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Surround Sound - Part Two: Dolby Surround And Dolby ProLogic

Surround Sound Transitions from the Cinema to the Home


The Emergence Of Dolby Surround

In the mid-70's Dolby Labs, with breakthrough film soundtracks such as Tommy, Star Wars, and Close Encounters of the Third Kind, unveiled a new surround sound process that was more easily adaptable for home use. Also, with the advent of the HiFi Stereo VCR and Stereo TV Broadcasting in the 1980's, there was an additional avenue for which to gain public acceptance of surround sound: Home Theater. Up to that point, listening to the sound portion of a TV Broadcast or VCR tape was like listening to a tabletop AM radio.

Dolby Surround Sound - Practical For The Home

With the ability encode the same surround information into a two channel signal that was encoded in the original Movie or TV soundtrack, software and hardware manufacturers had a new incentive to make affordable Surround sound components. Add-on Dolby Surround processors became available for those that already owned Stereo-only receivers. As the popularity of this experience reached into the more and more homes, more affordable Dolby Surround sound receivers and amplifiers became available, finally making Surround sound a permanent part of the Home Entertainment experience.

Dolby Surround Basics

The Dolby Surround process involves encoding four channels of information--Front Left, Center, Front Right, and Rear Surround into a two channel signal. A decoding chip then decodes the four channels and sends them to the appropriate destination, the Left, Right, Rear, and Phantom Center (center channel is derived from the L/R front channels).

The result of Dolby Surround mixing is a more balanced listening environment in which the main sounds derive from the left and right channels, the vocal or dialog emanates from the center phantom channel, and the ambience or effects information comes in from behind the listener.

In musical recordings encoded with this process the sound has a more natural feel, with better acoustical cues. In movie soundtracks the sensation of sounds moving from front to rear and left to right adds more realism to the viewing/listening experience by placing the viewer in the action. Dolby Surround is easily useful in both musical and film sound recording.

The Limitation Of Dolby Surround

Dolby Surround does have its limitations however, with the rear channel being basically passive, it lacks precise directionality. Also, overall separation between channels is much less than a typical Stereophonic recording.

Dolby Pro Logic

Dolby Pro Logic addresses the limitations of standard Dolby Surround by adding firmware and hardware elements in the decoding chip that emphasize important directional cues in a movie soundtrack. In other words, the decoding chip will add emphasis to directional sounds by increasing the output of the directional sounds in their respective channels.

This process, although not important in musical recordings, is very effective for film soundtracks and adds more accuracy to effects such as explosions, planes flying overhead, etc.. There is greater separation between channels. In addition, Dolby Pro Logic extracts a dedicated Center Channel that more accurately centers the dialog (this necessitates a center channel speaker for full effect) in a movie soundtrack.

The Limitation Of Dolby Pro-logic

Although Dolby Pro-Logic is an excellent refinement of Dolby Surround, its effects are derived strictly in the reproduction process, and even though the rear surround channel employs two speakers, they are still passing a monophonic signal, limiting rear-to-front and side-to-front motion and sound placement cues.

Continue On To Page 3 - Dolby Digital and Dolby Digital EX

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