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Dolby AC-3 Digital Surround Sound


With laserdisc, you can now experience in your home all of the excitement and heightened realism found in today's state-of-the-art digital movie theaters--with Dolby AC-3 Digital Surround! AC-3 marks the first major upgrade in home theater sound since Dolby Pro-Logic's introduction in 1987.
In order to fully appreciate the benefits provided by Dolby AC-3, it helps to understand the current Dolby Pro-Logic format. Dolby Surround is a format by which 4 channels of audio (left front, right front, center and surround) are processed and combined into a standard 2-channel stereo signal. A Dolby Pro-Logic surround sound decoder is then required to extract and convert each of the four channels back to it's original form. However, the decoder is unable to fully separate each of the channels from the others, causing sound intended only for one specific channel to bleed into the others. In addition, with all Pro-Logic systems, the two surround sound speakers receive an identical mono signal, not stereo, and the surround signal does not contain any low or high frequencies, only midrange. The overall result of Pro-Logic's limitations is reduced channel separation, imaging and soundstage depth; resulting in a less dynamic, realistic and involving recreation of the film's original soundtrack. Dolby Pro-Logic surround primarily provides a presence of ambient sound behind and around the listener and has the ability to direct sound from front to back, or across the front soundstage--but it is unable to recreate or locate specific sounds behind the listener or direct sounds diagonally from the left rear to the right front or move them around in a circle...that's where Dolby AC-3 Digital Surround comes in.
AC-3 effectively overcomes Pro-Logics limitations by featuring six discrete channels of audio (left front, right front, center, left rear, right rear and subwoofer) with true stereo, full-range surround channels. Because all six channels are independent from the recording process to the speakers without ever being mixed together, they do not have the ability to bleed together. This, along with AC-3's ability to direct sounds from virtually any direction, creates an incredible, breathtaking presentation--literally immersing the listener in a 3-Dimensional, 360 degree soundfield.
AC-3 also incorporates dynamic range compression and the ability to electronically "time align" each speaker. Dynamic range compression allows for low volume listening without losing the subtle dynamics and ambient characteristics of the soundtrack, perfect for late-night viewing. Electronic "time alignment" compensates for speakers that can't be placed exactly in the proper configuration. For example, the proper position for a center channel speaker is slightly behind the left and right front speakers; but if this is not possible and the center channel must be placed out in front of the left and right speakers, then the AC-3 decoder can electronically move the center channel into proper alignment by delaying the center channel signal. This allows for the seamless integration of the three front speakers and produces incredibly precise imaging and soundstage depth.
The AC-3 signal is encoded onto the area previously occupied by the laserdisc's right analog channel, leaving the standard Dolby Surround Sound audio on the disc's two digital tracks intact, as well as a mono version of the film's soundtrack on the left analog track. Therefore, AC-3 laserdiscs are fully compatible with existing Dolby Pro-Logic equipment, though the benefit of the AC-3 soundtrack will not be realized without an AC-3 decoder. But in the meantime, consumers can go on enjoying their existing equipment and their current and future laserdiscs with full confidence that nothing has become obsolete.
The necessary equipment required to implement Dolby AC-3 surround sound is a laserdisc player with an AC-3 RF output and an AC-3 decoder--either included in an audio/video receiver or as an add-on processor. The RF output is a recent addition to laserdisc players and is used to connect the laserdisc player to an AC-3 decoder. If you are wondering why the laserdisc player's digital output is not used to transfer the AC-3 signal to the decoder, the reason is that the digital output is internally connected to the laserdisc players two digital tracks, not the analog track where the AC-3 signal is recorded.

Image Entertainment, Inc.