm established standards for movie theaters to provide the best audio experience to accompany a film. Those standards are now applied to audio equipment for the home theater. With the proper THX certified equipment, a program will play back with 7.1 channels; either discrete or synthesized depending upon the program encoding. THX delivers the industry standard for full wrap-around surround sound.


DTS: What is it?

DTS stands for Digital Theater Systems and is a competitor with Dolby Laboratories. They both do the same thing, encode and decode signals to provide surround sound. One key difference is that DTS uses less compression than Dolby and, some would argue, achieves improved audio quality. A drawback of the lower compression rates is that it leaves less room on a DVD for “bonus material” such as commentaries and foreign languages.

DTS competes head-to-head with Dolby Digital 5.1. Like its competitor, it delivers 5.1 channels of sound (left, center, right, right surround, left surround and a low frequency effects (LFE) channel (the “.1”).

DTS-ES, ES stands for “extended sound” and adds a discrete rear surround channel. The rear channel can be played through one or two speakers. DTS has the advantage over THX because the THX rear surround signal is matrixed while DTS has a full-range discrete channel. However, presently, only a few DVDs include this encoding.

DTS Neo:6 is an advanced matrix decoder. It will take any two-channel source and expand it into five or six channels, depending on the user’s speaker layout. Two-channel sources include VHS tapes, broadcast television, stereo CDs and DVDs. DTS Neo:6 provides separate, optimized modes for stereo music materials and matrix surround motion picture soundtracks. DTS Neo:6 also decodes a center-surround channel from Extended Surround matrix soundtracks.


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