Although the previously outlined Dolby surround sound formats are designed to decode surround that is already encoded on DVDs or other material, there are thousands of music CDs, VHS movies, Laserdiscs, and television broadcasts that contain only simple analog two channel stereo or Dolby Surround encoding.
Surround Sound For Music
Also, with surround schemes such as Dolby Digital and Dolby Digital-EX primarily designed for movie viewing, there is a lack of an effective surround process for music listening. In fact, many discriminating audiophiles reject much of the surround sound schemes, including the new SACD (Super Audio CD) and DVD-Audio multi-channel audio formats, in favor of traditional two-channel stereo playback.
Manufacturers, such as Yamaha, have developed sound enhancement technologies (referred to as DSP - Digital Soundfield Processing) that can can place the source material in a virtual sound environment, such as a jazz club, concert hall, or stadium, but cannot "convert" two or four channel material into a 5.1 format.
The Benefits Of The Dolby Pro Logic II Decoding Process
With this in mind, Dolby Labs has come to the rescue with an enhancement to its original Dolby Pro-Logic technology that can create a "simulated" 5.1 channel surround environment from a 4-Channel Dolby Surround signal (dubbed Pro-Logic II). Although not a discrete format, such as Dolby Digital 5.1 or DTS, in which each channel goes though its own encoding/decoding process, Pro Logic II makes an effective use of matrixing to deliver an adequate 5.1 representation of a film or music soundtrack. With advancements in technology since the original Pro-Logic scheme was developed over 10 years ago, channel separation is more distinct, giving Pro Logic II the character of a discrete 5.1 channel scheme, such as Dolby Digital 5.1.
Extracting Surround Sound From Stereo Sources
Another benefit of Dolby Pro Logic II, is the ability to adequately create a surround listening experience from two-channel stereo music recordings. I, for one, have been less than satisfied trying to listen to two-channel music recordings in surround sound, using standard Pro Logic. Vocal balance, instrument placement, and transient sounds always seem to be somewhat unbalanced. There are, of course, many CD's that are Dolby Surround or DTS encoded, which are mixed for surround listening, but the vast majority are not and thus, can benefit from the application of Dolby Pro-Logic II enhancement.
Dolby Pro Logic II also has several settings that allow the listener to adjust the soundstage to suit specific tastes. These settings are:
Dimension control, which allows users to adjust the soundstage either towards the front or towards the rear.
Center Width Control, which Allows variable adjustment of the center image so it may be heard only from the Center speaker, only from the Left/Right speakers as a "phantom" center image, or various combinations of all three front speakers.
Panorama Mode which extends the front stereo image to include the Surround speakers for a wraparound effect.
A final advantage of a Pro-Logic II decoder is that it can also perform as a "regular" 4-channel Pro-Logic decoder, so, in essence, receivers that include Pro-Logic decoders can, instead, include Pro Logic II decoders, giving the consumer more flexibility, without having to having the expense of requiring two different Pro-Logic decoders in the same unit.
Dolby Pro Logic IIx
Lastly, a more recent variant of Dolby Pro Logic II is Dolby Pro Logic IIx, which expands the extracting capabilities of Dolby Pro Logic II, including its preference settings, to 6.1 or 7.1 channels on Dolby Pro Logic IIx-equipped receivers and preamps. Dolby Pro Logic IIx serves to deliver the listening experience to a greater number of channels without having to remix and reissue the original source material. This makes your record and CD collection easily adaptable to the latest surround sound listening environments.
Dolby Prologic IIz processing is an enhancement that extends surround sound vertically. Dolby Prologic IIz offers the option of adding two more front speakers that are placed above the left and right main speakers. This feature adds a "vertical" or overhead component to the surround sound field (great for rain, helicopter, plane flyover effects). Dolby Prologic IIz can be added to either a 5.1 channel or 7.1 channel setup. For more details, For more details, check out the official Dolby Prologic IIz page.
NOTE: Yamaha offers a similar technology on some of its home theater receivers called Presence.
Although the trend towards surround sound relies on adding additional channels and speakers, the requirement of multiple speakers around an entire room is not always practical. With that in mind, Dolby Labs has developed a way to create a fairly accurate surround experience that gives the illusion that you are listening to a complete surround speaker system, but is utilizing just two speakers and a subwoofer.
Dolby Virtual Speaker, when used with standard stereo sources, such as CD, creates a wider sound stage. However, when stereo sources are combined with Dolby Prologic II, or Dolby Digital encoded DVDs are played, Dolby Virtual speaker creates a 5.1 channel sound image using technology that takes into account sound reflection and how humans hear sound in a natural environment, enabling the surround sound signal to be reproduced without needing five or six speakers.
Audyssey, a company that develops and markets automatic speaker room equalization and correction software, has developed its own immersive surround sound technology: DSX (Dynamic Surround Expansion).
DSX adds front vertical-height speakers, similar to Prologic 11z, but also incorporates the addition of left/right wide speakers positioned between the front left and right and surround left and right speakers. For a more detailed explanation and speaker setup illustrations, check out the Official Audyssey DSX Page
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